Frank_D_Petruzella_Programmable_Logic_C.pdf

PROGRAMMABLE
LOGIC
CONTROLLERS
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PROGRAMMABLE
LOGIC
CONTROLLERS
F o u r t h E d i t i o n
F r a n k D .
P e t r u z e l l a
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PROGRAMMABLE LOGIC CONTROLLERS
Published by McGraw-Hill, a business unit of The McGraw-Hill Companies, Inc., 1221 Avenue of the
Americas, New York, NY, 10020. Copyright © 2011 by The McGraw-Hill Companies, Inc. All rights
reserved. Previous editions © 1989, 1998, and 2005. No part of this publication may be reproduced or
distributed in any form or by any means, or stored in a database or retrieval system, without the prior written
consent of The McGraw-Hill Companies, Inc., including, but not limited to, in any network or other electronic
storage or transmission, or broadcast for distance learning.
Some ancillaries, including electronic and print components, may not be available to customers outside the
United States.
This book is printed on acid-free paper.
1 2 3 4 5 6 7 8 9 0 DOW/DOW 1 0 9 8 7 6 5 4 3 2 1 0
ISBN 978-0-07-351088-0
MHID 0-07-351088-2
Vice president/Editor in chief: Elizabeth Haefele
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Compositor: MPS Limited, A Macmillan Company
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Cover credit: Cover Image Used with Permission of Rockwell Automation, Inc.
Library of Congress Cataloging-in-Publication Data
Petruzella, Frank D.
Programmable logic controllers / Frank D. Petruzella. — 4th ed.
p. cm.
Includes index.
ISBN-13: 978-0-07-351088-0 (alk. paper)
ISBN-10: 0-07-351088-2 (alk. paper)
1. Programmable controllers. I. Title.
TJ223.P76P48 2011
629.8’95—dc22
2010025533
The Internet addresses listed in the text were accurate at the time of publication. The inclusion of a Web site
does not indicate an endorsement by the authors or McGraw-Hill, and McGraw-Hill does not guarantee the
accuracy of the information presented at these sites.
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v
Contents
3.8 ASCII Code. . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.9 Parity Bit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.10 Binary Arithmetic . . . . . . . . . . . . . . . . . . . . . . 51
Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Chapter 4 Fundamentals of Logic 57
4.1 The Binary Concept . . . . . . . . . . . . . . . . . . . . 58
4.2 AND, OR, and NOT Functions. . . . . . . . . . . . 58
The AND Function . . . . . . . . . . . . . . . . . . . . . 58
The OR Function. . . . . . . . . . . . . . . . . . . . . . . 59
The NOT Function . . . . . . . . . . . . . . . . . . . . . 60
The Exclusive-OR (XOR) Function. . . . . . . . . 61
4.3 Boolean Algebra . . . . . . . . . . . . . . . . . . . . . . . 61
4.4 Developing Logic Gate Circuits
from Boolean Expressions . . . . . . . . . . . . . . . 63
4.5 Producing the Boolean Equation for a
Given Logic Gate Circuit . . . . . . . . . . . . . . . . 63
4.6 Hardwired Logic versus Programmed
Logic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.7 Programming Word Level Logic
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Chapter 5 Basics of PLC Programming 71
5.1 Processor Memory Organization . . . . . . . . . . 72
Program Files . . . . . . . . . . . . . . . . . . . . . . . . . 72
Data Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.2 Program Scan . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.3 PLC Programming Languages . . . . . . . . . . . . 79
5.4 Relay-Type Instructions . . . . . . . . . . . . . . . . . 81
5.5 Instruction Addressing . . . . . . . . . . . . . . . . . . 84
5.6 Branch Instructions. . . . . . . . . . . . . . . . . . . . . 85
5.7 Internal Relay Instructions . . . . . . . . . . . . . . . 87
5.8 Programming Examine If Closed and
Examine If Open Instructions. . . . . . . . . . . . . 88
5.9 Entering the Ladder Diagram . . . . . . . . . . . . . 89
5.10 Modes of Operation . . . . . . . . . . . . . . . . . . . . 91
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii
About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xiii
Walkthrough. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv
Chapter 1 Programmable Logic Controllers
(PLCs): An Overview 1
1.1 Programmable Logic Controllers . . . . . . . . . . . 2
1.2 Parts of a PLC. . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Principles of Operation. . . . . . . . . . . . . . . . . . . 8
1.4 Modifying the Operation . . . . . . . . . . . . . . . . 11
1.5 PLCs versus Computers . . . . . . . . . . . . . . . . . 11
1.6 PLC Size and Application. . . . . . . . . . . . . . . . 12
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Chapter 2 PLC Hardware Components 17
2.1 The I/O Section. . . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Discrete I/O Modules . . . . . . . . . . . . . . . . . . . 22
2.3 Analog I/O Modules . . . . . . . . . . . . . . . . . . . . 27
2.4 Special I/O Modules . . . . . . . . . . . . . . . . . . . . 29
2.5 I/O Specifications . . . . . . . . . . . . . . . . . . . . . . 32
Typical Discrete I/O Module Specifications . . 32
Typical Analog I/O Module Specifications . . . 33
2.6 The Central Processing Unit (CPU) . . . . . . . . 33
2.7 Memory Design . . . . . . . . . . . . . . . . . . . . . . . 35
2.8 Memory Types . . . . . . . . . . . . . . . . . . . . . . . . 36
2.9 Programming Terminal Devices . . . . . . . . . . . 37
2.10 Recording and Retrieving Data. . . . . . . . . . . . 38
2.11 Human Machine Interfaces (HMIs) . . . . . . . . 38
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Chapter 3 Number Systems and Codes 43
3.1 Decimal System . . . . . . . . . . . . . . . . . . . . . . . 44
3.2 Binary System. . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3 Negative Numbers. . . . . . . . . . . . . . . . . . . . . . 46
3.4 Octal System. . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.5 Hexadecimal System. . . . . . . . . . . . . . . . . . . . 48
3.6 Binary Coded Decimal (BCD) System. . . . . . 48
3.7 Gray Code. . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
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vi Contents
Chapter 9 Program Control Instructions 176
9.1 Master Control Reset Instruction . . . . . . . . . 177
9.2 Jump Instruction . . . . . . . . . . . . . . . . . . . . . . 180
9.3 Subroutine Functions . . . . . . . . . . . . . . . . . . 181
9.4 Immediate Input and Immediate Output
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . 184
9.5 Forcing External I/O Addresses . . . . . . . . . . 187
9.6 Safety Circuitry. . . . . . . . . . . . . . . . . . . . . . . 190
9.7 Selectable Timed Interrupt . . . . . . . . . . . . . . 193
9.8 Fault Routine. . . . . . . . . . . . . . . . . . . . . . . . . 194
9.9 Temporary End Instruction . . . . . . . . . . . . . . 194
9.10 Suspend Instruction. . . . . . . . . . . . . . . . . . . . 195
Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
Chapter 10 Data Manipulation Instructions 200
10.1 Data Manipulation . . . . . . . . . . . . . . . . . . . . 201
10.2 Data Transfer Operations . . . . . . . . . . . . . . . 201
10.3 Data Compare Instructions . . . . . . . . . . . . . . 209
10.4 Data Manipulation Programs . . . . . . . . . . . . 213
10.5 Numerical Data I/O Interfaces . . . . . . . . . . . 216
10.6 Closed-Loop Control . . . . . . . . . . . . . . . . . . 218
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
Chapter 11 Math Instructions 226
11.1 Math Instructions . . . . . . . . . . . . . . . . . . . . . 227
11.2 Addition Instruction . . . . . . . . . . . . . . . . . . . 227
11.3 Subtraction Instruction . . . . . . . . . . . . . . . . . 229
11.4 Multiplication Instruction . . . . . . . . . . . . . . . 230
11.5 Division Instruction . . . . . . . . . . . . . . . . . . . 231
11.6 Other Word-Level Math Instructions . . . . . . 233
11.7 File Arithmetic Operations . . . . . . . . . . . . . . 235
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
Chapter 12 Sequencer and Shift
Register Instructions 242
12.1 Mechanical Sequencers. . . . . . . . . . . . . . . . . 243
12.2 Sequencer Instructions . . . . . . . . . . . . . . . . . 245
12.3 Sequencer Programs . . . . . . . . . . . . . . . . . . . 248
12.4 Bit Shift Registers. . . . . . . . . . . . . . . . . . . . . 254
12.5 Word Shift Operations . . . . . . . . . . . . . . . . . 260
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
Chapter 6 Developing Fundamental PLC
Wiring Diagrams and Ladder
Logic Programs 95
6.1 Electromagnetic Control Relays. . . . . . . . . . . 96
6.2 Contactors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.3 Motor Starters . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.4 Manually Operated Switches . . . . . . . . . . . . . 99
6.5 Mechanically Operated Switches . . . . . . . . . 100
6.6 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Proximity Sensor. . . . . . . . . . . . . . . . . . . . . . 101
Magnetic Reed Switch. . . . . . . . . . . . . . . . . . 104
Light Sensors. . . . . . . . . . . . . . . . . . . . . . . . . 104
Ultrasonic Sensors . . . . . . . . . . . . . . . . . . . . 106
Strain/Weight Sensors . . . . . . . . . . . . . . . . . . 106
Temperature Sensors. . . . . . . . . . . . . . . . . . . 107
Flow Measurement . . . . . . . . . . . . . . . . . . . . 107
Velocity and Position Sensors . . . . . . . . . . . . 108
6.7 Output Control Devices . . . . . . . . . . . . . . . . 108
6.8 Seal-In Circuits . . . . . . . . . . . . . . . . . . . . . . . 110
6.9 Latching Relays . . . . . . . . . . . . . . . . . . . . . . 111
6.10 Converting Relay Schematics into PLC
Ladder Programs. . . . . . . . . . . . . . . . . . . . . . 116
6.11 Writing a Ladder Logic Program
Directly from a Narrative Description . . . . . 119
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Chapter 7 Programming Timers 125
7.1 Mechanical Timing Relays . . . . . . . . . . . . . . 126
7.2 Timer Instructions. . . . . . . . . . . . . . . . . . . . . 128
7.3 On-Delay Timer Instruction . . . . . . . . . . . . . 129
7.4 Off-Delay Timer Instruction . . . . . . . . . . . . . 133
7.5 Retentive Timer. . . . . . . . . . . . . . . . . . . . . . . 136
7.6 Cascading Timers . . . . . . . . . . . . . . . . . . . . . 140
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Chapter 8 Programming Counters 149
8.1 Counter Instructions . . . . . . . . . . . . . . . . . . . 150
8.2 Up-Counter . . . . . . . . . . . . . . . . . . . . . . . . . . 152
One-Shot Instruction. . . . . . . . . . . . . . . . . . . 155
8.3 Down-Counter. . . . . . . . . . . . . . . . . . . . . . . . 159
8.4 Cascading Counters . . . . . . . . . . . . . . . . . . . 162
8.5 Incremental Encoder-Counter
Applications . . . . . . . . . . . . . . . . . . . . . . . . . 165
8.6 Combining Counter and Timer Functions . . . 166
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
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Contents vii
Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
Tags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
Creating Tags . . . . . . . . . . . . . . . . . . . . . . . . 325
Monitoring and Editing Tags . . . . . . . . . . . . 326
Array. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
Review Questions . . . . . . . . . . . . . . . . . . . . . 328
Part 2 Bit-Level Programming . . . . . . . . . . . . . . . 329
Program Scan . . . . . . . . . . . . . . . . . . . . . . . . 329
Creating Ladder Logic . . . . . . . . . . . . . . . . . 330
Tag-Based Addressing . . . . . . . . . . . . . . . . . 331
Adding Ladder Logic to the Main Routine. . 332
Internal Relay Instructions . . . . . . . . . . . . . . 334
Latch and Unlatch Instructions. . . . . . . . . . . 334
One-Shot Instruction. . . . . . . . . . . . . . . . . . . 335
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
Part 3 Programming Timers . . . . . . . . . . . . . . . . . 338
Timer Predefined Structure. . . . . . . . . . . . . . 338
On-Delay Timer (TON) . . . . . . . . . . . . . . . . 339
Off-Delay Timer (TOF) . . . . . . . . . . . . . . . . 342
Retentive Timer On (RTO) . . . . . . . . . . . . . . 344
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
Part 4 Programming Counters . . . . . . . . . . . . . . . 347
Counters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
Count-Up (CTU) Counter. . . . . . . . . . . . . . . 348
Count-Down (CTD) Counter . . . . . . . . . . . . 350
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
Part 5 Math, Comparison, and Move
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . 353
Math Instructions . . . . . . . . . . . . . . . . . . . . . 353
Comparison Instructions. . . . . . . . . . . . . . . . 355
Move Instructions . . . . . . . . . . . . . . . . . . . . . 357
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
Part 6 Function Block Programming . . . . . . . . . . 361
Function Block Diagram (FBD) . . . . . . . . . . 361
FBD Programming . . . . . . . . . . . . . . . . . . . . 365
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
Chapter 13 PLC Installation Practices,
Editing, and Troubleshooting 268
13.1 PLC Enclosures. . . . . . . . . . . . . . . . . . . . . . . 269
13.2 Electrical Noise. . . . . . . . . . . . . . . . . . . . . . . 271
13.3 Leaky Inputs and Outputs. . . . . . . . . . . . . . . 272
13.4 Grounding. . . . . . . . . . . . . . . . . . . . . . . . . . . 272
13.5 Voltage Variations and Surges. . . . . . . . . . . . 274
13.6 Program Editing and Commissioning. . . . . . 275
13.7 Programming and Monitoring . . . . . . . . . . . 276
13.8 Preventive Maintenance . . . . . . . . . . . . . . . . 278
13.9 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . 279
Processor Module . . . . . . . . . . . . . . . . . . . . . 279
Input Malfunctions . . . . . . . . . . . . . . . . . . . . 279
Output Malfunctions . . . . . . . . . . . . . . . . . . . 281
Ladder Logic Program . . . . . . . . . . . . . . . . . 281
13.10 PLC Programming Software. . . . . . . . . . . . . 286
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
Chapter 14 Process Control, Network
Systems, and SCADA 291
14.1 Types of Processes . . . . . . . . . . . . . . . . . . . . 292
14.2 Structure of Control Systems . . . . . . . . . . . . 294
14.3 On/Off Control . . . . . . . . . . . . . . . . . . . . . . . 296
14.4 PID Control. . . . . . . . . . . . . . . . . . . . . . . . . . 297
14.5 Motion Control . . . . . . . . . . . . . . . . . . . . . . . 301
14.6 Data Communications. . . . . . . . . . . . . . . . . . 303
Data Highway. . . . . . . . . . . . . . . . . . . . . . . . 308
Serial Communication . . . . . . . . . . . . . . . . . 308
DeviceNet . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
ControlNet. . . . . . . . . . . . . . . . . . . . . . . . . . . 310
EtherNet/IP. . . . . . . . . . . . . . . . . . . . . . . . . . 311
Modbus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
Fieldbus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
PROFIBUS-DP. . . . . . . . . . . . . . . . . . . . . . . 313
14.7 Supervisory Control
and Data Acquisition (SCADA) . . . . . . . . . . 313
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
Chapter 15 ControlLogix Controllers 317
Part 1 Memory and Project Organization . . . . . . 318
Memory Layout . . . . . . . . . . . . . . . . . . . . . . 318
Configuration . . . . . . . . . . . . . . . . . . . . . . . . 318
Project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
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ix
Preface
Programmable logic controllers (PLCs) continue to
evolve as new technologies are added to their capabili-
ties. The PLC started out as a replacement for hardwired
relay control systems. Gradually, various math and logic
manipulation functions were added. Today PLCs are the
controller of choice for the vast majority of automated
processes. PLCs now incorporate smaller cases, faster
CPUs, networking, and various Internet technologies.
This Fourth Edition of Programmable Logic Control-
lers continues to provide an up-to-date introduction to all
aspects of PLC programming, installation, and mainte-
nance procedures. No previous knowledge of PLC sys-
tems or programming is assumed. As one reviewer of this
edition put it: “I honestly believe that someone with little
or no background to PLC systems could take this book
and teach themselves PLCs.”
The primary source of information for a particular PLC
is always the accompanying user manuals provided by the
manufacturer. This textbook is not intended to replace the
vendor’s reference material but rather to complement,
clarify, and expand on this information. With the current
number of different types of PLCs on the market it is not
practical to cover the specifics of all manufacturers and
models in a single text. With this in mind, the text dis-
cusses PLCs in a generic sense. Although the content is
of a nature to allow the information to be applied to a
variety of PLCs from different manufacturers, this book,
for the most part, uses the Allen-Bradley SLC 500 and
ControlLogix controller instruction sets for the program-
ming examples. The underlying PLC principles and
concepts covered in the text are common to most manu-
facturers and serve to maximize the knowledge gained
through attending PLC training programs offered by dif-
ferent vendors.
The text is written at a level and format understandable
to students being introduced to PLCs for the first time.
Feedback from instructors indicates that the information
is well organized, to the point, and easy to understand.
The content of this new Fourth Edition has been updated
and reflects the changes in technology since the publica-
tion of the previous edition.
Each chapter begins with a brief introduction outlin-
ing chapter coverage and learning objectives. When ap-
plicable, the relay equivalent of the virtual programmed
instruction is explained first, followed by the appropriate
PLC instruction. Chapters conclude with a set of review
questions and problems. The review questions are closely
related to the chapter objectives and require students to
recall and apply information covered in the chapter. The
problems range from easy to difficult, thus challenging
students at various levels of competence.
This new Fourth Edition has been revised to include a
number of new features:
How Programs Operate When the operation of a
program is called for, a bulleted list is used to summa-
rize its execution. The list is used in place of lengthy
paragraphs and is especially helpful when explaining
the different steps in the execution of a program.
Representation of I/O Field Devices Recognition
of the input and output field devices associated with
the program helps in understanding the overall opera-
tion of the program. With this in mind, in addition
to their symbols, we provide drawings and photos of
field input and output devices.
New ControlLogix Chapter Some instructors have
felt that students tend to get confused when switch-
ing back and forth from SLC 500 Logic to Logix
5000–based programming within the same chapter.
For this reason, a new Chapter 15 has been added that
is devoted entirely to the Allen-Bradley ControlLogix
family of controllers and the RSLogix 5000 software.
Each part of the new Chapter 15 is treated as a sepa-
rate unit of study and includes ControlLogix:
• Memory and Project Organization
• Bit-Level Programming
• Programming Timers
• Programming Counters
• Math, Comparison, and Move Instructions
• Function Block Programming
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x Preface
Chapter changes in this edition include:
Chapter 1
• Drawings and photos of real world field input and
output devices have been added.
• 50% more figures have been added to this chapter
to increase visual appeal and illustrate key concepts
further.
• Most recent photographs from major PLC
manufactures.
• Revisions to chapter review questions and problems.
Chapter 2
• Information on the latest selection of PLC hardware
components.
• Human machine interfacing with Pico controllers
added.
manufactures.
Chapter 3
• Improvement in sizing and placement of drawings
make explanations of the different number systems
easier to follow.
Chapter 4
make explanations easier to follow.
• Drawings and photos of real world field input
and output devices have been added to the logic
diagrams.
Chapter 5
• Information on the ControlLogix memory organiza-
tion relocated to chapter 15.
• Program scan process explained in greater detail.
• Extended coverage of relay type instructions.
• Instruction addressing examined in greater detail.
• Addressing of a micro PLC illustrated.
Chapter 6
output devices have been included in the ladder
logic programs.
• Wiring of field inputs and outputs to a micro PLC
illustrated.
• Additional coverage of hardwired motor control
circuits and their PLC equivalent.
Chapter 7
• Information on the ControlLogix timers relocated to
chapter 15.
logic programs.
• Bulleted lists used to summarize program execution.
manufactures.
Chapter 8
• Information on the ControlLogix counters relocated
to chapter 15.
logic programs.
manufactures.
Chapter 9
logic programs.
• Forcing of inputs and outputs covered in greater
detail.
• Differences between a safety PLC and a standard
PLC explained.
manufactures.
Chapter 10
logic programs.
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Preface xi
• Examines communications at all levels in an indus-
trial network in much greater detail.
• Fundamentals of PLC motion control have been
added.
• New photographs from major PLC manufactures.
Chapter 15
• Completely new chapter that concentrates on the
fundamentals of ControlLogix technology.
• Includes Memory and Project Organization, Bit
Level Programming, Timers, Counters, Math
Instructions, and Function Block Programming.
Ancillaries
• Activity Manual for Programmable Logic
Controllers, Fourth Edition.
This manual contains:
Tests made up of multiple choice, true/false,
and completion-type questions for each of the
chapters.
Generic programming hands-on exercises de-
signed to offer students real-world programming
experience. These assignments are designed for
use with any brand of PLC.
• LogixPro PLC Lab Manual for use with
Programmable Logic Controllers, Fourth
Edition
This manual contains:
LogixPro 500 simulator software CD. The
LogixPro simulation software converts the stu-
dent’s computer into a PLC and allows the stu-
dent to write ladder logic programs and verify
their real-world operation.
Over 250 LogixPro student lab exercises se-
quenced to support material covered in the text.
• Instructor’s Resource Center is available to in-
structors who adopt Programmable Logic Control-
lers, Fourth Edition. It includes:
Textbook answers to all questions and problems.
Activity Manual answers to all tests.
Computer Simulation Lab Manual answers for
all programming exercises.
PowerPoint presentations for each chapter.
EZ Test testing software with text-coordinated
question banks.
ExamView text-coordinated question banks.
• Analog control covered in more detail.
• PID control process explained in a simplified
manner.
manufactures.
Chapter 11
logic programs.
make explanations of the different math instructions
easier to follow.
manufactures.
Chapter 12
output devices have been included in the sequencer
programs.
• Improvements to sequencer line drawings designed
to make this instruction easier to follow.
manufactures.
Chapter 13
• Safety issues examined in greater detail.
• Extended coverage of practical troubleshooting
techniques.
• Improvement to PLC grounding diagrams makes
this function easier to follow.
manufactures.
Chapter 14
• All pertinent information from Chapters 14 and 15
of the 3rd edition have been incorporated into this
chapter.
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xii
Acknowledgments
Ralph Neidert
NECA/IBEW Local 26 JATC
Chrys Panayiotou
Indian River State College
Don Pelster
Nashville State Technical Community College
Dale Petty
Washtenaw Community College
Sal Pisciotta
Florence-Darlington Technical College
Roy E. Pruett
Bluefield State College
Melvin Roberts
Camden County College
Farris Saifkani
Northeast Wisconsin Technical College
David Setser
Johnson County Community College
Richard Skelton
Jackson State Community College
Amy Stephenson
Pitt Community College
William Sutton
ITT Technical Institute
John Wellin
Rochester Institute of Technology.
Last but not least, special thanks to Wade Wittmus of
Lakeshore Technical College, not only for his extended
help with reviews but also for his outstanding work on
the supplements.
Frank D. Petruzella
I would like to thank the following reviewers for their
comments and suggestions:
Wesley Allen
Jefferson State Community College
Bo Barry
University of North Carolina–Charlotte
David Barth
Edison Community College
Michael Brumbach
York Technical College
Fred Cope
Northeast State Technical Community College
Warren Dejardin
Northeast Wisconsin Technical College
Montie Fleshman
New River Community College
Steven Flinn
Illinois Central College
Brent Garner
McNeese State University
John Haney
Snead State Community College
Thomas Heraly
Milwaukee Area Technical College
John Lukowski
Michigan Technical University
John Martini
University of Arkansas–Fort Smith
Steven McPherson
Sauk Valley Community College
Max Neal
Griffin Technical College
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xiii
About the Author
Frank D. Petruzella has extensive practical
experience in the electrical control field, as well
as many years of experience teaching and author-
ing textbooks. Before becoming a full time edu-
cator, he was employed as an apprentice and
electrician in areas of electrical installation and
maintenance. He holds a Master of Science degree
from Niagara University, a Bachelor of Science
degree from the State University of New York
College–Buffalo, as well as diplomas in Electrical
Power and Electronics from the Erie County
Technical Institute.
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E
E
P
rogrammable Logic Controllers makes it
easy to learn PLCs from the ground up! Up-
to-the-minute revisions include all the new-
est developments in programming, installing, and
maintaining processes. Clearly developed chapters
deliver the organizing objectives, explanatory con-
tent with helpful diagrams and illustrations, and
closing review problems that evaluate retention of
the chapter objectives.
CHAPTER OBJECTIVES overview the chapter, letting stu-
dents and instructors focus on the main points to better grasp
concepts and retain information.
Here, drawings and photos of real-world input and
output devices have been included
In Chapter 14, students not only read about but can also see how
HMIs fit into an overall PLC system, giving them a practical
introduction to the topics
Chapter Objectives
After completing this chapter, you will be able to:
2.1 List and describe the function of the hardware
components used in PLC systems
2.2 Describe the basic circuitry and applications for
discrete and analog I/O modules, and interpret typical
I/O and CPU specifications
2.3 Explain I/O addressing
2.4 Describe the general classes and types of PLC
memory devices
2.5 List and describe the different types of PLC
peripheral support devices available
pet10882_ch02_017-042.indd 17
Figure 8-20 Alarm monitor program.
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:5
1.0
1
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
1
0
Ladder logic program Output
L2
T4:6
C5:1
DN
T4:5
DN
C5:1
DN
FS
FS
SS
C5:1
RES
Light
Light
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:6
1.0
1
0
OFF ON
L1
Inputs
FS
SS
DN
T4:5
DN
EN
EN
CU
DN
DN
DN
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Chapter content includes rich illustrative detail and extensive
visual aids, allowing students to grasp concepts more quickly
and understand practical applications
Figure 14-7 Human machine interface (HMI).
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xiv
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E
E
Additional coverage of communications and control networks
utilizes clear graphics to demonstrate how things work
BULLETED LISTS break down processes to helpfully sum-
marize execution of tasks
Figure 14-48 Typical SCADA system.
Host computer
SCADA/HMI
software
Data
transfer
I/O
control Field
devices
I/O
control Field
devices
I/O
control Field
devices
PLC
PLC
PLC
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An entirely new chapter on ControlLogix has
been added to familiarize students with the
entire Allen-Bradley family of controllers
and RSLogix 5000 software
Figure 15-1 Programmable automation controllers (PACs).
Source: Image Used with Permission of Rockwell Automation, Inc.
scan time.
The scan is normally a continuous and sequential pro-
cess of reading the status of inputs, evaluating the control
logic, and updating the outputs. Figure 5-8 shows an over-
view of the data flow during the scan process. For each
rung executed, the PLC processor will:
• Examine the status of the input image table bits.
• Solve the ladder logic in order to determine logical
continuity.
pet10882_ch05_071-094.indd 77 6/30/10 5:07 PM
• Update the appropriate output image table bits, if
necessary.
• Copy the output image table status to all of the out-
put terminals. Power is applied to the output device
if the output image table bit has been previously set
to a 1.
• Copy the status of all of the input terminals to the
input image table. If an input is active (i.e., there is
electrical continuity), the corresponding bit in the
input image table will be set to a 1.
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Diagrams, such as this one illustrating an overview of the func-
tion block programming language, help students put the pieces
together
10000
0
DN_PL
0
TT_PL
0
EN_PL
Status_Timer.DN
DN_PL
<Local:2:O.Data.3>
Status_Timer.TT
Input
L1
TT_PL
<Local:2:O.Data.2>
Status_Timer.EN
EN
Timer On Delay
Timer
Preset
Accum
Status_Timer
10000
0
EN_PL
<Local:2:O.Data.1>
DN
Timer_Sw
<Local:1:I.Data.6>
TON
Ladder logic
FBD equivalent
TONR_01
...
TONR
Timer On Delay with Reset
TimerEnable ACC
PRE
Reset
EN
TT
DN
Timer_Sw
Outputs L2
TT_PL
EN_PL
0
ACC_Value
0
10000
Timer_Sw
DN_PL
Figure 15-95 Comparison between ladder logic and the FBD equivalent for a
10 second TON and TONR timer.
xv
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E
E
END-OF-CHAPTER REVIEWS are structured to reinforce
chapter objectives
EXAMPLE PROBLEMS help bring home the applicability
of chapter concepts
1. Convert each of the following binary numbers to
decimal numbers:
a. 10
b. 100
c. 111
d. 1011
e. 1100
f. 10010
g. 10101
h. 11111
i. 11001101
j. 11100011
2. Convert each of the following decimal numbers to
binary numbers:
a. 7
b. 19
c. 28
d. 46
e. 57
f. 86
g. 94
h. 112
i. 148
j. 230
3. Convert each of the following octal numbers to
decimal numbers:
a. 36
b. 104
c 120
6. Convert each of the following hexadecimal num-
bers to binary numbers:
a. 4C
b. E8
c. 6D2
d. 31B
BCD:
a. 146
b. 389
c. 1678
d. 2502
8. What is the most important characteristic of the
Gray code?
9. What makes the binary system so applicable to
computer circuits?
10. Define the following as they apply to the binary
memory locations or registers:
a. Bit
b. Byte
c. Word
d. LSB
e. MSB
11. State the base used for each of the following num-
ber systems:
a. Octal
b. Decimal
c. Binary
CHAPTER 3 REVIEW QUESTIONS
pet10882_ch03_043-056.indd 55 6/30/10 5:06 PM
1. The following binary PLC coded information is to
be programmed using the hexadecimal code. Convert
each piece of binary information to the appropriate
hexadecimal code for entry into the PLC from the
keyboard.
a. 0001 1111
b. 0010 0101
c. 0100 1110
d. 0011 1001
2. The encoder circuit shown in Figure 3-17 is used
to convert the decimal digits on the keyboard to a
binary code. State the output status (HIGH/LOW) of
A-B-C-D when decimal number
a. 2 is pressed.
b. 5 is pressed.
CHAPTER 3 PROBLEMS
c. 7 is pressed.
d. 8 is pressed.
3. If the bits of a 16-bit word or register are numbered
according to the octal numbering system, beginning
with 00, what consecutive numbers would be used to
represent each of the bits?
4. Express the decimal number 18 in each of the fol-
lowing number codes:
a. Binary
b. Octal
c. Hexadecimal
d. BCD
Figure 3-17 Diagram for Problem 2.
Decimal
number
pressed
Binary-coded
0100 output
High
High
Low
Low
Low
Encoder
A
B
C
D
4 input
High
7 8 9
4 5 6
1 2
0
3
0
1
2
3
4
5
6
7
8
9
xvi
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E
E
ANCILLARIES THAT WORK
Expanded on and updated from the previous edition, this new edition includes an outstanding instructor support package:
• ExamView and EZ Test question test banks for each chapter.
• PowerPoint lessons with animations that help visualize the actual process.
• Activity Manual contains true/false, completion, matching, and multiple-choice questions for every chapter in the text. So that
students get a better understanding of programmable logic controllers, the manual also includes a wide range of programming
assignments and additional practice exercises.
• On-line Instructor’s Resource Center.
In addition, for students, this edition also has available:
• LogixPro PLC Lab Manual for use with Programmable Logic Con-
trollers Fourth Edition, with LogixPro PLC Simulator. This manual
contains:
• LogixPro 500 simulator software CD. The LogixPro simulation
software converts the student’s computer into a PLC and allows the
student to write ladder logic programs and verify their real-world
operation.
• Over 250 LogixPro student lab exercises sequenced to support ma-
terial covered in the text.
xvii
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1
1
Programmable Logic
Controllers (PLCs)
An Overview
This chapter gives a brief history of the evolution
of the programmable logic controller, or PLC.
The reasons for changing from relay control sys-
tems to PLCs are discussed. You will learn the
basic parts of a PLC, how a PLC is used to con-
trol a process, and the different kinds of PLCs
and their applications. The ladder logic language,
which was developed to simplify the task of pro-
gramming PLCs, is introduced.
Chapter Objectives
1.1 Define what a programmable logic controller (PLC) is
and list its advantages over relay systems
1.2 Identify the main parts of a PLC and describe their
functions
1.3 Outline the basic sequence of operation for a PLC
1.4 Identify the general classifications of PLCs
Image Used with Permission of Rockwell Automation, Inc.
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2 Chapter 1 Programmable Logic Controllers (PLCs)
Programmable controllers offer several advantages
over a conventional relay type of control. Relays have to
be hardwired to perform a specific function. When the
system requirements change, the relay wiring has to be
changed or modified. In extreme cases, such as in the auto
industry, complete control panels had to be replaced since
it was not economically feasible to rewire the old panels
with each model changeover. The programmable control-
ler has eliminated much of the hardwiring associated with
conventional relay control circuits (Figure 1-2). It is small
and inexpensive compared to equivalent relay-based pro-
cess control systems. Modern control systems still include
relays, but these are rarely used for logic.
In addition to cost savings, PLCs provide many other
benefits including:
• Increased Reliability. Once a program has been
written and tested, it can be easily downloaded
to other PLCs. Since all the logic is contained in
the PLC’s memory, there is no chance of making
a logic wiring error (Figure 1-3). The program
takes the place of much of the external wiring that
would normally be required for control of a process.
Hardwiring, though still required to connect field
devices, is less intensive. PLCs also offer the
reliability associated with solid-state components.
• More Flexibility. It is easier to create and change a pro-
gram in a PLC than to wire and rewire a circuit. With a
PLC the relationships between the inputs and outputs
are determined by the user program instead of the
manner in which they are interconnected (Figure 1-4).
Original equipment manufacturers can provide system
updates by simply sending out a new program. End
users can modify the program in the field, or if desired,
security can be provided by hardware features such as
key locks and by software passwords.
• Lower Cost. PLCs were originally designed to re-
place relay control logic, and the cost savings have
been so significant that relay control is becoming
1.
1 Programmable Logic Controllers
Programmable logic controllers (Figure 1-1) are now the
most widely used industrial process control technology.
A programmable logic controller (PLC) is an industrial
grade computer that is capable of being programmed to
perform control functions. The programmable controller
has eliminated much of the hardwiring associated with
conventional relay control circuits. Other benefits include
easy programming and installation, high control speed,
network compatibility, troubleshooting and testing conve-
nience, and high reliability.
The programmable logic controller is designed for
multiple input and output arrangements, extended tem-
perature ranges, immunity to electrical noise, and resis-
tance to vibration and impact. Programs for the control
and operation of manufacturing process equipment and
machinery are typically stored in battery-backed or non-
volatile memory. A PLC is an example of a real-time sys-
tem since the output of the system controlled by the PLC
depends on the input conditions.
The programmable logic controller is, then, basically
a digital computer designed for use in machine control.
Unlike a personal computer, it has been designed to op-
erate in the industrial environment and is equipped with
special input/output interfaces and a control programming
language. The common abbreviation used in industry for
these devices, PC, can be confusing because it is also the
abbreviation for “personal computer.” Therefore, most
manufacturers refer to their programmable controller as a
PLC, which stands for “programmable logic controller.”
Initially the PLC was used to replace relay logic, but its
ever-increasing range of functions means that it is found in
many and more complex applications. Because the struc-
ture of a PLC is based on the same principles as those
employed in computer architecture, it is capable not only
of performing relay switching tasks but also of performing
other applications such as timing, counting, calculating,
comparing, and the processing of analog signals.
Figure 1-1 Programmable logic controller.
Source: (a–b) Courtesy GE Intelligent Platforms.
(a) (b)
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Programmable Logic Controllers (PLCs) Chapter 1 3
obsolete except for power applications. Generally,
if an application has more than about a half-dozen
control relays, it will probably be less expensive to
install a PLC.
• Communications Capability. A PLC can communi-
cate with other controllers or computer equipment to
perform such functions as supervisory control, data
gathering, monitoring devices and process parameters,
and download and upload of programs (Figure 1-5).
User program
PLC
Figure 1-3 All the logic is contained in the PLC’s memory.
Figure 1-4 Relationships between the inputs and outputs
are determined by the user program.
Contactor Light Solenoid
Outputs
Inputs
Pushbutton Limit switch Sensor
• Faster Response Time. PLCs are designed for high-
speed and real-time applications (Figure 1-6). The
programmable controller operates in real time,
which means that an event taking place in the
field will result in the execution of an operation or
output. Machines that process thousands of items
per second and objects that spend only a fraction
of a second in front of a sensor require the PLC’s
quick-response capability.
• Easier to Troubleshoot. PLCs have resident diag-
nostics and override functions that allow users to
(a)
(b)
Figure 1-2 Relay- and PLC-based control panels. (a) Relay-
based control panel. (b) PLC-based control panel.
Source: (a) Courtesy Mid-Illini Technical Group, Inc.; (b) Photo courtesy Ramco
Electric, Ltd.
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easily trace and correct software and hardware prob-
lems. To find and fix problems, users can display the
control program on a monitor and watch it in real
time as it executes (Figure 1-7).
1.2 Parts of a PLC
A typical PLC can be divided into parts, as illustrated in
Figure 1-8. These are the central processing unit (CPU),
the input/output (I/O) section, the power supply, and the
programming device. The term architecture can refer to
PLC hardware, to PLC software, or to a combination of
both. An open architecture design allows the system to be
connected easily to devices and programs made by other
manufacturers. Open architectures use off-the-shelf com-
ponents that conform to approved standards. A system
with a closed architecture is one whose design is propri-
etary, making it more difficult to connect to other systems.
Most PLC systems are in fact proprietary, so you must be
sure that any generic hardware or software you may use
is compatible with your particular PLC. Also, although
the principal concepts are the same in all methods of pro-
gramming, there might be slight differences in address-
ing, memory allocation, retrieval, and data handling for
different models. Consequently, PLC programs cannot be
interchanged among different PLC manufacturers.
There are two ways in which I/Os (Inputs/Outputs) are
incorporated into the PLC: fixed and modular. Fixed I/O
(Figure 1-9) is typical of small PLCs that come in one
package with no separate, removable units. The processor
and I/O are packaged together, and the I/O terminals will
have a fixed number of connections built in for inputs and
outputs. The main advantage of this type of packaging is
lower cost. The number of available I/O points varies and
usually can be expanded by buying additional units of
fixed I/O. One disadvantage of fixed I/O is its lack of flex-
ibility; you are limited in what you can get in the quanti-
ties and types dictated by the packaging. Also, for some
models, if any part in the unit fails, the whole unit has to
be replaced.
Modular I/O (Figure 1-10) is divided by compartments
into which separate modules can be plugged. This fea-
ture greatly increases your options and the unit’s flexibil-
ity. You can choose from the modules available from the
manufacturer and mix them any way you desire. The basic
modular controller consists of a rack, power supply, pro-
cessor module (CPU), input/output (I/O modules), and an
operator interface for programming and monitoring. The
modules plug into a rack. When a module is slid into the
rack, it makes an electrical connection with a series of con-
tacts called the backplane, located at the rear of the rack.
The PLC processor is also connected to the backplane and
can communicate with all the modules in the rack.
The power supply supplies DC power to other modules
that plug into the rack (Figure 1-11). For large PLC systems,
this power supply does not normally supply power to the
field devices. With larger systems, power to field devices is
Figure 1-7 Control program can be displayed on a monitor
in real time.
PLC Monitor
Figure 1-5 PLC communication module.
Source: Photo courtesy Automation Direct, www.automationdirect.com.
Figure 1-6 High-speed counting.
Source: Courtesy Banner Engineering Corp.
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(b) Fixed type
Power supply
Communications
Input
section
Output
section
Memory
CPU
M
(a) Modular type
Central
Processing
Unit (CPU)
Programming device
Memory
Input
sensing
devices
Output
load
devices
Program Data
Optical
isolation
Input
module
Output
module
Processor Module
Optical
isolation
Power supply
module
Figure 1-8 Typical parts of a programmable logic controller.
Source: (a) Courtesy Mitsubishi Automation; (b) Image Used with Permission of Rockwell Automation, Inc.
Figure 1-9 Fixed I/O conﬁguration.
PL
Input
connections
Common power bus
Common return bus
Output
connections
Processor PLC
provided by external alternating current (AC) or direct cur-
rent (DC) supplies. For some small micro PLC systems, the
power supply may be used to power field devices.
The processor (CPU) is the “brain” of the PLC. A typi-
cal processor (Figure 1-12) usually consists of a micro-
processor for implementing the logic and controlling the
communications among the modules. The processor re-
quires memory for storing the results of the logical op-
erations performed by the microprocessor. Memory is also
required for the program EPROM or EEPROM plus RAM.
The CPU controls all PLC activity and is designed so
that the user can enter the desired program in relay ladder
logic. The PLC program is executed as part of a repeti-
tive process referred to as a scan (Figure 1-13). A typical
PLC scan starts with the CPU reading the status of inputs.
Then, the application program is executed. Once the pro-
gram execution is completed, the CPU performs internal
diagnostic and communication tasks. Next, the status of
all outputs is updated. This process is repeated continu-
ously as long as the PLC is in the run mode.
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The I/O system forms the interface by which field de-
vices are connected to the controller (Figure 1-14). The
purpose of this interface is to condition the various sig-
nals received from or sent to external field devices. Input
devices such as pushbuttons, limit switches, and sensors
are hardwired to the input terminals. Output devices such
as small motors, motor starters, solenoid valves, and in-
dicator lights are hardwired to the output terminals. To
electrically isolate the internal components from the
input and output terminals, PLCs commonly employ an
optical isolator, which uses light to couple the circuits to-
gether. The external devices are also referred to as “field”
or “real-world” inputs and outputs. The terms field or
real world are used to distinguish actual external devices
that exist and must be physically wired from the internal
user program that duplicates the function of relays, tim-
ers, and counters.
Figure 1-11 The power supply supplies DC power to other
modules that plug into the rack.
Source: This material and associated copyrights are proprietary to, and used
with the permission of Schneider Electric.
Power supply
Figure 1-12 Typical PLC processor modules.
Execu
t
e
p
r
o
g
r
a
m
Diagnost
i
c
s
&
communi
c
a
t
i
o
n
R
e
ad
inputs
U
p
d
a
t
e
o
u
tputs
Figure 1-13 Typical PLC scan cycle.
Output module
Input module
Processor
module
Power
supply
Combination
I/O module
Module
slides into
the rack
Figure 1-10 Modular I/O conﬁguration.
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A programming device is used to enter the desired pro-
gram into the memory of the processor. The program can
be entered using relay ladder logic, which is one of the
most popular programming languages. Instead of words,
ladder logic programming language uses graphic symbols
that show their intended outcome. A program in ladder
logic is similar to a schematic for a relay control circuit.
It is a special language written to make it easy for peo-
ple familiar with relay logic control to program the PLC.
Hand-held programming devices (Figure 1-15) are some-
times used to program small PLCs because they are inex-
pensive and easy to use. Once plugged into the PLC, they
can be used to enter and monitor programs. Both compact
hand-held units and laptop computers are frequently used
on the factory floor for troubleshooting equipment, modi-
fying programs, and transferring programs to multiple
machines.
A personal computer (PC) is the most commonly used
programming device. Most brands of PLCs have software
available so that a PC can be used as the programming
device. This software allows users to create, edit, docu-
ment, store, and troubleshoot ladder logic programs (Fig-
ure 1-16). The computer monitor is able to display more
logic on the screen than can hand-held types, thus sim-
plifying the interpretation of the program. The personal
computer communicates with the PLC processor via a se-
rial or parallel data communications link, or Ethernet. If
the programming unit is not in use, it may be unplugged
and removed. Removing the programming unit will not
affect the operation of the user program.
A program is a user-developed series of instructions
that directs the PLC to execute actions. A programming
language provides rules for combining the instructions
so that they produce the desired actions. Relay ladder
logic (RLL) is the standard programming language used
Input module
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
IN 0
IN 2
IN 4
IN 6
IN 8
IN 10
IN 12
IN 14
DC
COM
IN 1
IN 3
IN 5
IN 7
IN 9
IN 11
IN 13
IN 15
DC
COM
Output module
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
VAC
OUT 1
OUT 3
OUT 5
OUT 7
OUT 9
OUT 11
OUT 13
OUT 15
OUT 0
OUT 2
OUT 4
OUT 6
OUT 8
OUT 10
OUT 12
OUT 14
AC
COM
24 VDC
input
module
240 VAC
output
module
24 VDC
Field device
power supply
+ –
240 VAC
M
Field device
power supply
L2 L1
R
Y
Figure 1-14 Typical PLC input/output (I/O) system connections.
Figure 1-15 Typical hand-held programming device.
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with PLCs. Its origin is based on electromechanical relay
control. The relay ladder logic program graphically rep-
resents rungs of contacts, coils, and special instruction
blocks. RLL was originally designed for easy use and un-
derstanding for its users and has been modified to keep up
with the increasing demands of industry’s control needs.
1.3 Principles of Operation
To get an idea of how a PLC operates, consider the simple
process control problem illustrated in Figure 1-17. Here a
mixer motor is to be used to automatically stir the liquid
in a vat when the temperature and pressure reach preset
values. In addition, direct manual operation of the motor
is provided by means of a separate pushbutton station.
The process is monitored with temperature and pressure
sensor switches that close their respective contacts when
conditions reach their preset values.
This control problem can be solved using the relay
method for motor control shown in the relay ladder dia-
gram of Figure 1-18. The motor starter coil (M) is energized
when both the pressure and temperature switches are closed
or when the manual pushbutton is pressed.
Now let’s look at how a programmable logic controller
might be used for this application. The same input field
devices (pressure switch, temperature switch, and push-
button) are used. These devices would be hardwired to an
appropriate input module according to the manufacturer’s
addressing location scheme. Typical wiring connections
for a 120 VAC modular configured input module is shown
in Figure 1-19.
The same output field device (motor starter coil) would
also be used. This device would be hardwired to an appropri-
ate output module according to the manufacturer’s addressing
location scheme. Typical wiring connections for a 120 VAC
modular configured output module is shown in Figure 1-20.
Figure 1-16 Typical PC software used to create a ladder logic program.
Pressure
sensor
switch
Motor
Temperature
sensor switch
Manual pushbutton station
Figure 1-17 Mixer process control problem.
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L1 L2
M
OL
Manual
pushbutton
120 VAC
Motor
starter
coil
Temperature
switch
Pressure
switch
Figure 1-18 Process control relay ladder diagram.
Common
0
1
2
3
4
5
6
7
Input
module
L1 N
120 VAC
Manual
pushbutton
Temperature
Pressure
Figure 1-19 Typical wiring connections for a 120 VAC
modular conﬁgured input module.
Output
module
120 VAC
N
L1
L1
0
1
2
3
4
5
6
7
Motor
starter coil
M
OL
Figure 1-20 Typical wiring connections for a 120 VAC
modular conﬁgured output module.
Next, the PLC ladder logic program would be con-
structed and entered into the memory of the CPU. A
typical ladder logic program for this process is shown in
Figure 1-21. The format used is similar to the layout of
the hardwired relay ladder circuit. The individual symbols
represent instructions, whereas the numbers represent the
instruction location addresses. To program the controller,
you enter these instructions one by one into the proces-
sor memory from the programming device. Each input
and output device is given an address, which lets the PLC
know where it is physically connected. Note that the I/O
address format will differ, depending on the PLC model
and manufacturer. Instructions are stored in the user pro-
gram portion of the processor memory. During the pro-
gram scan the controller monitors the inputs, executes the
control program, and changes the output accordingly.
For the program to operate, the controller is placed
in the RUN mode, or operating cycle. During each op-
erating cycle, the controller examines the status of input
devices, executes the user program, and changes outputs
accordingly. Each symbol can be thought of as a set of
normally open contacts. The symbol is considered to
represent a coil that, when energized, will close a set of
contacts. In the ladder logic program of Figure 1-21, the
coil O/1 is energized when contacts I/1 and I/2 are closed
or when contact I/3 is closed. Either of these conditions
provides a continuous logic path from left to right across
the rung that includes the coil.
A programmable logic controller operates in real time
in that an event taking place in the field will result in an
operation or output taking place. The RUN operation for
the process control scheme can be described by the fol-
lowing sequence of events:
• First, the pressure switch, temperature switch, and
pushbutton inputs are examined and their status is
recorded in the controller’s memory.
• A closed contact is recorded in memory as logic 1
and an open contact as logic 0.
• Next the ladder diagram is evaluated, with each
internal contact given an OPEN or CLOSED status
according to its recorded 1 or 0 state.
• When the states of the input contacts provide logic
continuity from left to right across the rung, the
output coil memory location is given a logic 1 value
and the output module interface contacts will close.
• When there is no logic continuity of the program
rung, the output coil memory location is set to logic 0
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and the output module interface contacts will be
open.
• The completion of one cycle of this sequence by the
controller is called a scan. The scan time, the time
required for one full cycle, provides a measure of
the speed of response of the PLC.
• Generally, the output memory location is updated
during the scan but the actual output is not
updated until the end of the program scan during
the I/O scan.
Figure 1-22 shows the typical wiring required to im-
plement the process control scheme using a fixed PLC
O/1
Motor
starter
coil
I/1
I/1
Pressure
switch
I/2
I/2
O/1
Temperature
switch
I/3
I/3
Manual
pushbutton
L2
L1
Inputs Output
Program
Monitor
inputs
Checks the
inputs
Execute
program
Change
outputs
...
Executes control
program
...
And updates the
outputs
...
M
OL
Figure 1-21 Process control PLC ladder logic program with typical addressing scheme.
I1
I1 I2 I3
L1 L2
I2 Q1
I3
L1
L2
M Starter
Inputs
Pressure
Temp
PB
Outputs
Program
Q1 Q2 Q3 Q4
Figure 1-22 Typical wiring required to implement the process control scheme
using a ﬁxed PLC controller.
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controller. In this example the Allen-Bradley Pico con-
troller equipped with 8 inputs and 4 outputs is used to
control and monitor the process. Installation can be sum-
marized as follows:
• Fused power lines, of the specified voltage type and
level, are connected to the controller’s L1 and L2
terminals.
• The pressure switch, temperature switch, and push-
button field input devices are hardwired between
L1 and controller input terminals I1, I2, and I3,
respectively.
• The motor starter coil connects directly to L2 and in
series with Q1 relay output contacts to L1.
• The ladder logic program is entered using the front
keypad and LCD display.
• Pico programming software is also available that
allows you to create as well as test your program
using a personal computer.
1.4 Modifying the Operation
One of the important features of a PLC is the ease with
which the program can be changed. For example, assume
that the original process control circuit for the mixing op-
eration must be modified as shown in the relay ladder dia-
gram of Figure 1-23. The change requires that the manual
pushbutton control be permitted to operate at any pres-
sure, but not unless the specified temperature setting has
been reached.
If a relay system were used, it would require some re-
wiring of the circuit shown in Figure 1-23 to achieve the
desired change. However, if a PLC system were used, no
rewiring would be necessary. The inputs and outputs are
still the same. All that is required is to change the PLC
ladder logic program as shown in Figure 1-24.
1.5 PLCs versus Computers
The architecture of a PLC is basically the same as that
of a personal computer. A personal computer (PC) can
be made to operate as a programmable logic control-
ler if you provide some way for the computer to re-
ceive information from devices such as pushbuttons
or switches. You also need a program to process the
inputs and decide the means of turning load devices
off and on.
However, some important characteristics distinguish
PLCs from personal computers. First, unlike PCs, the
PLC is designed to operate in the industrial environ-
ment with wide ranges of ambient temperature and
humidity. A well-designed industrial PLC installation,
such as that shown in Figure 1-25, is not usually af-
fected by the electrical noise inherent in most industrial
locations.
Unlike the personal computer, the PLC is programmed
in relay ladder logic or other easily learned languages.
The PLC comes with its program language built into its
memory and has no permanently attached keyboard, CD
drive, or monitor. Instead, PLCs come equipped with ter-
minals for input and output field devices as well as com-
munication ports.
Computers are complex computing machines capable
of executing several programs or tasks simultaneously
and in any order. Most PLCs, on the other hand, execute a
single program in an orderly and sequential fashion from
first to last instruction.
PLC control systems have been designed to be easily
installed and maintained. Troubleshooting is simplified
by the use of fault indicators and messaging displayed
on the programmer screen. Input/output modules for
connecting the field devices are easily connected and
replaced.
L1 L2
Manual
pushbutton
120 VAC
Motor
starter
coil
Temperature
switch
Pressure
switch
M
OL
Figure 1-23 Relay ladder diagram for the modiﬁed
process.
O/1
Motor
starter
coil
I/1
Pressure
switch
I/2
Temperature
switch
I/3
Manual
pushbutton
Figure 1-24 PLC ladder logic program for the modiﬁed
process.
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Software associated with a PLC but written and run on
a personal computer falls into the following two broad
categories:
• PLC software that allows the user to program and
document gives the user the tools to write a PLC
program—using ladder logic or another program-
ming language—and document or explain the
program in as much detail as is necessary.
• PLC software that allows the user to monitor
and control the process is also called a human
machine interface (HMI). It enables the user to
view a process—or a graphical representation of a
process—on a monitor, determine how the system
is running, trend values, and receive alarm condi-
tions (Figure 1-26). Many operator interfaces do
not use PLC software. PLCs can be integrated with
HMIs but the same software does not program both
devices.
Most recently automation manufacturers have responded
to the increased requirements of industrial control systems
by blending the advantages of PLC-style control with
that of PC-based systems. Such a device has been termed
a programmable automation controller, or PAC (Fig-
ure 1-27). Programmable automation controllers combine
PLC ruggedness with PC functionality. Using PACs, you
can build advanced systems incorporating software capa-
bilities such as advanced control, communication, data
logging, and signal processing with rugged hardware per-
forming logic, motion, process control, and vision.
1.6 PLC Size and Application
The criteria used in categorizing PLCs include functional-
ity, number of inputs and outputs, cost, and physical size
(Figure 1-28). Of these, the I/O count is the most impor-
tant factor. In general, the nano is the smallest size with
less than 15 I/O points. This is followed by micro types
(15 to 128 I/O points), medium types (128 to 512 I/O
points), and large types (over 512 I/O points).
Matching the PLC with the application is a key factor
in the selection process. In general it is not advisable to
Figure 1-26 PLC operator interface and monitor.
Source: Courtesy Rogers Machinery Company, Inc.
Figure 1-27 Programmable automation controller (PAC).
Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.
(a)
(b)
Figure 1-25 PLC installed in an industrial environment.
Source: (a–b) Courtesy Automation IG.
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buy a PLC system that is larger than current needs dic-
tate. However, future conditions should be anticipated to
ensure that the system is the proper size to fill the current
and possibly future requirements of an application.
There are three major types of PLC application: single-
ended, multitask, and control management. A single-
ended or stand-alone PLC application involves one PLC
controlling one process (Figure 1-29). This would be a
stand-alone unit and would not be used for communicat-
ing with other computers or PLCs. The size and sophisti-
cation of the process being controlled are obvious factors
in determining which PLC to select. The applications
could dictate a large processor, but usually this category
requires a small PLC.
A multitask PLC application involves one PLC control-
ling several processes. Adequate I/O capacity is a signifi-
cant factor in this type of installation. In addition, if the PLC
would be a subsystem of a larger process and would have to
communicate with a central PLC or computer, provisions
for a data communications network are also required.
A control management PLC application involves one
PLC controlling several others (Figure 1-30). This kind
of application requires a large PLC processor designed to
communicate with other PLCs and possibly with a com-
puter. The control management PLC supervises several
PLCs by downloading programs that tell the other PLCs
what has to be done. It must be capable of connection to
all the PLCs so that by proper addressing it can communi-
cate with any one it wishes to.
Memory is the part of a PLC that stores data, instruc-
tions, and the control program. Memory size is usually
expressed in K values: 1 K, 6 K, 12 K, and so on. The
measurement kilo, abbreviated K, normally refers to
1000 units. When dealing with computer or PLC memory,
however, 1 K means 1024, because this measurement is
based on the binary number system (210 = 1024). Depend-
ing on memory type, 1 K can mean 1024 bits, 1024 bytes,
or 1024 words.
Although it is common for us to measure the memory
capacity of PLCs in words, we need to know the num-
ber of bits in each word before memory size can be accu-
rately compared. Modern computers usually have a word
size of 16, 32, or 64 bits. For example, a PLC that uses
8-bit words has 49,152 bits of storage with a 6 K word
capacity (8 3 6 3 1024 5 49,152), whereas a PLC using
32-bit words has 196,608 bits of storage with the same
6 K memory (32 3 6 3 1024 5 196,608). The amount
Figure 1-28 Typical range of sizes of programmable
controllers.
Source: Courtesy Siemens.
Figure 1-29 Single-ended PLC application.
Figure 1-30 Control management PLC application.
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Table 1-1 Typical PLC Instructions
Instruction Operation
XIC (Examine ON) . . . . . . . . . . . . Examine a bit for an ON condition
XIO (Examine OFF) . . . . . . . . . . . Examine a bit for an OFF condition
OTE (Output Energize). . . . . . . . . Turn ON a bit (nonretentive)
OTL (Output Latch) . . . . . . . . . . . Latch a bit (retentive)
OTU (Output Unlatch) . . . . . . . . . Unlatch a bit (retentive)
TOF (Timer Off-Delay) . . . . . . . . . Turn an output ON or OFF after its rung has been OFF for a preset time interval
TON (Timer On-Delay) . . . . . . . . . Turn an output ON or OFF after its rung has been ON for a preset time interval
CTD (Count Down) . . . . . . . . . . . Use a software counter to count down from a specified value
CTU (Count Up). . . . . . . . . . . . . . Use a software counter to count up to a specified value
of memory required depends on the application. Factors
affecting the memory size needed for a particular PLC
installation include:
• Number of I/O points used
• Size of control program
• Data-collecting requirements
• Supervisory functions required
• Future expansion
The instruction set for a particular PLC lists the differ-
ent types of instructions supported. Typically, this ranges
from 15 instructions on smaller units up to 100 instruc-
tions on larger, more powerful units (see Table 1-1).
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1. What is a programmable logic controller (PLC)?
2. Identify four tasks in addition to relay switching
operations that PLCs are capable of performing.
3. List six distinct advantages that PLCs offer over
conventional relay-based control systems.
4. Explain the differences between open and propri-
etary PLC architecture.
5. State two ways in which I/O is incorporated into
the PLC.
6. Describe how the I/O modules connect to the pro-
cessor in a modular-type PLC configuration.
7. Explain the main function of each of the following
major components of a PLC:
a. Processor module (CPU)
b. I/O modules
c. Programming device
d. Power supply module
8. What are the two most common types of PLC pro-
gramming devices?
9. Explain the terms program and programming lan-
guage as they apply to a PLC.
10. What is the standard programming language used
with PLCs?
11. Answer the following with reference to the process
control relay ladder diagram of Figure 1-18 of this
chapter:
a. When do the pressure switch contacts close?
b. When do the temperature switch contacts close?
c. How are the pressure and temperature switches
connected with respect to each other?
d. Describe the two conditions under which the
motor starter coil will become energized.
e. What is the approximate value of the voltage
drop across each of the following when their
contacts are open?
(1) Pressure switch
(2) Temperature switch
(3) Manual pushbutton
12. The programmable controller operates in real time.
What does this mean?
13. Answer the following with reference to the process
control PLC ladder logic diagram of Figure 1-21 of
this chapter:
a. What do the individual symbols represent?
b. What do the numbers represent?
c. What field device is the number I/2 identified
with?
d. What field device is the number O/1 identified
with?
e. What two conditions will provide a continuous
path from left to right across the rung?
f. Describe the sequence of operation of the
controller for one scan of the program.
14. Compare the method by which the process control
operation is changed in a relay-based system to the
method used for a PLC-based system.
15. Compare the PLC and PC with regard to:
a. Physical hardware differences
b. Operating environment
c. Method of programming
d. Execution of program
16. What two categories of software written and run on
PCs are used in conjunction with PLCs?
17. What is a programmable automation controller
(PAC)?
18. List four criteria by which PLCs are categorized.
19. Compare the single-ended, multitask, and control
management types of PLC applications.
20. What is the memory capacity, expressed in bits, for
a PLC that uses 16-bit words and has an 8 K word
capacity?
21. List five factors affecting the memory size needed
for a particular PLC installation.
22. What does the instruction set for a particular PLC
refer to?
1. Given two single-pole switches, write a program
that will turn on an output when both switch A and
switch B are closed.
CHAPTER 1 PROBLEMS
2. Given two single-pole switches, write a program that
will turn on an output when either switch A or switch
B is closed.
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5. Write a program for the relay ladder diagram shown
in Figure 1-32.
3. Given four NO (Normally Open) pushbuttons (A-
B-C-D), write a program that will turn a lamp on if
pushbuttons A and B or C and D are closed.
4. Write a program for the relay ladder diagram shown
in Figure 1-31.
120 VAC
S3
TS1
PB1
S2
S1 PS1
L1
Figure 1-32 Circuit for Problem 5.
LS2
120 VAC
S1 LS1
L1
Figure 1-31 Circuit for Problem 4.
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17
This chapter exposes you to the details of PLC
hardware and modules that make up a PLC con-
trol system. The chapter’s illustrations show the
various subparts of a PLC as well as general
connection paths. In this chapter we discuss
the CPU and memory hardware components,
including the various types of memory that are
available, and we describe the hardware of the
input/output section, including the difference be-
tween the discrete and analog types of modules.
Chapter Objectives
2.1 List and describe the function of the hardware
components used in PLC systems
2.2 Describe the basic circuitry and applications for
discrete and analog I/O modules, and interpret typical
I/O and CPU specifications
2.3 Explain I/O addressing
2.4 Describe the general classes and types of PLC
memory devices
2.5 List and describe the different types of PLC
peripheral support devices available
2
PLC Hardware Components
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18 Chapter 2 PLC Hardware Components
communicate operating signals through the output inter-
face to the process devices under its control.
Allen-Bradley controllers make a distinction between
a PLC chassis and rack as illustrated in Figure 2-2. The
hardware assembly that houses I/O modules, processor
modules, and power supplies is referred to as the chassis.
Chassis come in different sizes according to the number
of slots they contain. In general, they can have 4, 8, 12,
or 16 slots.
A logical rack is an addressable unit consisting of 128
input points and 128 output points. A rack uses 8 words in
the input image table file and 8 words in the output image
table file. A word in the output image table file and its cor-
responding word in the input image table file are called an
I/O group. A rack can contain a maximum of 8 I/O groups
(numbered from 0 through 7) for up to 128 discrete I/O.
There can be more than one rack in a chassis and more
than one chassis in a rack.
One benefit of a PLC system is the ability to locate
the I/O modules near the field devices, as illustrated in
Figure 2-3, in order to minimize the amount of wiring
2.
1 The I/O Section
The input/output (I/O) section of a PLC is the section to
which all field devices are connected and provides the in-
terface between them and the CPU. Input/output arrange-
ments are built into a fixed PLC while modular types use
external I/O modules that plug into the PLC.
Figure 2-1 illustrates a rack-based I/O section made
up of individual I/O modules. Input interface modules
accept signals from the machine or process devices and
convert them into signals that can be used by the con-
troller. Output interface modules convert controller sig-
nals into external signals used to control the machine
or process. A typical PLC has room for several I/O
modules, allowing it to be customized for a particular
application by selecting the appropriate modules. Each
slot in the rack is capable of accommodating any type
of I/O module.
The I/O system provides an interface between the hard-
wired components in the field and the CPU. The input
interface allows status information regarding processes to
be communicated to the CPU, and thus allows the CPU to
Figure 2-3 Remote I/O rack.
Power
Processor
Input
Output
Local I/O
Stop/Start
Hopper
Motor
Power
Input
Output
Remote I/O
Communication
Sensor
On/Off
control
Figure 2-1 Rack-based I/O section.
Power
supply
0 1 2 3 4 5 6 Slot
I/O modules
Processor
module
Figure 2-2 Allen-Bradley PLC chassis and rack.
0 1 2 3 4 5 6 7
8-slot
chassis
Processor
128 I/O rack
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PLC Hardware Components Chapter 2 19
Slot—The slot number is the physical location of the
I/O module. This may be a combination of the rack
number and the slot number when using expansion
racks.
Word and Bit—The word and bit are used to identify
the actual terminal connection in a particular I/O mod-
ule. A discrete module usually uses only one word,
and each connection corresponds to a different bit that
makes up the word.
With a rack/slot address system the location of a mod-
ule within a rack and the terminal number of a module
to which an input or output device is connected will
determine the device’s address. Figure 2-4 illustrates
the Allen-Bradley PLC-5 controller addressing format.
The following are typical examples of input and output
addresses:
required. The processor receives signals from the remote
input modules and sends signals back to their output mod-
ules via the communication module.
A rack is referred to as a remote rack when it is lo-
cated away from the processor module. To communi-
cate with the processor, the remote rack uses a special
communications network. Each remote rack requires a
unique station number to distinguish one from another.
The remote racks are linked to the local rack through a
communications module. Cables connect the modules
with each other. If fiber optic cable is used between the
CPU and I/O rack, it is possible to operate I/O points
from distances greater than 20 miles with no voltage
drop. Coaxial cable will allow remote I/O to be in-
stalled at distances greater than two miles. Fiber optic
cable will not pick up noise caused by adjacent high
power lines or equipment normally found in an indus-
trial environment. Coaxial cable is more susceptible to
this type of noise.
The PLC’s memory system stores information about
the status of all the inputs and outputs. To keep track of
all this information, it uses a system called addressing. An
address is a label or number that indicates where a certain
piece of information is located in a PLC’s memory. Just
as your home address tells where you live in your city, a
device’s or a piece of data’s address tells where informa-
tion about it resides in the PLC’s memory. That way, if a
PLC wants to find out information about a field device, it
knows to look in its corresponding address location. Ex-
amples of addressing schemes include rack/slot-based,
versions of which are used in Allen-Bradley PLC-5 and
SLC 500 controllers, tag-based used in Allen-Bradley
ControlLogix controllers, and PC-based control used in
soft PLCs.
In general, rack/slot-based addressing elements include:
Type—The type determines if an input or output is
being addressed.
I1:27/17 Input, file 1, rack 2, group 7, bit 17
O0:34/07 Output, file 0, rack 3, group 4, bit 7
I1:0/0 Input, file 1, rack 0, group 0, bit 0 (Short
form blank 5 0)
O0:1/1 Output, file 0, rack 0, group 1, bit 1 (Short
form blank 5 0)
Figure 2-4 Allen-Bradley PLC-5 rack/slot-based addressing format.
O 0 : 0 1 / 00
Terminal number (bit address)
Bit delimiter
Group (0–7); 8 groups in a rack
Rack no.
File delimiter
File no.
File type (O—output; Ι—input)
Figure 2-5 illustrates the Allen-Bradley SLC 500 con-
troller addressing format. The address is used by the pro-
cessor to identify where the device is located to monitor or
control it. In addition, there is some means of connecting
field wiring on the I/O module housing. Connecting the
field wiring to the I/O housing allows easier disconnec-
tion and reconnection of the wiring to change modules.
Lights are also added to each module to indicate the ON
or OFF status of each I/O circuit. Most output modules
also have blown fuse indicators. The following are typical
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in Figure 2-9. A module is made up of a printed circuit
board and a terminal assembly. The printed circuit board
contains the electronic circuitry used to interface the
circuit of the processor with that of the input or output
device. Modules are designed to plug into a slot or con-
nector in the I/O rack or directly into the processor. The
terminal assembly, which is attached to the front edge of
the printed circuit board, is used for making field-wiring
connections. Modules contain terminals for each input
and output connection, status lights for each of the inputs
and outputs, and connections to the power supply used to
power the inputs and outputs. Terminal and status light
arrangements vary with different manufacturers.
Most PLC modules have plug-in wiring terminal strips.
The terminal block is plugged into the actual module as il-
lustrated in Figure 2-10. If there is a problem with a mod-
ule, the entire strip is removed, a new module is inserted,
and the terminal strip is plugged into the new module.
Unless otherwise specified, never install or remove I/O
modules or terminal blocks while the PLC is powered.
A module inserted into the wrong slot could be dam-
aged by improper voltages connected through the wiring
arm. Most faceplates and I/O modules are keyed to pre-
vent putting the wrong faceplate on the wrong module.
In other words, an output module cannot be placed in the
slot where an input module was originally located.
Input and output modules can be placed anywhere in a
rack, but they are normally grouped together for ease of
wiring. I/O modules can be 8, 16, 32, or 64 point cards
(Figure 2-11). The number refers to the number of inputs
or outputs available. The standard I/O module has eight
inputs or outputs. A high-density module may have up to
64 inputs or outputs. The advantage with the high-density
module is that it is possible to install up to 64 inputs or
outputs in one slot for greater space savings. The only dis-
advantage is that the high-density output modules cannot
handle as much current per output.
examples of SLC 500 real-world general input and output
addresses:
Figure 2-5 Allen-Bradley SLC 500 rack/slot-based addressing format.
Memory
address
Real-world
address
I 1 3 0
. /
:
File type
File number
Element number
Slot number
Module type
Subelement number
For terminals above #15
0 1
Bit number
Terminal number
O:4/15 Output module in slot 4, terminal 15
I:3/8 Input module in slot 3, terminal 8
O:6.0 Output module, slot 6
I:5.0 Input module, slot 5
Every input and output device connected to a discrete
I/O module is addressed to a specific bit in the PLC’s
memory. A bit is a binary digit that can be either 1 or 0.
Analog I/O modules use a word addressing format, which
allows the entire words to be addressed. The bit part of
the address is usually not used; however, bits of the digital
representation of the analog value can be addressed by the
programmer if necessary. Figure 2-6 illustrates bit level
and word level addressing as it applies to an SLC 500
controller.
Figure 2-7 illustrates the Allen-Bradley ControlLogix
tag-based addressing format. With Logix5000 controllers,
you use a tag (alphanumeric name) to address data (vari-
ables). Instead of a fixed numeric format the tag name
itself identifies the data. The field devices are assigned
tag names that are referenced when the PLC ladder logic
program is developed.
PC-based control runs on personal or industrial hard-
ened computers. Also known as soft PLCs, they simulate
the functions of a PLC on a PC, allowing open architecture
systems to replace proprietary PLCs. This implementa-
tion uses an input/output card (Figure 2-8) in conjunction
with the PC as an interface for the field devices.
Combination I/O modules can have both input and out-
put connections in the same physical module as illustrated
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Figure 2-6 SLC 500 bit level and word level addressing. (a) Bit level
addressing. (b) Word level addressing.
1
Inputs
Data files
Program files
Output
I:1/0
0
I:1/1
Processor memory
Input
addressing
Output
addressing
I:1/1
I:1/1
I:1/0 O:3/0
O:3/0
I:1/0
0
0:3/0
(a)
Processor
O0:2.0
(address)
Meter
analog
output
Thermocouple
analog input
I0:2.0
(address)
0
Analog
module
1 2
1 2 0
Type Slot
Address
Word Bit Inputs
O 2 0
Type Slot
Address
Word Bit
0
1
Outputs
0
1
Not
used
Not
used
IN 0
IN 0
OUT 0
OUT 0
Output Input
Power
Analog
2:20
(b)
Figure 2-7 Allen-Bradley ControlLogix tag-based addressing format.
Start
I_PBO
<Local:6:1.Data.0>
Input instruction
Base address
Alias tag pointing
to base address
Description assigned
to alias tag
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Figure 2-9 Typical combination I/O module.
0
1
2
3
4
5
6
7
Status
Inputs
Status
indicators Input Output
Input
connections
Output
connections
0
1
2
3
4
5
6
7
Outputs
Power supply
connections
Power supply
connections
Figure 2-10 Plug-in terminal block.
Terminal
block
Module
Figure 2-8 Typical PC interface card.
Source: Photo © Beckhoff Automation GmbH.
2.2 Discrete I/O Modules
The most common type of I/O interface module is the dis-
crete type (Figure 2-12). This type of interface connects
field input devices of the ON/OFF nature such as selec-
tor switches, pushbuttons, and limit switches. Likewise,
output control is limited to devices such as lights, relays,
solenoids, and motor starters that require simple ON/OFF
switching. The classification of discrete I/O covers bit-
oriented inputs and outputs. In this type of input or output,
each bit represents a complete information element in itself
and provides the status of some external contact or advises
of the presence or absence of power in a process circuit.
Each discrete I/O module is powered by some field-
supplied voltage source. Since these voltages can be of
different magnitude or type, I/O modules are available at
various AC and DC voltage ratings, as listed in Table 2-1.
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The modules themselves receive their voltage and cur-
rent for proper operation from the backplane of the rack
enclosure into which they are inserted, as illustrated in Fig-
ure 2-13. Backplane power is provided by the PLC module
power supply and is used to power the electronics that re-
side on the I/O module circuit board. The relatively higher
Figure 2-11 16, 32, and 64 point I/O modules.
Source: (all ) Photos courtesy Omron Industrial Automation, www.ia.omron.com.
Figure 2-12 Discrete input and output devices.
Indicator
lights
Signaling
column
Relays Motor
starter
Pushbuttons Selector
switch
Limit
switch
Discrete inputs
Discrete outputs
Proximity
switches
Figure 2-13 Modules receive their voltage and current
from the backplane.
Backplane power
Power
supply
Input Interfaces Output Interfaces
12 V AC/DC /24 V AC/DC 12–48 V AC
48 V AC/DC 120 V AC
120 V AC/DC 230 V AC
230 V AC/DC 120 V DC
5 V DC (TTL level) 230 V DC
5 V DC (TTL level)
24 V DC
Table 2-1 Common Ratings for Discrete
I/O Interface Modules
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• The zener diode (ZD
) voltage rating sets the mini-
mum threshold level of voltage that can be detected.
• When light from the LED strikes the phototransis-
tor, it switches into conduction and the status of
the pushbutton is communicated in logic to the
processor.
• The optical isolator not only separates the higher
AC input voltage from the logic circuits but also
prevents damage to the processor due to line volt-
age transients. In addition, this isolation also helps
reduce the effects of electrical noise, common in the
industrial environment, which can cause erratic op-
eration of the processor.
• For fault diagnosis, an input state LED indicator
is on when the input pushbutton is closed. This in-
dicator may be wired on either side of the optical
isolator.
• An AC/DC type of input module is used for both
AC and DC inputs as the input polarity does not
matter.
• A PLC input module will have either all inputs iso-
lated from each other with no common input con-
nections or groups of inputs that share a common
connection.
currents required by the loads of an output module are
normally provided by user-supplied power. Module power
supplies typically may be rated for 3 A, 4 A, 12 A, or 16 A
depending on the type and number of modules used.
Figure 2-14 shows the block diagrams for one input of
a typical alternating current (AC) discrete input module.
The input circuit is composed of two basic sections: the
power section and the logic section. An optical isolator is
used to provide electrical isolation between the field wir-
ing and the PLC backplane internal circuitry. The input
LED turns on or off, indicating the status of the input de-
vice. Logic circuits process the digital signal to the pro-
cessor. Internal PLC control circuitry typically operates at
5 VDC or less volts.
A simplified diagram for a single input of a discrete AC
input module is shown in Figure 2-15. The operation of
the circuit can be summarized as follows:
• The input noise filter consisting of the capacitor and
resistors R1 and R2 removes false signals that are
due to contact bounce or electrical interference.
• When the pushbutton is closed, 120 VAC is applied
to the bridge rectifier input.
• This results in a low-level DC output voltage that is
applied across the LED of the optical isolator.
Figure 2-14 Discrete AC input module block diagram.
L1
(120 VAC)
Input
signal
L2
Logic
circuits
To processor
(low VDC)
Input status
indicator
Zener
diode
level
detection
Power
Logic
Opto-electrical
isolation
Bridge
rectifier
Figure 2-15 Simpliﬁed diagram for a single input of a discrete AC input module.
Bridge
rectifier
Input module
terminal strip
Filter
Fuse
Z
Threshold
detector
Internal module circuit
D
Optical
isolator
LED input
status indicator
Digital
logic
circuit
Common
Field wiring
L2
R3
L1
PB
R2
R1
C
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• When the processor calls for an output load to be
energized, a voltage is applied across the LED of
the opto-isolator.
• The LED then emits light, which switches the pho-
totransistor into conduction.
• This in turn triggers the triac AC semiconductor
switch into conduction allowing current to flow to
the output load.
• Since the triac conducts in either direction, the out-
put to the load is alternating current.
• The triac, rather than having ON and OFF status,
actually has LOW and HIGH resistance levels,
respectively. In its OFF state (HIGH resistance),
a small leakage current of a few milliamperes still
flows through the triac.
• As with input circuits, the output interface is usually
provided with LEDs that indicate the status of each
output.
• Fuses are normally required for the output module,
and they are provided on a per circuit basis, thus al-
lowing for each circuit to be protected and operated
separately. Some modules also provide visual indi-
cators for fuse status.
• The triac cannot be used to switch a DC load.
Discrete input modules perform four tasks in the PLC
control system. They:
• Sense when a signal is received from a field device.
• Convert the input signal to the correct voltage level
for the particular PLC.
• Isolate the PLC from fluctuations in the input
signal’s voltage or current.
• Send a signal to the processor indicating which sen-
sor originated the signal.
Figure 2-16 shows the block diagram for one output of
a typical discrete output module. Like the input module,
it is composed of two basic sections: the power section
and the logic section, coupled by an isolation circuit. The
output interface can be thought of as an electronic switch
that turns the output load device on and off. Logic circuits
determine the output status. An output LED indicates the
status of the output signal.
A simplified diagram for a single output of a discrete
AC output module is shown in Figure 2-17. The operation
of the circuit can be summarized as follows:
• As part of its normal operation, the digital logic cir-
cuits of the processor sets the output status accord-
ing to the program.
Figure 2-16 Discrete AC output module block diagram.
Digital signal
from processor
Output status
indicator
Logic
circuits
Electronic
switch
Load
120 VAC
Logic
Power
L1
L2
Opto-electrical
isolation
Figure 2-17 Simpliﬁed diagram for a single output of a discrete AC
output module.
Optical
isolator
Field wiring
Triac
switch
Fuse
Output module
terminal strip
L1
Internal module circuit
LED output
status indicator
Digital
logic
circuit
Load L2
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whereas transistor outputs can be used only for control
of DC devices. The discrete relay contact output module
uses electromechanical as the switching element. These
relay outputs can be used with AC or DC devices, but they
have a much slower switching time compared to solid-
state outputs. Allen-Bradley modules are color-coded for
identification as follows:
• For fault diagnosis, the LED output status indica-
tor is on whenever the PLC is commanding that the
output load be switched on.
Individual AC outputs are usually limited by the size of
the triac to 1 A or 2 A. The maximum current load for any
one module is also specified. To protect the output module
circuits, specified current ratings should not be exceeded.
For controlling larger loads, such as large motors, a standard
control relay is connected to the output module. The con-
tacts of the relay can then be used to control a larger load or
motor starter, as shown in Figure 2-18. When a control relay
is used in this manner, it is called an interposing relay.
Discrete output modules are used to turn field output
devices either on or off. These modules can be used to
control any two-state device, and they are available in AC
and DC versions and in various voltage ranges and cur-
rent ratings. Output modules can be purchased with tran-
sistor, triac, or relay output as illustrated in Figure 2-19.
Triac outputs can be used only for control of AC devices,
Figure 2-19 Relay, transistor, and triac switching elements.
Contact AC/DC
⫹
⫺
Coil
Load
Relay output
AC
Load
Triac output
DC
Load
Transistor output
Figure 2-18 Interposing relay connection.
Source: Photo courtesy Tyco Electronics, www.tycoelectronics.com.
CR
Interposing
relay coil
OL
M
Motor
starter coil
T1 T2
L1
L2
L3
M M
L1 L2
CR
T3
M
Motor
Color Type of I/O
Red AC inputs/outputs
Blue DC inputs/outputs
Orange Relay outputs
Green Specialty modules
Black I/O wiring; terminal blocks
are not removable
Certain DC I/O modules specify whether the module
is designed for interfacing with current-source or current-
sink devices. If the module is a current-sourcing module,
then the input or output device must be a current-sinking
device. Conversely, if the module is specified as current-
sinking, then the connected device must be current-
sourcing. Some modules allow the user to select whether
the module will act as current sinking or current sourcing,
thereby allowing it to be set to whatever the field devices
require.
The internal circuitry of some field devices requires
that they be used in sinking or sourcing circuits. In gen-
eral, sinking (NPN) and sourcing (PNP) are terms used to
describe a current signal flow relationship between field
input and output devices in a control system and their
power supply. Figure 2-20 illustrates the current flow re-
lationship between sinking and sourcing inputs to a DC
input module.
Figure 2-21 illustrates the current flow relationship
between sinking and sourcing outputs to a DC output
module. DC input and output circuits are commonly con-
nected with field devices that have some form of internal
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current signal from the level transmitter field device. This
input is converted from an analog to a digital value for
use by the processor. The circuitry of the analog output
module accepts the digital value from the processor and
converts it back to an analog signal that drives the field
tank level meter.
Analog input modules normally have multiple input
channels that allow 4, 8, or 16 devices to be interface to
the PLC. The two basic types of analog input modules
are voltage sensing and current sensing. Analog sen-
sors measure a varying physical quantity over a specific
range and generate a corresponding voltage or current
signal. Common physical quantities measured by a PLC
analog module include temperature, speed, level, flow,
weight, pressure, and position. For example, a sensor may
solid-state circuitry that needs a DC signal voltage to
function. Field devices connected to the positive (1) side
of the field power supply are classified as sourcing field
devices. Conversely, field devices connected to the nega-
tive (2) side or DC common of the field power supply are
sinking field devices.
2.3 Analog I/O Modules
Earlier PLCs were limited to discrete or digital I/O in-
terfaces, which allowed only on/off-type devices to be
connected. This limitation meant that the PLC could have
only partial control of many process applications. Today,
however, a complete range of both discrete and analog
interfaces are available that will allow controllers to be
applied to practically any type of control process.
Discrete devices are inputs and outputs that have only
two states: on and off. In comparison, analog devices rep-
resent physical quantities that can have an infinite number
of values. Typical analog inputs and outputs vary from
0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts.
Figure 2-22 illustrates how PLC analog input and output
modules are used in measuring and displaying the level
of fluid in a tank. The analog input interface module con-
tains the circuitry necessary to accept an analog voltage or
Figure 2-20 Sinking and sourcing inputs.
0
1
2
3
4
5
6
7
Common
Sinking input
module
⫹
⫺
Power
supply
Sourcing
sensor
Current
0
1
2
3
4
5
6
7
Common
Sourcing input
module
⫺
⫹
Power
supply
Sinking
sensor
Current
Figure 2-21 Sinking and sourcing outputs.
0
1
2
3
4
5
6
7
Common
Sourcing output
module
Current
Sinking output
module
Current
⫹
⫺
Power
supply
Sourcing
field device
⫺
⫹
Power
supply
Sinking
field device
0
1
2
3
4
5
6
7
Common
Figure 2-22 Analog input and output to a PLC.
Level
transmitter
PLC Level
indicator
Processor
Analog
input
module
Analog
output
module
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measure temperature over a range of 0 to 500°C, and out-
put a corresponding voltage signal that varies between 0
and 50 mV.
Figure 2-23 illustrates an example of a voltage sensing
input analog module used to measure temperature. The
connection diagram applies to an Allen-Bradley Micro-
Logic 4-channel analog thermocouple input module. A
varying DC voltage in the low millivolt range, propor-
tional to the temperature being monitored, is produced by
the thermocouple. This voltage is amplified and digitized
by the analog input module and then sent to the processor
on command from a program instruction. Because of the
low voltage level of the input signal, a twisted shielded
pair cable is used in wiring the circuit to reduce unwanted
electrical noise signals that can be induced in the conduc-
tors from other wiring. When using an ungrounded ther-
mocouple, the shield must be connected to ground at the
module end. To obtain accurate readings from each of the
channels, the temperature between the thermocouple wire
and the input channel must be compensated for. A cold
junction compensating (CJC) thermistor is integrated in
the terminal block for this purpose.
The transition of an analog signal to digital values is
accomplished by an analog-to-digital (A/D) converter, the
main element of the analog input module. Analog volt-
age input modules are available in two types: unipolar
and bipolar. Unipolar modules can accept an input signal
that varies in the positive direction only. For example, if
the field device outputs 0 V to 110 V, then the unipolar
modules would be used. Bipolar signals swing between a
maximum negative value and a maximum positive value.
For example, if the field device outputs 210 V to 110 V
a bipolar module would be used. The resolution of an ana-
log input channel refers to the smallest change in input
signal value that can be sensed and is based on the num-
ber of bits used in the digital representation. Analog input
modules must produce a range of digital values between
a maximum and minimum value to represent the analog
signal over its entire span. Typical specifications are as
follows:
Ungrounded
thermocouple
⫹
⫺
⫹
⫺
Grounded
thermocouple
IN 0⫹
IN 0⫺
IN 1⫹
IN 1⫹
IN 3⫺
IN 3⫹
IN 2⫺
IN 2⫹
CJC⫺
CJC⫹
Figure 2-23 MicroLogix 4-channel analog thermocouple input module.
Span
of
analog
input
Bipolar
10 V 210 to 110 V
5 V 25 to 15 V
Unipolar
10 V 0 to 110 V
5 V 0 to 15 V
Resolution 0.3 mV
When connecting voltage sensing inputs, close adher-
ence to specified requirements regarding wire length is
important to minimize signal degrading and the effects
of electromagnetic noise interference induced along the
connecting conductors. Current input signals, which
are not as sensitive to noise as voltage signals, are typi-
cally not distance limited. Current sensing input modules
typically accept analog data over the range of 4 mA to
20 mA, but can accommodate signal ranges of –20 mA
to 120 mA. The loop power may be supplied by the sen-
sor or may be provided by the analog output module as
illustrated in Figure 2-24. Shielded twisted pair cable is
normally recommended for connecting any type analog
input signal.
The analog output interface module receives from the
processor digital data, which are converted into a propor-
tional voltage or current to control an analog field device.
The transition of a digital signal to analog values is accom-
plished by a digital-to-analog (D/A) converter, the main
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available is 0 to 100 kHz, which means the module would
be able to count 100,000 pulses per second.
THUMBWHEEL MODULE
The thumbwheel module allows the use of thumbwheel
switches (Figure 2-27) for feeding information to the PLC
to be used in the control program.
TTL MODULE
The TTL module (Figure 2-28) allows the transmitting
and receiving of TTL (Transistor-Transistor-Logic) sig-
nals. This module allows devices that produce TTL-level
signals to communicate with the PLC’s processor.
Figure 2-24 Sensor and analog module supplied power.
Sensor supplied power Module supplied power
Sensor
4–20 mA loop
Power
supply
⫹ ⫺
⫺
⫹ 4–20 mA loop
Sensor
⫹ ⫺
Power
supply
⫺
⫹
Figure 2-25 Typical analog I/O control system.
Valve
Analog output
Analog input
Level
sensor
PLC
element of the analog output module. An analog output
signal is a continuous and changing signal that is varied
under the control of the PLC program. Common devices
controlled by a PLC analog output module include instru-
ments, control valves, chart recorder, electronic drives,
and other types of control devices that respond to analog
signals.
Figure 2-25 illustrates the use of analog I/O modules in
a typical PLC control system. In this application the PLC
controls the amount of fluid placed in a holding tank by
adjusting the percentage of the valve opening. The ana-
log output from the PLC is used to control the flow by
controlling the amount of the valve opening. The valve
is initially open 100 percent. As the fluid level in the tank
approaches the preset point, the processor modifies the
output, which adjusts the valve to maintain a set point.
2.4 Special I/O Modules
Many different types of I/O modules have been developed
to meet special needs. These include:
HIGH-SPEED COUNTER MODULE
The high-speed counter module is used to provide an in-
terface for applications requiring counter speeds that sur-
pass the capability of the PLC ladder program. High-speed
counter modules are used to count pulses (Figure 2-26)
from sensors, encoders, and switches that operate at very
high speeds. They have the electronics needed to count
independently of the processor. A typical count rate
Figure 2-26 High-speed counter module.
Source: Courtesy Control Technology Corporation.
High-speed pulses
Figure 2-27 Thumbwheel switch.
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ENCODER-COUNTER MODULE
An encoder-counter module allows the user to read the
signal from an encoder (Figure 2-29) on a real-time basis
and stores this information so it can be read later by the
processor.
BASIC OR ASCII MODULE
The BASIC or ASCII module (Figure 2-30) runs user-
written BASIC and C programs. These programs are in-
dependent of the PLC processor and provide an easy, fast
interface between remote foreign devices and the PLC
processor. Typical applications include interfaces to bar
code readers, robots, printers, and displays.
STEPPER-MOTOR MODULE
The stepper-motor module provides pulse trains to a
stepper-motor translator, which enables control of a step-
per motor (Figure 2-31). The commands for the module
are determined by the control program in the PLC.
BCD-OUTPUT MODULE
The BCD-output module enables a PLC to operate devices
that require BCD-coded signals such as seven-segment
displays (Figure 2-32).
Figure 2-28 TTL module.
Source: Courtesy Control Technology, Inc.
Figure 2-29 Encoder.
Source: Photo courtesy of Allied Motion Technologies, Inc.
Figure 2-30 BASIC module.
Figure 2-31 Stepper-motor.
Source: Courtesy Sherline Products.
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Figure 2-33 PID module.
Source: Courtesy Red Lion Controls.
Some special modules are referred to as intelligent I/O
because they have their own microprocessors on board
that can function in parallel with the PLC. These include:
PID MODULE
The proportional-integral-derivative (PID) module (Fig-
ure 2-33) is used in process control applications that
incorporate PID algorithms. An algorithm is a complex
program based on mathematical calculations. A PID mod-
ule allows process control to take place outside the CPU.
This arrangement prevents the CPU from being burdened
with complex calculations. The basic function of this
module is to provide the control action required to main-
tain a process variable such as temperature, flow, level, or
speed within set limits of a specified set point.
MOTION AND POSITION CONTROL
MODULE
Motion and position control modules are used in applica-
tions involving accurate high-speed machining and pack-
aging operations. Intelligent position and motion control
modules permit PLCs to control stepper and servo motors.
These systems require a drive, which contains the power
electronics that translate the signals from the PLC module
into signals required by the motor (Figure 2-34).
COMMUNICATION MODULES
Serial communications modules (Figure 2-35) are used to
establish point-to-point connections with other intelligent
devices for the exchange of data. Such connections are
normally established with computers, operator stations,
process control systems, and other PLCs. Communication
modules allow the user to connect the PLC to high-speed
local networks that may be different from the network
communication provided with the PLC.
Figure 2-32 Seven-segment display.
Source: Courtesy Red Lion Controls.
Figure 2-34 PLC servo module.
PLC servo module
Outputs
Inputs
Servo
drive
Servo
motor
Encoder
Figure 2-35 Serial communications module.
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2.5 I/O Speciﬁcations
Manufacturers’ specifications provide information about
how an interface device is correctly and safely used.
These specifications place certain limitations not only on
the I/O module but also on the field equipment that it can
operate. Some PLC systems support hot swappable I/O
modules designed to be changed with the power on and
the PLC operating. The following is a list of some typi-
cal manufacturers’ I/O specifications, along with a short
description of what is specified.
Typical Discrete I/O Module
Speciﬁcations
NOMINAL INPUT VOLTAGE
This discrete input module voltage value specifies the
magnitude (e.g., 5 V, 24 V, 230 V) and type (AC or DC) of
user-supplied voltage that a module is designed to accept.
Input modules are typically designed to operate correctly
without damage within a range of plus or minus 10 per-
cent of the input voltage rating. With DC input modules,
the input voltage may also be expressed as an operating
range (e.g., 24–60 volts DC) over which the module will
operate.
INPUT THRESHOLD VOLTAGES
This discrete input module specification specifies two val-
ues: a minimum ON-state voltage that is the minimum
voltage at which logic 1 is recognized as absolutely ON;
and a maximum OFF-state voltage which is the voltage at
which logic 0 is recognized as absolutely OFF.
NOMINAL CURRENT PER INPUT
This value specifies the minimum input current that the
discrete input devices must be capable of driving to op-
erate the input circuit. This input current value, in con-
junction with the input voltage, functions as a threshold
to protect against detecting noise or leakage currents as
valid signals.
AMBIENT TEMPERATURE RATING
This value specifies what the maximum temperature of
the air surrounding the I/O modules should be for best
operating conditions.
INPUT ON/OFF DELAY
Also known as response time, this value specifies the
maximum time duration required by an input module’s
circuitry to recognize that a field device has switched ON
(input ON-delay) or switched OFF (input OFF-delay).
This delay is a result of filtering circuitry provided to
protect against contact bounce and voltage transients.
This input delay is typically in the 9 to 25 millisecond
range.
OUTPUT VOLTAGE
This AC or DC value specifies the magnitude (e.g., 5 V,
115 V, 230 V) and type (AC or DC) of user-supplied voltage
at which a discrete output module is designed to operate.
The output field device that the module interfaces to the
PLC must be matched to this specification. Output modules
are typically designed to operate within a range of plus or
minus 10 percent of the nominal output voltage rating.
OUTPUT CURRENT
These values specify the maximum current that a single
output and the module as a whole can safely carry under
load (at rated voltage). This rating is a function of the
module’s components and heat dissipation characteris-
tics. A device drawing more than the rated output current
results in overloading, causing the output fuse to blow. As
an example, the specification may give each output a cur-
rent limit of 1 A. The overall rating of the module current
will normally be less than the total of the individuals. The
overall rating might be 6 A because each of the eight de-
vices would not normally draw their 1 A at the same time.
Other names for the output current rating are maximum
continuous current and maximum load current.
INRUSH CURRENT
An inrush current is a momentary surge of current that an
AC or DC output circuit encounters when energizing in-
ductive, capacitive, or filament loads. This value specifies
the maximum inrush current and duration (e.g., 20 A for
0.1 s) for which an output circuit can exceed its maximum
continuous current rating.
SHORT CIRCUIT PROTECTION
Short circuit protection is provided for AC and DC output
modules by either fuses or some other current-limiting
circuitry. This specification will designate whether the
particular module’s design has individual protection for
each circuit or if fuse protection is provided for groups
(e.g., 4 or 8) of outputs.
LEAKAGE CURRENT
This value specifies the amount of current still conducting
through an output circuit even after the output has been
turned off. Leakage current is a characteristic exhibited
by solid-state switching devices such as transistors and
triacs and is normally less than 5 milliamperes. Leakage
current is normally not large enough to falsely trigger an
output device but must be taken into consideration when
switching very low current sensitive devices.
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ELECTRICAL ISOLATION
Recall that I/O module circuitry is electrically isolated to
protect the low-level internal circuitry of the PLC from
high voltages that can be encountered from field device
connections. The specification for electrical isolation,
typically 1500 or 2500 volts AC, rates the module’s ca-
pacity for sustaining an excessive voltage at its input or
output terminals. Although this isolation protects the
logic side of the module from excessive input or output
voltages or current, the power circuitry of the module may
be damaged.
POINTS PER MODULE
This specification defines the number of field inputs or
outputs that can be connected to a single module. Most
commonly, a discrete module will have 8, 16, or 32 cir-
cuits; however, low-end controllers may have only 2 or
4 circuits. Modules with 32 or 64 input or output bits are
referred to as high-density modules. Some modules pro-
vide more than one common terminal, which allows the
user to use different voltage ranges on the same card as
well as to distribute the current more effectively.
BACKPLANE CURRENT DRAW
This value indicates the amount of current the module
requires from the backplane. The sum of the backplane
current drawn for all modules in a chassis is used to select
the appropriate chassis power supply rating.
Typical Analog I/O Module
Speciﬁcations
CHANNELS PER MODULE
Whereas individual circuits on discrete I/O modules are
referred to as points, circuits on analog I/O modules are
often referred to as channels.These modules normally have
4, 8, or 16 channels. Analog modules may allow for either
single-ended or differential connections. Single-ended
connections use a single ground terminal for all channels
or for groups of channels. Differential connections use a
separate positive and negative terminal for each channel.
If the module normally allows 16 single-ended connec-
tions, it will generally allow only 8 differential connec-
tions. Single-ended connections are more susceptible to
electrical noise.
INPUT CURRENT/VOLTAGE RANGE(S)
These are the voltage or current signal ranges that an ana-
log input module is designed to accept. The input ranges
must be matched accordingly to the varying current or
voltage signals generated by the analog sensors.
OUTPUT CURRENT/VOLTAGE RANGE(S)
This specification defines the current or voltage signal
ranges that a particular analog output module is designed
to output under program control. The output ranges must
be matched according to the varying voltage or current
signals that will be required to drive the analog output
devices.
INPUT PROTECTION
Analog input circuits are usually protected against acci-
dentally connecting a voltage that exceeds the specified
input voltage range.
RESOLUTION
The resolution of an analog I/O module specifies how ac-
curately an analog value can be represented digitally. This
specification determines the smallest measurable unit of
current or voltage. The higher the resolution (typically
specified in bits), the more accurately an analog value can
be represented.
INPUT IMPEDANCE AND CAPACITANCE
For analog I/Os, these values must be matched to the ex-
ternal device connected to the module. Typical ratings are
in Megohm (MV) and picofarads (pF).
COMMON-MODE REJECTION
Noise is generally caused by electromagnetic interfer-
ence, radio frequency interference, and ground loops.
Common-mode noise rejection applies only to differ-
ential inputs and refers to an analog module’s ability to
prevent noise from interfering with data integrity on a
single channel and from channel to channel on the mod-
ule. Noise that is picked up equally in parallel wires is
rejected because the difference is zero. Twisted pair wires
are used to ensure that this type of noise is equal on both
wires. Common-mode rejection is normally expressed in
decibels or as a ratio.
2.6 The Central Processing
Unit (CPU)
The central processing unit (CPU) is built into single-
unit fixed PLCs while modular rack types typically use
a plug-in module. CPU, controller, and processor are all
terms used by different manufacturers to denote the same
module that performs basically the same functions. Pro-
cessors vary in processing speed and memory options. A
processor module can be divided into two sections: the
CPU section and the memory section (Figure 2-36). The
CPU section executes the program and makes the deci-
sions needed by the PLC to operate and communicate
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with other modules. The memory section electronically
stores the PLC program along with other retrievable digi-
tal information.
The PLC power supply provides the necessary power
(typically 5 VDC) to the processor and I/O modules
plugged into the backplane of the rack (Figure 2-37).
Power supplies are available for most voltage sources
encountered. The power supply converts 115 VAC or
230 VAC into the usable DC voltage required by the CPU,
memory, and I/O electronic circuitry. PLC power supplies
are normally designed to withstand momentary losses of
power without affecting the operation of the PLC. Hold-
up time, which is the length of time a PLC can tolerate
a power loss, typically ranges from 10 milliseconds to
3 seconds.
The CPU contains the similar type of microproces-
sor found in a personal computer. The difference is that
the program used with the microprocessor is designed to
facilitate industrial control rather than provide general-
purpose computing. The CPU executes the operating
system, manages memory, monitors inputs, evaluates the
user logic (ladder program), and turns on the appropriate
outputs.
The CPU of a PLC system may contain more than one
processor. One advantage of using multiprocessing is that
the overall operating speed is improved. Each processor
has its own memory and programs, which operate simul-
taneously and independently. In such configurations the
scan of each processor is parallel and independent thus
reducing the total response time. Fault-tolerant PLC sys-
tems support dual processors for critical processes. These
systems allow the user to configure the system with redun-
dant (two) processors, which allows transfer of control to
the second processor in the event of a processor fault.
Associated with the processor unit will be a number of
status LED indicators to provide system diagnostic infor-
mation to the operator (Figure 2-38). Also, a keyswitch
may be provided that allows you to select one of the fol-
lowing three modes of operation: RUN, PROG, and REM.
RUN Position
• Places the processor in the Run mode
• Executes the ladder program and energizes output
devices
• Prevents you from performing online program edit-
ing in this position
• Prevents you from using a programmer/operator in-
terface device to change the processor mode
PROG Position
• Places the processor in the Program mode
• Prevents the processor from scanning or executing
the ladder program, and the controller outputs are
de-energized
• Allows you to perform program entry and editing
• Prevents you from using a programmer/operator in-
terface device to change the processor mode
REM Position
• Places the processor in the Remote mode: either the
REMote Run, REMote Program, or REMote Test
mode
• Allows you to change the processor mode from a
programmer/operator interface device
• Allows you to perform online program editing
The processor module also contains circuitry to com-
municate with the programming device. Somewhere on
the module you will find a connector that allows the PLC
to be connected to an external programming device. The
decision-making capabilities of PLC processors go far
beyond simple logic processing. The processor performs
Figure 2-36 Sections of a PLC processor module.
Source: Courtesy Mitsubishi Automation.
Communication
interface
Processor module
Discrete
I/O
devices
Analog
I/O
devices
I/O modules
Memory
CPU
Power
supply
Figure 2-37 PLC power supply.
Backplane
Power
Power
supply
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other functions such as timing, counting, latching, com-
paring, motion control and complex math functions.
PLC processors have changed constantly due to ad-
vancements in computer technology and greater demand
from applications. Today, processors are faster and have
additional instructions added as new models are intro-
duced. Because PLCs are microprocessor based, they can
be made to perform tasks that a computer can do. In ad-
dition to their control functions, PLCs can be networked
to do supervisory control and data acquisition (SCADA).
Many electronic components found in processors and
other types of PLC modules are sensitive to electrostatic
voltages that can degrade their performance or damage
them. The following static control procedures should be
followed when handling and working with static-sensitive
devices and modules:
• Ground yourself by touching a conductive surface
before handling static-sensitive components.
• Wear a wrist strap that provides a path to bleed off
any charge that may build up during work.
• Be careful not to touch the backplane connector or
connector pins of the PLC system (always handle
the circuit cards by the edge if possible).
• Be careful not to touch other circuit components in
a module when you configure or replace its internal
components.
• When not in use, store modules in its static-shield bag.
• If available, use a static-safe work station.
2.7 Memory Design
Memory is the element that stores information, programs,
and data in a PLC. The user memory of a PLC includes
space for the user program as well as addressable memory
locations for storage of data. Data are stored in memory
locations by a process called writing. Data are retrieved
from memory by what is referred to as reading.
The complexity of the program determines the amount
of memory required. Memory elements store individual
pieces of information called bits (for binary digits). The
amount of memory capacity is specified in increments of
1000 or in “K” increments, where 1 K is 1024 bytes of
memory storage (a byte is 8 bits).
The program is stored in the memory as 1s and 0s,
which are typically assembled in the form of 16-bit
words. Memory sizes are commonly expressed in thou-
sands of words that can be stored in the system; thus 2 K
is a memory of 2000 words, and 64 K is a memory of
64,000 words. The memory size varies from as small as
1 K for small systems to 32 MB for very large systems
(Figure 2-39). Memory capacity is an important prereq-
uisite for determining whether a particular processor will
handle the requirements of the specific application.
Memory location refers to an address in the CPU’s
memory where a binary word can be stored. A word usu-
ally consists of 16 bits. Each binary piece of data is a bit
and eight bits make up one byte (Figure 2-40). Memory
utilization refers to the number of memory locations re-
quired to store each type of instruction. A rule of thumb
Figure 2-38 Typical processor module.
Keyswitch
Memory
module
Hardware
address
Battery (battery provides
backup power for the
CMOS RAM)
FORCE
SLC 5/05 CPU
Front view
Side view
RUN
ENET
FLT
RS232
BATT
PROG
RUN REM
Channel 0
RS232
(DH485, DF1,
or ASCII)
Channel 1
ethernet
(10Base-T)
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for memory locations is one location per coil or contact.
One K of memory would then allow a program containing
1000 coils and contacts to be stored in memory.
The memory of a PLC may be broken into sections that
have specific functions. Sections of memory used to store
the status of inputs and outputs are called input status files
or tables and output status files or tables (Figure 2-41).
These terms simply refer to a location where the status of
an input or output device is stored. Each bit is either a 1 or
0, depending on whether the input is open or closed. A
closed contact would have a binary 1 stored in its respec-
tive location in the input table, whereas an open contact
would have a 0 stored. A lamp that is ON would have
a 1 stored in its respective location in the output table,
whereas a lamp that is OFF would have a 0 stored. Input
and output image tables are constantly being revised by
the CPU. Each time a memory location is examined, the
table changes if the contact or coil has changed state.
PLCs execute memory-checking routines to be sure
that the PLC memory has not been corrupted. This mem-
ory checking is undertaken for safety reasons. It helps en-
sure that the PLC will not execute if memory is corrupted.
2.8 Memory Types
Memory can be placed into two general categories: vola-
tile and nonvolatile. Volatile memory will lose its stored
information if all operating power is lost or removed.
Volatile memory is easily altered and is quite suitable for
most applications when supported by battery backup.
Nonvolatile memory has the ability to retain stored
information when power is removed accidentally or in-
tentionally. As the name implies, programmable logic
controllers have programmable memory that allows users
to develop and modify control programs. This memory is
made nonvolatile so that if power is lost, the PLC holds
its programming.
Read Only Memory (ROM) stores programs, and data
cannot be changed after the memory chip has been manu-
factured. ROM is normally used to store the programs and
data that define the capabilities of the PLC. ROM memory
is nonvolatile, meaning that its contents will not be lost if
power is lost. ROM is used by the PLC for the operating
system. The operating system is burned into ROM by the
PLC manufacturer and controls the system software that
the user uses to program the PLC.
Random Access Memory (RAM), sometimes re-
ferred to as read-write (R/W) memory, is designed so that
information can be written into or read from the memory.
RAM is used as a temporary storage area of data that may
need to be quickly changed. RAM is volatile, meaning
that the data stored in RAM will be lost if power is lost. A
battery backup is required to avoid losing data in the event
of a power loss (Figure 2-42). Most PLCs use CMOS-
RAM technology for user memory. CMOS-RAM chips
have very low current draw and can maintain memory
with a lithium battery for an extended time, two to five
Figure 2-39 Typical PLC memory sizes.
MicroLogic 1000
Controller
1 K memory
Up to 20 inputs
Up to 14 outputs
1 K
SLC 500
Controller
Up to 64 K memory
Up to 4096 inputs
and outputs
64 K
ControlLogix
Controller
2 to 32 M memory
Up to 128,000 inputs
and outputs
32 M
Figure 2-40 Memory bit, byte, and word.
Bit
1 1 1 1
0 0 0
0 0 0 0 0 0 0 0
0
Word
Byte
Figure 2-41 Input and output tables.
Processor memory
Input image table
Input
module
Input devices
0000000000000010
0000000000000000
0000000000000000
0000000000000000
0000000000000000
Processor memory
Output image table
Output
module
0000000000000000
0000000000000000
0000000000000000
0000000000000000
0000000010000000
Output OFF
Output ON
Closed
Open
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years in many cases. Some processors have a capacitor
that provides at least 30 minutes of battery backup when
the battery is disconnected and power is OFF.
Erasable Programmable Read-Only Memory
(EPROM) provides some level of security against unau-
thorized or unwanted changes in a program. EPROMs are
designed so that data stored in them can be read, but not
easily altered without special equipment. For example,
UV EPROMs (ultraviolet erasable programmable read-
only memory) can only be erased with an ultraviolet light.
EPROM memory is used to back up, store, or transfer
PLC programs.
Electrically erasable programmable read-only mem-
ory (EEPROM) is a nonvolatile memory that offers the
same programming flexibility as does RAM. The EEPROM
can be electrically overwritten with new data instead of
being erased with ultraviolet light. Because the EEPROM
is nonvolatile memory, it does not require battery backup.
It provides permanent storage of the program and can be
changed easily using standard programming devices. Typi-
cally, an EEPROM memory module is used to store, back
up, or transfer PLC programs (Figure 2-43).
Flash EEPROMs are similar to EEPROMs in that they
can only be used for backup storage. The main difference
comes in the flash memory: they are extremely fast at sav-
ing and retrieving files. In addition, they do not need to
be physically removed from the processor for reprogram-
ming; this can be done using the circuitry within the pro-
cessor module in which they reside. Flash memory is also
sometimes built into the processor module (Figure 2-44),
where it automatically backs up parts of RAM. If power
fails while a PLC with flash memory is running, the PLC
will resume running without having lost any working data
after power is restored.
2.9 Programming Terminal Devices
A programming terminal device is needed to enter, mod-
ify, and troubleshoot the PLC program. PLC manufac-
turers use various types of programming devices. The
simplest type is the hand-held type programmer shown
in Figure 2-45. This proprietary programming device has
a connecting cable so that it can be plugged into a PLC’s
programming port. Certain controllers use a plug-in panel
rather than a hand-held device.
Figure 2-42 Battery used to back up processor RAM.
⫹
⫺
Figure 2-43 EEPROM memory module is used to store,
back up, or transfer PLC programs.
EEPROM
(nonvolatile)
Program
backup
Parameters
RAM
(volatile)
Executed
program
Current
data
Memory
bits,
timers,
counters
Figure 2-44 Flash memory card installed in a socket on
the processor.
Flash
Card
Processor Module
Figure 2-45 Hand-held programming terminal.
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Hand-held programmers are compact, inexpensive,
and easy to use. These units contain multifunction keys
and a liquid-crystal display (LCD) or light-emitting
diode (LED) window. There are usually keys for instruc-
tion entering and editing, and navigation keys for mov-
ing around the program. Hand-held programmers have
limited display capabilities. Some units will display only
the last instruction that has been programmed, whereas
other units will display from two to four rungs of ladder
logic. So-called intelligent hand-held programmers are
designed to support a certain family of PLCs from a spe-
cific manufacturer.
The most popular method of PLC programming is to
use a personal computer (PC) in conjunction with the
manufacturer’s programming software (Figure 2-46).
Typical capabilities of the programming software include
online and offline program editing, online program moni-
toring, program documentation, diagnosing malfunctions
in the PLC, and troubleshooting the controlled system.
Hard-copy reports generated in the software can be
printed on the computer’s printer. Most software pack-
ages will not allow you to develop programs on another
manufacturer’s PLC. In some cases, a single manufac-
turer will have multiple PLC families, each requiring its
own software to program.
2.
10 Recording and Retrieving Data
Printers are used to provide hard-copy printouts of the
processor’s memory in ladder program format. Lengthy
ladder programs cannot be shown completely on a screen.
Typically, a screen shows a maximum of five rungs at a
time. A printout can show programs of any length and
analyze the complete program.
The PLC can have only one program in memory at
a time. To change the program in the PLC, it is neces-
sary either to enter a new program directly from the
keyboard or to download one from the computer hard
drive (Figure 2-47). Some CPUs support the use of a
memory cartridge that provides portable EEPROM stor-
age for the user program (Figure 2-48). The cartridge
can be used to copy a program from one PLC to another
similar type PLC.
2.
11 Human Machine Interfaces
(HMIs)
A human machine interface (HMI) can be connected to
communicate with a PLC and to replace pushbuttons, se-
lector switches, pilot lights, thumbwheels, and other op-
erator control panel devices (Figure 2-49). Luminescent
touch-screen keypads provide an operator interface that
operates like traditional hardwired control panels.
Human machine interfaces give the ability to the op-
erator and to management to view the operation in real
Figure 2-46 Personal computer used as the programming
device.
Serial port
Laptop computer
Processor
Software
Figure 2-47 Copying programs to a computer hard drive.
A
C
Copy
(Disk drive)
(Internal hard drive)
A
C
Figure 2-48 Memory cartridge provides portable storage
for user program.
Memory cartridge
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time. Through personal computer–based setup software,
you can configure display screens to:
• Replace hardwired pushbuttons and pilot lights
with realistic-looking icons. The machine operator
need only touch the display panel to activate the
pushbuttons.
• Show operations in graphic format for easier viewing.
• Allow the operator to change timer and counter pre-
sets by touching the numeric keypad graphic on the
touch screen.
• Show alarms, complete with time of occurrence and
location.
• Display variables as they change over time.
The Allen-Bradley Pico GFX-70 controller, shown in
Figure 2-50, serves as a controller with HMI capabilities.
This device consists of three modular parts: an HMI,
processor/power supply, and I/O modules.
The display/keypad can be used as an operator inter-
face or can be linked to control operations to provide real-
time feedback. It has the ability to show text, date and
time, as well as custom messages and bitmap graphics,
allowing operators to acknowledge fault messages, enter
values, and initiate actions. Users can create both the
control program and HMI functionality using a personal
computer with PicoSoft Pro software installed or the con-
troller’s on-board display buttons.
Figure 2-49 Human Machine Interfaces (HMIs).
Figure 2-50 Allen-Bradley Pico GFX-70 controller.
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1. What is the function of a PLC input interface
module?
2. What is the function of a PLC output interface
module?
3. Define the term logical rack.
4. With reference to a PLC rack:
a. What is a remote rack?
b. Why are remote racks used?
5. How does the processor identify the location of a
specific input or output device?
6. List the three basic elements of rack/slot-based
addressing.
7. Compare bit level and word level addressing.
8. In what way does tag-based addressing differ from
rack/slot-based addressing?
9. What do PC-based control systems use to interface
with field devices?
10. What type of I/O modules have both inputs and
outputs connected to them?
11. In addition to field devices what other connections
are made to a PLC module?
12. Most PLC modules use plug-in wiring terminal
strips. Why?
13. What are the advantage and the disadvantage of
using high-density modules?
14. With reference to PLC discrete input modules:
a. What types of field input devices are suitable for
use with them?
b. List three examples of discrete input devices.
15. With reference to PLC discrete output modules:
a. What types of field output devices are suitable
for use with them?
b. List three examples of discrete output devices.
16. Explain the function of the backplane of a PLC
rack.
17. What is the function of the optical isolator circuit
used in discrete I/O module circuits?
18. Name the two distinct sections of an I/O module.
19. List four tasks performed by a discrete input
module.
20. What electronic element can be used as the switch-
ing device for a 120 VAC discrete output interface
module?
21. With reference to discrete output module current
ratings:
a. What is the maximum current rating for a typical
120 VAC output module?
b. Explain one method of handling outputs with
larger current requirements.
22. What electronic element can be used as the switch-
ing device for DC discrete output modules?
23. A discrete relay type output module can be used to
switch either AC or DC load devices. Why?
24. With reference to sourcing and sinking I/O
modules:
a. What current relationship are the terms sourcing
and sinking used to describe?
b. If an I/O module is specified as a current-sinking
type, then which type of field device (sinking or
sourcing) it is electrically compatible with?
25. Compare discrete and analog I/O modules with
respect to the type of input or output devices with
which they can be used.
26. Explain the function of the analog-to-digital (A/D)
converter circuit used in analog input modules.
27. Explain the function of the digital-to-analog (D/A)
converter circuit used in analog output modules.
28. Name the two general sensing classifications for
analog input modules.
29. List five common physical quantities measured by
a PLC analog input module.
30. What type of cable is used when connecting a ther-
mocouple to a voltage sensing analog input mod-
ule? Why?
31. Explain the difference between a unipolar and bi-
polar analog input module.
32. The resolution of an analog input channel is speci-
fied as 0.3 mV. What does this tell you?
33. In what two ways can the loop power for current
sensing input modules be supplied?
34. List three field devices that are commonly controlled
by a PLC analog output module.
35. State one application for each of the following spe-
cial I/O modules:
a. High-speed counter module
b. Thumbwheel module
c. TTL module
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44. List five important procedures to follow when han-
dling static-sensitive PLC components.
45. Define each of the following terms as they apply to
the memory element of a PLC:
a. writing
b. reading
c. bits
d. location
e. utilization
46. With reference to the I/O image tables:
a. What information is stored in PLC input and
output tables?
b. What is the input status of a closed switch
stored as?
c. What is the input status of an open switch
stored as?
d. What is the status of an output that is ON
stored as?
e. What is the status of an output that is OFF
stored as?
47. Why do PLCs execute memory-checking
routines?
48. Compare the memory storage characteristics of
volatile and nonvolatile memory elements.
49. What information is normally stored in the ROM
memory of a PLC?
50. What information is normally stored in the RAM
memory of a PLC?
51. What information is normally stored in an
EEPROM memory module?
52. What are the advantages of a processor that utilizes
a flash memory card?
53. List three functions of a PLC programming termi-
nal device.
54. Give one advantage and one limitation to the use of
hand-held programming devices.
55. What is required for a personal computer to be used
as a PLC programming terminal?
56. List four important capabilities of PLC program-
ming software.
57. How many programs can a PLC have stored in
memory at any one time?
58. Outline four functions that an HMI interface screen
can be configured to perform.
d. Encoder-counter module
e. BASIC or ASCII module
f. Stepper-motor module
g. BCD-output module
36. List one application for each of the following intel-
ligent I/O modules:
a. PID module
b. Motion and position control module
c. Communication module
37. Write a short explanation for each of the following
discrete I/O module specifications:
a. Nominal input voltage
b. Input threshold voltages
c. Nominal current per input
d. Ambient temperature rating
e. Input ON/OFF delay
f. Output voltage
g. Output current
h. Inrush current
i. Short circuit protection
j. Leakage current
k. Electrical isolation
l. Points per module
m. Backplane current draw
38. Write a short explanation for each of the following
analog I/O module specifications:
a. Channels per module
b. Input current/voltage range(s)
c. Output current/voltage range(s)
d. Input protection
e. Resolution
f. Input impedance and capacitance
g. Common-mode rejection
39. Compare the function of the CPU and memory sec-
tions of a PLC processor.
40. With reference to the PLC chassis power supply:
a. What conversion of power takes place within the
power supply circuit?
b. Explain the term hold-up time as it applies to the
power supply.
41. Explain the purpose of a redundant PLC processor.
42. Describe three typical modes of operation that can
be selected by the keyswitch of a processor.
43. State five other functions, in addition to simple
logic processing, that PLC processors are capable
of performing.
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1. A discrete 120 VAC output module is to be used to
control a 230 VDC solenoid valve. Draw a diagram
showing how this could be accomplished using an
interposing relay.
2. Assume a thermocouple, which supplies the input
to an analog input module, generates a linear volt-
age of from 20 mV to 50 mV when the temperature
changes from 750°F to 1250°F. How much voltage
will be generated when the temperature of the
thermocouple is at 1000°F?
3. With reference to I/O module specifications:
a. If the ON-delay time of a given discrete input
module is specified as 12 milliseconds, how much
is this expressed in seconds?
b. If the output leakage current of a discrete output
module is specified as 950 μA, how much is this
expressed in amperes?
CHAPTER 2 PROBLEMS
c. If the ambient temperature rating for an I/O
module is specified as 60°C, how much is this
expressed in degrees Fahrenheit?
4. Create a five-digit code using the SLC 500 rack/
slot-based addressing format for each of the
following:
a. A pushbutton connected to terminal 5 of module
group 2 located on rack 1.
b. A lamp connected to terminal 3 of module
group 0 located on rack 2.
5. Assume the triac of an AC discrete output module
fails in the shorted state. How would this affect the
device connected to this output?
6. A personal computer is to be used to program sev-
eral different PLCs from different manufacturers.
What would be required?
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43
Using PLCs requires us to become familiar with
other number systems besides decimal. Some
PLC models and individual PLC functions use
other numbering systems. This chapter deals
with some of these numbering systems, including
binary, octal, hexadecimal, BCD, Gray, and ASCII.
The basics of each system, as well as conversion
from one system to another, are explained.
Chapter Objectives
3.1 Define the decimal, binary, octal, and hexadecimal
numbering systems and be able to convert from one
numbering or coding system to another
3.2 Explain the BCD, Gray, and ASCII code systems
3.3 Define the terms bit, byte, word, least significant bit
(LSB), and most significant bit (MSB) as they apply to
binary memory locations
3.4 Add, subtract, multiply, and divide binary numbers
3
Number Systems and Codes
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44 Chapter 3 Number Systems and Codes
3.
1 Decimal System
Knowledge of different number systems and digital codes
is quite useful when working with PLCs or with most any
type of digital computer. This is true because a basic re-
quirement of these devices is to represent, store, and oper-
ate on numbers. In general, PLCs work on binary numbers
in one form or another; these are used to represent various
codes or quantities.
The decimal system, which is most common to us, has a
base of 10. The radix or base of a number system determines
the total number of different symbols or digits used by that
system. For instance, in the decimal system, 10 unique num-
bers or digits—i.e., the digits 0 through 9—are used: the
total number of symbols is the same as the base, and the
symbol with the largest value is 1 less than the base.
The value of a decimal number depends on the dig-
its that make up the number and the place value of each
digit. A place (weight) value is assigned to each position
that a digit would hold from right to left. In the decimal
system the first position, starting from the rightmost po-
sition, is 0; the second is 1; the third is 2; and so on up
to the last position. The weighted value of each position
can be expressed as the base (10 in this case) raised to
the power of the position. For the decimal system then,
the position weights are 1, 10, 100, 1000, and so on. Fig-
ure 3-1 illustrates how the value of a decimal number can
be calculated by multiplying each digit by the weight of
its position and summing the results.
3.2 Binary System
The binary system uses the number 2 as the base. The
only allowable digits are 0 and 1. With digital circuits it is
easy to distinguish between two voltage levels (i.e., 15 V
and 0 V), which can be related to the binary digits 1 and 0
(Figure 3-2). Therefore the binary system can be applied
quite easily to PLCs and computer systems.
Since the binary system uses only two digits, each
position of a binary number can go through only two
changes, and then a 1 is carried to the immediate left po-
sition. Table 3-1 shows a comparison among four com-
mon number systems: decimal (base 10), octal (base 8),
hexadecimal (base 16), and binary (base 2). Note that all
numbering systems start at zero.
The decimal equivalent of a binary number can be de-
termined in a manner similar to that used for a decimal
number. This time the weighted values of the positions
are 1, 2, 4, 8, 16, 32, 64, and so on. The weighted value,
instead of being 10 raised to the power of the position, is
2 raised to the power of the position. Figure 3-3 illustrates
how the binary number 10101101 is converted to its deci-
mal equivalent: 173.
Each digit of a binary number is known as a bit. In a
PLC the processor-memory element consists of hundreds
or thousands of locations. These locations, or registers,
Decimal Octal Hexadecimal Binary
0 . . . . . . 0 . . . . . . . . 0 . . . . . . . . . . 0
1 . . . . . . 1 . . . . . . . . 1 . . . . . . . . . . 1
2 . . . . . . 2 . . . . . . . . 2 . . . . . . . . . .10
3 . . . . . . 3 . . . . . . . . 3 . . . . . . . . . .11
4 . . . . . . 4 . . . . . . . . 4 . . . . . . . . . 100
5 . . . . . . 5 . . . . . . . . 5 . . . . . . . . . 101
6 . . . . . . 6 . . . . . . . . 6 . . . . . . . . . 110
7 . . . . . . 7 . . . . . . . . 7 . . . . . . . . . 111
8 . . . . . . 10 . . . . . . . . 8 . . . . . . . . 1000
9 . . . . . . 11 . . . . . . . . 9 . . . . . . . . 1001
10 . . . . . . 12 . . . . . . . . A . . . . . . . . 1010
11 . . . . . . 13 . . . . . . . . B . . . . . . . . 1011
12 . . . . . . 14 . . . . . . . . C . . . . . . . . 1100
13 . . . . . . 15 . . . . . . . . D . . . . . . . . 1101
14 . . . . . . 16 . . . . . . . . E . . . . . . . . 1110
15 . . . . . . 17 . . . . . . . . F . . . . . . . . 1111
16 . . . . . . 20 . . . . . . . . 10 . . . . . . . .10000
17 . . . . . . 21 . . . . . . . . 11 . . . . . . . .10001
18 . . . . . . 22 . . . . . . . . 12 . . . . . . . .10010
19 . . . . . . 23 . . . . . . . . 13 . . . . . . . .10011
20 . . . . . . 24 . . . . . . . . 14 . . . . . . . .10100
Table 3-1 Number System Comparisons
Figure 3-1 Weighted value in the decimal system.
2
1 9 6
0
3 2 1
2
60
900
1000
1962
2 10
6 10
9 10
1 10
0
1
2
3

2 1
6 10
9 100
1 1000
10
(Sum of products)
Decimal
number
10
Figure 3-2 Digital signal waveform.
+5
0
Volts
Time
High (H) (1)
Low (L) (0)
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Number Systems and Codes Chapter 3 45
are referred to as words. Each word is capable of storing
data in the form of binary digits, or bits. The number of
bits that a word can store depends on the type of PLC sys-
tem used. Sixteen-bit and 32-bit words are the most com-
mon. Bits can also be grouped within a word into bytes.
A group of 8 bits is a byte, and a group of 2 or more bytes
is a word. Figure 3-4 illustrates a 16-bit word made up
of 2 bytes. The least significant bit (LSB) is the digit that
represents the smallest value, and the most significant bit
(MSB) is the digit that represents the largest value. A bit
within the word can exist only in two states: a logical 1 (or
ON) condition, or a logical 0 (or OFF) condition.
PLC memory is organized using bytes, single words,
or double words. Older PLCs use 8-bit or 16-bit memory
words while newer systems, such as the ControlLogix
platform from Allen-Bradley, use 32-bit double words.
The size of the programmable controller memory relates
to the amount of user program that can be stored. If the
memory size is 1 K word (Figure 3-5), it can store 1024
words or 16,384 (1024 3 16) bits of information using 16-
bit words, or 32,768 (1024 3 32) bits using 32-bit words.
To convert a decimal number to its binary equivalent,
we must perform a series of divisions by 2. Figure 3-6
illustrates the conversion of the decimal number 47 to bi-
nary. We start by dividing the decimal number by 2. If
there is a remainder, it is placed in the LSB of the binary
number. If there is no remainder, a 0 is placed in the LSB.
The result of the division is brought down and the pro-
cess is repeated until the result of successive divisions has
been reduced to 0.
Even though the binary system has only two digits, it
can be used to represent any quantity that can be repre-
sented in the decimal system. All PLCs work internally in
the binary system. The processor, being a digital device,
understands only 0s and 1s, or binary.
Computer memory is, then, a series of binary 1s and
0s. Figure 3-7 shows the output status file for an Allen-
Bradley SLC 500 modular chassis, which is made up of
single bits grouped into 16-bit words. One 16-bit output
file word is reserved for each slot in the chassis. Each bit
represents the ON or OFF state of one output point. These
points are numbered 0 through 15 across the top row from
Figure 3-3 Converting a binary number to a decimal
number.
1
1 1 0
1
0
4
8
0
32
0
128
173
1 20
0 21
1 22
1 23
0 24
1 25
0 26
1 2
7

1 1
0 2
1 4
1 8
0 16
1 32
0 64
1 128
10
(Sum of products)
1
0
1
0
0
7 2 1
3
4
5
6
2
Decimal number
Binary
number
Figure 3-4 A 16-bit word.
16-bit word
Upper byte Lower byte
1
0 1 1 0 0 1 1 0 0 0 1 1 1 0 1
LSB
Bit
MSB
Figure 3-5 1-K word memory.
0 1 1 0 0 1 1 0 0 0 1 1 1 0 1 1
15
Bits
14 13 12 1110 9 8 7 6 5 4 3 2 1 0
0000
0001
0002
0003
0004
0005
1018
1019
1020
1021
1022
1023
Word
Addresses
Figure 3-6 Converting a decimal number to a binary
number.
1
1
1
1
0
1
2 = 23 with a remainder of
2 =
23
11
5
2
1
47 LSB
MSB
101111
with a remainder of
Decimal number
Binary number
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right to left. The column on the far right lists the output
module address. Although the table in Figure 3-7 illus-
trates sequentially addressed output status file words, in
reality a word is created in the table only if the processor
finds an output module residing in a particular slot. If the
slot is empty, no word will be created.
3.3 Negative Numbers
If a decimal number is positive, it has a plus sign; if a
number is negative, it has a minus sign. In binary number
systems, such as used in a PLC, it is not possible to use
positive and negative symbols to represent the polarity of
a number. One method of representing a binary number as
either a positive or negative value is to use an extra digit,
or sign bit, at the MSB side of the number. In the sign bit
position, a 0 indicates that the number is positive, and a
1 indicates a negative number (Table 3-2).
Another method of expressing a negative number in
a digital system is by using the complement of a binary
number. To complement a binary number, change all the
1s to 0s and all the 0s to 1s. This is known as the 1’s
complement form of a binary number. For example, the
1’s complement of 1001 is 0110.
The most common way to express a negative binary
number is to show it as a 2’s complement number. The
2’s complement is the binary number that results when
1 is added to the 1’s complement. This system is shown
in Table 3-3. A zero sign bit means a positive number,
whereas a 1 sign bit means a negative number.
Using the 2’s complement makes it easier for the PLC
to perform mathematical operations. The correct sign bit
is generated by forming the 2’s complement. The PLC
knows that a number retrieved from memory is a nega-
tive number if the MSB is 1. Whenever a negative num-
ber is entered from a keyboard, the PLC stores it as a 2’s
complement. What follows is the original number in true
binary followed by its 1’s complement, its 2’s comple-
ment, and finally, its decimal equivalent.
Magnitude
Sign
Decimal
Value
Same as
binary
numbers
0111 . . . . . . . . 17
0110 . . . . . . . . 16
0101 . . . . . . . . 15
0100 . . . . . . . . 14
0011 . . . . . . . . 13
0010 . . . . . . . . 12
0001 . . . . . . . . 11
0000 . . . . . . . . 0
1001 . . . . . . . . 21
1010 . . . . . . . . 22
1011 . . . . . . . . 23
1100 . . . . . . . . 24
1101 . . . . . . . . 25
1110 . . . . . . . . 26
1111 . . . . . . . . 27
Table 3-2 Signed Binary Numbers
Signed
Decimal
1’s
Complement
2’s
Complement
17 . . . . . . . . . . 0111 . . . . . . . . . . . 0111
16 . . . . . . . . . . 0110 . . . . . . . . . . . 0110
15 . . . . . . . . . . 0101 . . . . . . . . . . . 0101
14 . . . . . . . . . . 0100 . . . . . . . . . . . 0100
13 . . . . . . . . . . 0011 . . . . . . . . . . . 0011
12 . . . . . . . . . . 0010 . . . . . . . . . . . 0010
11 . . . . . . . . . . 0001 . . . . . . . . . . . 0001
0 . . . . . . . . . . 0000 . . . . . . . . . . . 0000
21 . . . . . . . . . . 1110 . . . . . . . . . . . 1111
22 . . . . . . . . . . 1101 . . . . . . . . . . . 1110
23 . . . . . . . . . . 1100 . . . . . . . . . . . 1101
24 . . . . . . . . . . 1011 . . . . . . . . . . . 1100
25 . . . . . . . . . . 1010 . . . . . . . . . . . 1011
26 . . . . . . . . . . 1001 . . . . . . . . . . . 1010
27 . . . . . . . . . . 1000 . . . . . . . . . . . 1001
Table 3-3 1’s and 2’s Complement
Representation of Positive and Negative
Numbers
Same as
binary
numbers
Figure 3-7 SLC 500 output status ﬁle.
15
1
0
1
0
1
14
1
0
0
0
1
13
0
1
1
0
1
12
0
1
0
0
0
11
0
0
1
0
1
10
0
1
1
0
0
9
1
0
0
0
0
8
0
0
0
0
1
7
1
0
1
0
1
6
1
0
1
1
1
5
1
0
1
0
0
4
1
0
0
0
0
3
0
1
0
1
1
2
0
1
0
0
1
1
0
1
0
0
0
0
1
1
1
0
1
Address
O:1
O:2
O:3
O:4
O:5
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3.4 Octal System
To express the number in the binary system requires
many more digits than in the decimal system. Too many
binary digits can become cumbersome to read or write.
To solve this problem, other related numbering systems
are used.
The octal numbering system, a base 8 system, is used
because 8 data bits make up a byte of information that
can be addressed. Figure 3-8 illustrates the addressing of
I/O modules using the octal numbering system. The digits
range from 0 to 7; therefore, numbers 8 and 9 are not al-
lowed. Allen-Bradley PLC-5 processors use octal-based
I/O addressing while the SLC 500 and Logix controllers
use decimal-base 10 addressing.
Octal is a convenient means of handling large binary
numbers. As shown in Table 3-4, one octal digit can be
used to express three binary digits.As in all other number-
ing systems, each digit in an octal number has a weighted
decimal value according to its position. Figure 3-9 illus-
trates how the octal number 462 is converted to its deci-
mal equivalent: 306.
Octal converts easily to binary equivalents. For ex-
ample, the octal number 462 is converted to its binary
equivalent by assembling the 3-bit groups, as illustrated
in Figure 3-10. Notice the simplicity of the notation: the
octal 462 is much easier to read and write than its binary
equivalent is.
Binary Octal
000 . . . . . . . . 0
001 . . . . . . . . 1
010 . . . . . . . . 2
011 . . . . . . . . 3
100 . . . . . . . . 4
101 . . . . . . . . 5
110 . . . . . . . . 6
111 . . . . . . . . 7
Table 3-4 Binary and Related Octal
Code
Figure 3-8 Addressing of I/O modules using the octal numbering system.
I/O module screw
terminal number
0
1
2
3
4
5
6
7
1 2 3 4
0 10 20 30
1 11 21 31
2 12 22 32
3 13 23 33
4 14 24 34
5 15 25 35
6 16 26 36
7 17 27 37
Slot number
and address
Power
supply
Slot 0 Slot 1 Slot 2 Slot 3 Slot 4
CPU Address
0–7
Address
10–17
Address
20–27
Address
30–37
0
2
4
6
1
3
5
7
O:3/22
I:2/16
0
2
4
6
1
3
5
7
0
2
4
6
1
3
5
7
0
2
4
6
1
3
5
7
Figure 3-9 Converting an octal number to a decimal
number.
2
4 6
2
48
256
306
2 80

6 81

4 82

2 1
6 8
4 64
8
(Sum of products)
Decimal number 10
Octal
number
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3.5 Hexadecimal System
The hexadecimal (hex) numbering system is used in pro-
grammable controllers because a word of data consists of
16 data bits, or two 8-bit bytes. The hexadecimal system
is a base 16 system, with A to F used to represent decimal
numbers 10 to 15 (Table 3-5). The hexadecimal number-
ing system allows the status of a large number of binary
bits to be represented in a small space, such as on a com-
puter screen or PLC programming device display.
The techniques used when converting hexadecimal to
decimal and decimal to hexadecimal are the same as those
used for binary and octal. To convert a hexadecimal num-
ber to its decimal equivalent, the hexadecimal digits in the
columns are multiplied by the base 16 weight, depending
on digit significance. Figure 3-11 illustrates how the con-
version would be done for the hex number 1B7.
Hexadecimal numbers can easily be converted to bi-
nary numbers. Conversion is accomplished by writing the
4-bit binary equivalent of the hex digit for each position,
as illustrated in Figure 3-12.
3.6 Binary Coded Decimal (BCD)
System
The binary coded decimal (BCD) system provides a con-
venient way of handling large numbers that need to be
input to or output from a PLC. As you can see from look-
ing at the various number systems, there is no easy way
to go from binary to decimal and back. The BCD system
provides a means of converting a code readily handled
by humans (decimal) to a code readily handled by the
equipment (binary). PLC thumbwheel switches and LED
displays are examples of PLC devices that make use of
the BCD number system. Table 3-6 shows examples of
numeric values in decimal, binary, BCD, and hexadeci-
mal representation.
The BCD system uses 4 bits to represent each decimal
digit. The 4 bits used are the binary equivalents of the
numbers from 0 to 9. In the BCD system, the largest deci-
mal number that can be displayed by any four digits is 9.
The BCD representation of a decimal number is ob-
tained by replacing each decimal digit by its BCD equiva-
lent. To distinguish the BCD numbering system from a
binary system, a BCD designation is placed to the right
of the units digit. The BCD representation of the decimal
number 7863 is shown in Figure 3-13.
A thumbwheel switch is one example of an input de-
vice that uses BCD. Figure 3-14 shows a single-digit
BCD thumbwheel. The circuit board attached to the
thumbwheel has one connection for each bit’s weight plus
Hexadecimal Binary Decimal
0 . . . . . . . . . . . . . 0000 . . . . . . . . . . . 0
1 . . . . . . . . . . . . . 0001 . . . . . . . . . . . 1
2 . . . . . . . . . . . . . 0010 . . . . . . . . . . . 2
3 . . . . . . . . . . . . . 0011 . . . . . . . . . . . 3
4 . . . . . . . . . . . . . 0100 . . . . . . . . . . . 4
5 . . . . . . . . . . . . . 0101 . . . . . . . . . . . 5
6 . . . . . . . . . . . . . 0110 . . . . . . . . . . . 6
7 . . . . . . . . . . . . . 0111 . . . . . . . . . . . 7
8 . . . . . . . . . . . . . 1000 . . . . . . . . . . . 8
9 . . . . . . . . . . . . . 1001 . . . . . . . . . . . 9
A . . . . . . . . . . . . . 1010 . . . . . . . . . . 10
B . . . . . . . . . . . . . 1011 . . . . . . . . . . 11
C. . . . . . . . . . . . . 1100 . . . . . . . . . . 12
D . . . . . . . . . . . . . 1101 . . . . . . . . . . 13
E . . . . . . . . . . . . . 1110 . . . . . . . . . . 14
F . . . . . . . . . . . . . 1111 . . . . . . . . . . 15
Table 3-5 Hexadecimal Numbering
System
Figure 3-11 Converting a hexadecimal number to a
decimal number.
7
1 B
7
176
256
439
7 160

11 161

1 162

7 1
11 16
1 256
(Sum of products)
Decimal number 10
Hex
number
Figure 3-12 Converting a hexadecimal number to a binary
number.
1
0 1
7
1 B
1
0 0 1
1 0
Hex number
0 1 1
Binary
number
Figure 3-10 Converting an octal number to a binary
number.
0
0 1
2
4 6
0
1 0 0
1 1 Binary
number
Octal
number
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a common connection. The operator dials in a decimal
digit between 0 and 9, and the thumbwheel switch outputs
the equivalent 4 bits of BCD data. In this example, the
number eight is dialed to produce the input bit pattern of
1000. A four-digit thumbwheel switch, similar to the one
shown, would control a total of 16 (4 3 4) PLC inputs.
Scientific calculators are available to convert num-
bers back and forth between decimal, binary, octal, and
Figure 3-13 The BCD representation of a decimal number.
3
7 8
1
0 0
4 bits used for
each decimal digit
1
0 1 0 0 1 0
6
1
1 0 1
0 1 BCD
BCD
number
Decimal number
Decimal Binary BCD Hexadecimal
0 . . . . . . . . . . . . . . . 0 . . . . . . . . . . . . . 0000 . . . . . . . . . . . . 0
1 . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . 0001 . . . . . . . . . . . . 1
2 . . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . 0010 . . . . . . . . . . . . 2
3 . . . . . . . . . . . . . . . 11 . . . . . . . . . . . . . 0011 . . . . . . . . . . . . 3
4 . . . . . . . . . . . . . . 100 . . . . . . . . . . . . . 0100 . . . . . . . . . . . . 4
5 . . . . . . . . . . . . . . 101 . . . . . . . . . . . . . 0101 . . . . . . . . . . . . 5
6 . . . . . . . . . . . . . . 110 . . . . . . . . . . . . . 0110 . . . . . . . . . . . . 6
7 . . . . . . . . . . . . . . 111 . . . . . . . . . . . . . 0111 . . . . . . . . . . . . 7
8 . . . . . . . . . . . . . 1000 . . . . . . . . . . . . . 1000 . . . . . . . . . . . . 8
9 . . . . . . . . . . . . . 1001 . . . . . . . . . . . . . 1001 . . . . . . . . . . . . 9
10 . . . . . . . . . . . . . 1010 . . . . . . . . . . 0001 0000 . . . . . . . . . . . . A
11 . . . . . . . . . . . . . 1011 . . . . . . . . . . 0001 0001 . . . . . . . . . . . . B
12 . . . . . . . . . . . . . 1100 . . . . . . . . . . 0001 0010 . . . . . . . . . . . . C
13 . . . . . . . . . . . . . 1101 . . . . . . . . . . 0001 0011 . . . . . . . . . . . . D
14 . . . . . . . . . . . . . 1110 . . . . . . . . . . 0001 0100 . . . . . . . . . . . . E
15 . . . . . . . . . . . . . 1111 . . . . . . . . . . 0001 0101 . . . . . . . . . . . . F
16 . . . . . . . . . . . . 1 0000 . . . . . . . . . . 0001 0110 . . . . . . . . . . . . 10
17 . . . . . . . . . . . . 1 0001 . . . . . . . . . . 0001 0111 . . . . . . . . . . . . 11
18 . . . . . . . . . . . . 1 0010 . . . . . . . . . . 0001 1000 . . . . . . . . . . . . 12
19 . . . . . . . . . . . . 1 0011 . . . . . . . . . . 0001 1001 . . . . . . . . . . . . 13
20 . . . . . . . . . . . . 1 0100 . . . . . . . . . . 0010 0000 . . . . . . . . . . . . 14
126 . . . . . . . . . . . 111 1110 . . . . . . . 0001 0010 0110 . . . . . . . . . . . . 7E
127 . . . . . . . . . . . 111 1111 . . . . . . . 0001 0010 0111 . . . . . . . . . . . . 7F
128 . . . . . . . . . . 1000 0000 . . . . . . . 0001 0010 1000 . . . . . . . . . . . . 80
510 . . . . . . . . . 1 1111 1110 . . . . . . . 0101 0001 0000 . . . . . . . . . . . 1FE
511 . . . . . . . . . 1 1111 1111 . . . . . . . 0101 0001 0001 . . . . . . . . . . . 1FF
512 . . . . . . . . . 10 0000 0000 . . . . . . . 0101 0001 0010 . . . . . . . . . . . 200
Table 3-6 Numeric Values in Decimal, Binary, BCD,
and Hexadecimal Representation
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Gray Code Binary
0000 . . . . . . . . . . 0000
0001 . . . . . . . . . . 0001
0011 . . . . . . . . . . 0010
0010 . . . . . . . . . . 0011
0110 . . . . . . . . . . 0100
0111 . . . . . . . . . . 0101
0101 . . . . . . . . . . 0110
0100 . . . . . . . . . . 0111
1100 . . . . . . . . . . 1000
1101 . . . . . . . . . . 1001
1111 . . . . . . . . . . 1010
1110 . . . . . . . . . . 1011
1010 . . . . . . . . . . 1100
1011 . . . . . . . . . . 1101
1001 . . . . . . . . . . 1110
1000 . . . . . . . . . . 1111
Table 3-7 Gray Code and Binary
Equivalent
Figure 3-15 PLC number conversion.
Thumbwheel or
other input
Input
module
BCD
to binary
PLC
Processor Binary
to BCD
Decimal
readout
1 7 6 5
Output
module
Convert-to-decimal PLC instruction
Destination
N7:23
O:20
TOD
To BCD
Source
Input A
4 1 9
6
Figure 3-14 BCD thumbwheel switch interfaced to a PLC.
1
2
4
8
C
1s Input = 0
2s Input = 0
4s Input = 0
8s Input = 1
+ –
Input
module
hexadecimal. In addition, PLCs contain number conver-
sion functions such as illustrated in Figure 3-15. BCD-
to-binary conversion is required for the input while
binary-to-BCD conversion is required for the output. The
PLC convert-to-decimal instruction will convert the bi-
nary bit pattern at the source address, N7:23, into a BCD
bit pattern of the same decimal value as the destination
address, O:20. The instruction executes every time it is
scanned, and the instruction is true.
Many PLCs allow you to change the format of the data
that the data monitor displays. For example, the change
radix function found on Allen-Bradley controllers allows
you to change the display format of data to binary, octal,
decimal, hexadecimal, or ASCII.
3.7 Gray Code
The Gray code is a special type of binary code that does
not use position weighting. In other words, each position
does not have a definite weight. The Gray code is set up
so that as we progress from one number to the next, only
one bit changes. This can be quite confusing for count-
ing circuits, but it is ideal for encoder circuits. For ex-
ample, absolute encoders are position transducers that use
the Gray code to determine angular position. The Gray
code has the advantage that for each “count” (each transi-
tion from one number to the next) only one digit changes.
Table 3-7 shows the Gray code and the binary equivalent
for comparison.
In binary, as many as four digits could change for a single
“count.” For example, the transition from binary 0111 to
1000 (decimal 7 to 8) involves a change in all four digits.
This kind of change increases the possibility for error in cer-
tain digital circuits. For this reason, the Gray code is consid-
ered to be an error-minimizing code. Because only one bit
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changes at a time, the speed of transition for the Gray code is
considerably faster than that for codes such as BCD.
Gray codes are used with position encoders for accu-
rate control of the motion of robots, machine tools, and
servomechanisms. Figure 3-16 shows an optical encoder
disk that uses a 4-bit Gray code to detect changes in angu-
lar position. In this example, the encoder disk is attached
to a rotating shaft and outputs a digital Gray code signal
that is used to determine the position of the shaft. A fixed
array of photo diodes senses the reflected light from each
of the cells across a row of the encoder path. Depending
on the amount of light reflected, each cell will output a
voltage corresponding to a binary 1 or 0. Thus, a different
4-bit word is generated for each row of the disk.
3.8 ASCII Code
ASCII stands for American Standard Code for Informa-
tion Interchange. It is an alphanumeric code because it in-
cludes letters as well as numbers. The characters accessed
by the ASCII code include 10 numeric digits; 26 lower-
case and 26 uppercase letters of the alphabet; and about
25 special characters, including those found on a standard
typewriter. Table 3-8 shows a partial listing of the ASCII
code. It is used to interface the PLC CPU with alphanu-
meric keyboards and printers.
The keystrokes on the keyboard of a computer are con-
verted directly intoASCII for processing by the computer.
Each time you press a key on a computer keyboard, a 7- or
8-bit word is stored in computer memory to represent the
alphanumeric, function, or control data represented by the
specific keyboard key that was depressed. ASCII input
modules convert ASCII code input information from
an external device to alphanumeric information that the
PLC can process. The communication interfacing is done
through either an RS-232 or RS-422 protocol. Modules
are available that will transmit and receiveASCII files and
that can be used to create an operator interface. The user
writes a program in the BASIC language that operates in
conjunction with the ladder logic as the program runs.
3.9 Parity Bit
Some PLC communication systems use a binary digit to
check the accuracy of data transmission. For example,
when data are transferred between PLCs, one of the bi-
nary digits may be accidentally changed from a 1 to a 0.
This can happen because of a transient or a noise or be-
cause of a failure in some portion of the transmission net-
work. A parity bit is used to detect errors that may occur
while a word is moved.
Parity is a system in which each character transmitted
contains one additional bit. That bit is known as a par-
ity bit. The bit may be a binary 0 or binary 1, depend-
ing on the number of 1s and 0s in the character itself.
Two systems of parity are normally used: odd and even.
Odd parity means that the total number of binary 1 bits in
the character, including the parity bit, is odd. Even parity
means that the number of binary 1 bits in the character, in-
cluding the parity bit, is even. Examples of odd and even
parity are shown in Table 3-9.
3.
10 Binary Arithmetic
Arithmetic circuit units form a part of the CPU. Mathe-
matical operations include addition, subtraction, multipli-
cation, and division. Binary addition follows rules similar
to decimal addition. When adding with binary numbers,
there are only four conditions that can occur:
0 1 0 1
10 10 11 11
0 1 1 0 carry 1
Figure 3-16 Optical encoder disk.
Source: Photo courtesy Baumer Electric.
0111
0110
0101
0100
1100
1101
1111
1110
1010
1011
1001
1000
0000
0001
0011
0010
1
1
1
0
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larger binary numbers, the resulting 1s are carried
into higher-order columns, as shown in the following
examples.
Decimal Equivalent binary
5 101
12 1 10
7 111
10 10 10
1 3 1 11
13 11 01
26 1 1010
112 1 1100
38 1 0 0110
In arithmetic functions, the initial numeric quantities
that are to be combined by subtraction are the minuend
carry
1
carry
1
carry
1
Character 7-Bit ASCII Character 7-Bit ASCII
A . . . . . . . . . . 100 0001 X . . . . . . . . . . 101 1000
B . . . . . . . . . . 100 0010 Y . . . . . . . . . . 101 1001
C . . . . . . . . . . 100 0011 Z . . . . . . . . . . 101 1010
D . . . . . . . . . . 100 0100 0 . . . . . . . . . . 011 0000
E . . . . . . . . . . 100 0101 1 . . . . . . . . . . 011 0001
F . . . . . . . . . . 100 0110 2 . . . . . . . . . . 011 0010
G . . . . . . . . . . 100 0111 3 . . . . . . . . . . 011 0011
H . . . . . . . . . . 100 1000 4 . . . . . . . . . . 011 0100
I . . . . . . . . . . 100 1001 5 . . . . . . . . . . 011 0101
J . . . . . . . . . . 100 1010 6 . . . . . . . . . . 011 0110
K . . . . . . . . . . 100 1011 7 . . . . . . . . . . 011 0111
L . . . . . . . . . . 100 1100 8 . . . . . . . . . . 011 1000
M . . . . . . . . . . 100 1101 9 . . . . . . . . . . 011 1001
N . . . . . . . . . . 100 1110 blank . . . . . . . . . 010 0000
O . . . . . . . . . . 100 1111 . . . . . . . . . . . 010 1110
P . . . . . . . . . . 101 0000 , . . . . . . . . . . 010 1100
Q . . . . . . . . . . 101 0001 1 . . . . . . . . . . 010 1011
R . . . . . . . . . . 101 0010 2 . . . . . . . . . . 010 1101
S . . . . . . . . . . 101 0011 # . . . . . . . . . . 010 0011
T . . . . . . . . . . 101 0100 ( . . . . . . . . . . 010 1000
U . . . . . . . . . . 101 0101 % . . . . . . . . . . 010 0101
V . . . . . . . . . . 101 0110 5 . . . . . . . . . . 011 1101
W . . . . . . . . . . 101 0111
Table 3-8 Partial Listing of ASCII Code
Character
Even
Parity Bit
Odd
Parity Bit
0000 . . . . . . . . . . 0 . . . . . . . . . . . 1
0001 . . . . . . . . . . 1 . . . . . . . . . . . 0
0010 . . . . . . . . . . 1 . . . . . . . . . . . 0
0011 . . . . . . . . . . 0 . . . . . . . . . . . 1
0100 . . . . . . . . . . 1 . . . . . . . . . . . 0
0101 . . . . . . . . . . 0 . . . . . . . . . . . 1
0110 . . . . . . . . . . 0 . . . . . . . . . . . 1
0111 . . . . . . . . . . 1 . . . . . . . . . . . 0
1000 . . . . . . . . . . 1 . . . . . . . . . . . 0
1001 . . . . . . . . . . 0 . . . . . . . . . . . 1
Table 3-9 Odd and Even Parity
The first three conditions are easy because they are
like adding decimals, but the last condition is slightly
different. In decimal, 1 1 1 5 2. In binary, a 2 is writ-
ten 10. Therefore, in binary, 1 1 1 5 0, with a carry of
1 to the next most significant place value. When adding
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and subtrahend. The result of the subtraction process is
called the difference, represented as:
A (minuend)
2B (subtrahend)
C (difference)
To subtract from larger binary numbers, subtract col-
umn by column, borrowing from the adjacent column
when necessary. Remember that when borrowing from
the adjacent column, there are now two digits, i.e., 0 bor-
row 1 gives 10.
The procedure for subtracting numbers using the 1’s
complement is as follows:
Step 1 Change the subtrahend to 1’s complement.
Step 2 Add the two numbers.
Step 3 Remove the last carry and add it to the number
(end-around carry).
Decimal Binary
10 1010 1010
2 6 20110 11001
4 100 10011
11
100
When there is a carry at the end of the result, the result
is positive. When there is no carry, then the result is nega-
tive and a minus sign has to be placed in front of it.
End-around carry
1’s complement
Subtract 1001 from 1101.
1101
21001
0100
1011
20111
0100
E X A M P L E
Binary numbers can also be negative. The procedure
for this calculation is identical to that of decimal numbers
because the smaller value is subtracted from the larger
value and a negative sign is placed in front of the result.
111
2100
2011
11011
210111
200100
E X A M P L E
There are other methods available for doing subtraction:
1’s complement
2’s complement
01101
1 2 00100 The 1’s complement
10001 There is no carry,
so we take the 1’s
complement and add
the minus sign:
201110
E X A M P L E
For subtraction using the 2’s complement, the 2’s com-
plement is added instead of subtracting the numbers. In
the result, if the carry is a 1, then the result is positive; if
the carry is a 0, then the result is negative and requires a
minus sign.
111
1010 The first 1 indicates that
the result is positive, so it
is disregarded:
010
E X A M P L E
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Binary numbers are multiplied in the same manner
as decimal numbers. When multiplying binary numbers,
there are only four conditions that can occur:
0 3 0 5 0
0 3 1 5 0
1 3 0 5 0
1 3 1 5 1
To multiply numbers with more than one digit, form
partial products and add them together, as shown in the
following example.
5 101
36 3110
30 000
101
101
11110
The process for dividing one binary number by an-
other is the same for both binary and decimal numbers, as
shown in the following example.
7 111
2q
_
14 10q
_
1110
10
11
10
10
10
00
The basic function of a comparator is to compare the
relative magnitude of two quantities. PLC data compari-
son instructions are used to compare the data stored in
two words (or registers). At times, devices may need to
be controlled when they are less than, equal to, or greater
than other data values or set points used in the applica-
tion, such as timer and counter values. The basic compare
instructions are as follows:
A 5 B (A equals B)
A . B (A is greater than B)
A , B (A is less than B)
111
1010 The first 1 indicates that
the result is positive, so it
is disregarded:
010
E X A M P L E
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1. Convert each of the following binary numbers to
decimal numbers:
a. 10
b. 100
c. 111
d. 1011
e. 1100
f. 10010
g. 10101
h. 11111
i. 11001101
j. 11100011
binary numbers:
a. 7
b. 19
c. 28
d. 46
e. 57
f. 86
g. 94
h. 112
i. 148
j. 230
decimal numbers:
a. 36
b. 104
c. 120
d. 216
e. 360
f. 1516
binary numbers:
a. 74
b. 130
c. 250
d. 1510
e. 2551
f. 2634
bers to decimal numbers:
a. 5A
b. C7
c. 9B5
d. 1A6
bers to binary numbers:
a. 4C
b. E8
c. 6D2
d. 31B
BCD:
a. 146
b. 389
c. 1678
d. 2502
8. What is the most important characteristic of the
Gray code?
9. What makes the binary system so applicable to
computer circuits?
10. Define the following as they apply to the binary
memory locations or registers:
a. Bit
b. Byte
c. Word
d. LSB
e. MSB
11. State the base used for each of the following num-
ber systems:
a. Octal
b. Decimal
c. Binary
d. Hexadecimal
12. Define the term sign bit.
13. Explain the difference between the 1’s complement
of a number and the 2’s complement.
14. What is ASCII code?
15. Why are parity bits used?
16. Add the following binary numbers:
a. 110 1 111
b. 101 1 011
c. 1100 1 1011
17. Subtract the following binary numbers:
a. 1101 2 101
b. 1001 2 110
c. 10111 2 10010
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19. Divide the following unsigned binary numbers:
a. 1010 4 10
b. 1100 4 11
c. 110110 4 10
18. Multiply the following binary numbers:
a. 110 3 110
b. 010 3 101
c. 101 3 11
1. The following binary PLC coded information is to
be programmed using the hexadecimal code. Convert
each piece of binary information to the appropriate
hexadecimal code for entry into the PLC from the
keyboard.
a. 0001 1111
b. 0010 0101
c. 0100 1110
d. 0011 1001
2. The encoder circuit shown in Figure 3-17 is used
to convert the decimal digits on the keyboard to a
binary code. State the output status (HIGH/LOW) of
A-B-C-D when decimal number
a. 2 is pressed.
b. 5 is pressed.
CHAPTER 3 PROBLEMS
c. 7 is pressed.
d. 8 is pressed.
3. If the bits of a 16-bit word or register are numbered
according to the octal numbering system, beginning
with 00, what consecutive numbers would be used to
represent each of the bits?
4. Express the decimal number 18 in each of the fol-
lowing number codes:
a. Binary
b. Octal
c. Hexadecimal
d. BCD
Figure 3-17 Diagram for Problem 2.
Decimal
number
pressed
Binary-coded
0100 output
High
High
Low
Low
Low
Encoder
A
B
C
D
4 input
High
7 8 9
4 5 6
1 2
0
3
0
1
2
3
4
5
6
7
8
9
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57
4
Fundamentals of Logic
This chapter gives an overview of digital logic
gates and illustrates how to duplicate this type
of control on a PLC. Boolean algebra, which is a
shorthand way of writing digital gate diagrams,
is discussed brieﬂy. Some small hand-held pro-
grammers have digital logic keys, such as AND,
OR, and NOT, and are programmed using Bool-
ean expressions.
Chapter Objectives
4.1 Describe the binary concept and the functions of gates
4.2 Draw the logic symbol, construct a truth table, and
state the Boolean equation for the AND, OR, and NOT
functions
4.3 Construct circuits from Boolean expressions and derive
Boolean equations for given logic circuits
4.4 Convert relay ladder schematics to ladder logic
programs
4.5 Develop elementary programs based on logic gate
functions
4.6 Program instructions that perform logical operations
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58 Chapter 4 Fundamentals of Logic
the outcome and a symbol that represents the operation.
For the purpose of this discussion, the outcome or out-
put is called Y and the signal inputs are called A, B, C,
and so on. Also, binary 1 represents the presence of a
signal or the occurrence of some event, and binary 0
represents the absence of the signal or nonoccurrence
of the event.
The AND Function
The symbol drawn in Figure 4-3 is that of an AND gate.
An AND gate is a device with two or more inputs and
one output. The AND gate output is 1 only if all in-
puts are 1. The AND truth table in Figure 4-3 shows
the resulting output from each of the possible input
combinations.
Logic gate truth tables show each possible input to the
gate or circuit and the resultant output depending upon the
combination of the input(s).
Since logic gates are digital ICs (Integrated Circuits)
their input and output signals can be in only one of two
possible digital states, i.e., logic 0 or logic 1. Thus, the
logic state of the output of a logic gate depends on the
logic states of each of its individual inputs. Figure 4-4
4.
1 The Binary Concept
The PLC, like all digital equipment, operates on the bi-
nary principle. The term binary principle refers to the idea
that many things can be thought of as existing in only one
of two states. These states are 1 and 0. The 1 and 0 can
represent ON or OFF, open or closed, true or false, high or
low, or any other two conditions. The key to the speed and
accuracy with which binary information can be processed
is that there are only two states, each of which is distinctly
different. There is no in-between state so when informa-
tion is processed the outcome is either yes or no.
A logic gate is a circuit with several inputs but only
one output that is activated by particular combinations of
input conditions. The two-state binary concept, applied
to gates, can be the basis for making decisions. The high
beam automobile lighting circuit of Figure 4-1 is an ex-
ample of a logical AND decision. For this application,
the high beam light can be turned on only when the light
switch AND the high beam switch are closed.
The dome light automobile circuit of Figure 4-2 is an
example of a logical OR decision. For this application, the
dome light will be turned on whenever the passenger door
switch OR the driver door switch is activated.
Logic is the ability to make decisions when one or
more different factors must be taken into account before
an action is taken. This is the basis for the operation of
the PLC, where it is required for a device to operate when
certain conditions have been met.
4.2 AND, OR, and NOT Functions
The operations performed by digital equipment are
based on three fundamental logic functions: AND, OR,
and NOT. Each function has a rule that will determine
Figure 4-1 The logical AND.
High beam
light
Light switch
High beam
switch
Inputs Output
AND
Figure 4-2 The logical OR.
Dome light
Passenger
door switch
Driver
door switch
Inputs Output
AND
Figure 4-3 AND gate.
A
B Output
Y
Two-input AND
gate symbol
Inputs
0
0
1
1
0
1
0
1
0
0
0
1
A B Y
Inputs
AND truth table
Output
Figure 4-4 AND logic gate digital signal states.
Truth table
0
0
1
1
A
0
1
0
1
B
0
0
0
1
Y
1
Output (Y ) 1
1
0
Output (Y ) 0
0
1
Output (Y ) 0
0
0
Output (Y ) 0
1
B
A
B
A
B
A
B
A
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Fundamentals of Logic Chapter 4 59
Figure 4-7 illustrates the four possible combinations of
inputs for a 2-input OR gate. The basic rules that apply to
an OR gate are:
• If one or more inputs are 1, the output is 1.
• If all inputs are 0, the output will be 0.
The OR logic gate operates similarly to control devices
connected in parallel, as illustrated in Figure 4-8. The
light will be on if switch A or switch B or both are closed.
illustrates the four possible combinations of inputs for a
2-input AND gate. The basic rules that apply to an AND
gate are:
• If all inputs are 1, the output will be 1.
• If any input is 0, the output will be 0.
The AND logic gate operates similarly to control de-
vices connected in series, as illustrated in Figure 4-5. The
light will be on only when both switch A and switch B
are closed.
The OR Function
The symbol drawn in Figure 4-6 is that of an OR gate.
An OR gate can have any number of inputs but only one
output. The OR gate output is 1 if one or more inputs are
1. The truth table in Figure 4-6 shows the resulting output
Y from each possible input combination.
Figure 4-5 AND logic gate operates similarly to control devices connected in series.
Truth table
SW-
Open
Open
Closed
Closed
(0)
(0)
(1)
(1)
Open
Closed
Open
Closed
(0)
(1)
(0)
(1)
SW-A B Light
Off
Off
Off
On
(0)
(0)
(0)
(1)
Hardwired circuit
Light
SW-A SW-B
SW-A
Light
SW-B
Logic representation
Figure 4-6 OR gate.
0
0
1
1
0
1
0
1
0
1
1
1
A B Y
Inputs Output
OR truth table
B Output
Y
Inputs
Two-input OR
gate symbol
A
Figure 4-7 OR logic gate digital signal states.
A B Y
Inputs Output
Truth table
0
0
1
1
0
1
0
1
0
1
1
1
1
1
Output (Y ) = 0
Output (Y ) = 1
Output (Y ) = 1
Output (Y ) = 1
B
A
1
0
B
A
0
1
B
A
0
0
B
A
Figure 4-8 OR logic gate operates similarly to control devices connected in parallel.
Open
Open
Closed
Closed
(0)
(0)
(1)
(1)
Open
Closed
Open
Closed
(0)
(1)
(0)
(1)
Off
On
On
On
(0)
(1)
(1)
(1)
SW-A SW-B Light
Truth table
Hardwired circuit
SW-B
SW-A
Light
Logic representation
SW-A
Light
SW-B
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The NOT Function
The symbol drawn in Figure 4-9 is that of a NOT func-
tion. Unlike the AND and OR functions, the NOT func-
tion can have only one input. The NOT output is 1 if the
input is 0. The output is 0 if the input is 1. The result
of the NOT operation is always the inverse of the input,
and the NOT function is, therefore, called an inverter. The
NOT function is often depicted by using a bar across the
top of the letter, indicating an inverted output. The small
circle at the output of the inverter is termed a state indica-
tor and indicates that an inversion of the logical function
has taken place.
The logical NOT function can be performed on a con-
tact input simply by using a normally closed instead of
a normally open contact. Figure 4-10 shows an example
Figure 4-9 NOT function.
Output
Input
A
0
1
NOT A
1
0
NOT truth table
A
A (NOT A)
Figure 4-10 NOT function constructed using a normally closed pushbutton.
Light
Not pressed
Pressed
On
Off
(1)
(0)
(0)
(1)
Truth table Logic representation
Hardwired circuit
Pushbutton
N.C. pushbutton

Light
Single input
Light
Figure 4-11 NOT function is most often used in
conjunction with an AND gate.
Low-pressure
indicator off (0)
(0)
(Pressure
switch closed)
(Power on)
Low-pressure
indicator on (1)
(1)
A 1
B 1
(Pressure
switch open)
(Power on)
Pressure
switch Power
Pressure
indicator
0
1
1
1
1
0
Truth table
A 1
B 0
Figure 4-12 NAND gate symbol and truth table.
Inputs Output
0
0
1
1
0
1
0
1
1
1
1
0
NAND truth table
A B Y
Output
Inputs
Two-input
NAND gate
B
Y
A
Figure 4-13 NOR gate symbol and truth table.
Inputs Output
0
0
1
1
0
1
0
1
1
0
NOR truth table
0
0
A B Y
Output
Inputs
Two-input
NOR gate
B
Y
A
of the NOT function constructed using a normally closed
pushbutton in series with a lamp. When the input pushbut-
ton is not actuated, the output lamp is ON. When the input
pushbutton is actuated, the output lamp switches OFF.
The NOT function is most often used in conjunction
with the AND or the OR gate. Figure 4-11 shows the NOT
function connected to one input of an AND gate for a low-
pressure indicator circuit. If the power is on (1) and the
pressure switch is not closed (0), the warning light will
be on (1).
The NOT symbol placed at the output of an AND gate
would invert the normal output result. An AND gate with
an inverted output is called a NAND gate. The NAND
gate symbol and truth table are shown in Figure 4-12. The
NAND function is often used in integrated circuit logic
arrays and can be used in programmable controllers to
solve complex logic.
The same rule about inverting the normal output result
applies if a NOT symbol is placed at the output of the OR
gate. The normal output is inverted, and the function is re-
ferred to as a NOR gate. The NOR gate symbol and truth
table are shown in Figure 4-13.
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only when one input or the other is HIGH, but not both.
The exclusive-OR gate is commonly used for the com-
parison of two binary numbers.
4.3 Boolean Algebra
The mathematical study of the binary number system and
logic is called Boolean algebra. The purpose of this al-
gebra is to provide a simple way of writing complicated
combinations of logic statements. There are many appli-
cations where Boolean algebra could be applied to solv-
ing PLC programming problems.
A typical Boolean instruction list (also referred to as
a statement list) is shown in Table 4-1. The instructions
The Exclusive-OR (XOR) Function
An often-used combination of gates is the exclusive-OR
(XOR) function. The XOR gate symbol and truth table are
shown in Figure 4-14. The output of this circuit is HIGH
Figure 4.14 The XOR gate symbol and truth table.
Truth table
Inputs Output
0
0
1
1
0
1
0
1
0
1
0
1
A B Y
Output
Inputs
B
Y
A
Boolean Instruction and Function Graphic Symbol
Store (STR)–Load (LD)
Begins a new rung or an additional branch in a rung with a normally open contact.
Store Not (STR NOT)–Load Not (LD NOT)
Begins a new rung or an additional branch in a rung with a normally closed contact.
Or (OR)
Logically ORs a normally open contact in parallel with another contact in a rung.
Or Not (OR NOT)
Logically ORs a normally closed contact in parallel with another contact in a rung.
And (AND)
Logically ANDs a normally open contact in series with another contact in a rung.
And Not (AND NOT)
Logically ANDs a normally closed contact in series with another contact in a rung.
And Store (AND STR)–And Load (AND LD)
Logically ANDs two branches of a rung in series.
Or Store (OR STR)–Or Load (OR LOAD)
Logically ORs two branches of a rung in parallel.
Out (OUT)
Reflects the status of the rung (on/off) and outputs the discrete (ON/OFF) state to
the specified image register point or memory location.
OUT
Or Out (OR OUT)
Reflects the status of the rung and outputs the discrete (ON/OFF) state to the
image register. Multiple OR OUT instructions referencing the same discrete point
can be used in the program.
OROUT
Output Not (OUT NOT)
Reflects the status of the rung and turns the output OFF for an ON execution
condition; turns the output ON for an OFF execution condition.
Table 4-1 Typical Boolean Instruction or Statement List
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NOT functions. Inputs are represented by capital letters
A, B, C, and so on, and the output by a capital Y. The
multiplication sign (3) or dot (•) represents the AND
operation, an addition sign (1) represents the OR op-
eration, the circle with an addition sign ! represents
the exclusive-OR operation, and a bar over the letter
__
A
represents the NOT operation. The Boolean equations
are used to express the mathematical function of the
logic gate.
PLC digital systems may be designed using Boolean
algebra. Circuit functions are represented by Boolean
equations. Figure 4-16 illustrates how logic operators
AND, NAND, OR, NOR, and NOT are used singly to
form logical statements. Figure 4-17 illustrates how basic
logic operators are used in combination to form Boolean
equations.
An understanding of the technique of writing simpli-
fied Boolean equations for complex logical statements
is a useful tool when creating PLC control programs.
Some laws of Boolean algebra are different from those
of ordinary algebra. These three basic laws illustrate
the close comparison between Boolean algebra and
Figure 4-15 Boolean algebra as related to AND, OR, and NOT functions.
Logic symbol Logic statement Boolean equation Boolean notations
A
B
A
B
A Y
Y
Y Y A B
Y A
Y A • B
or
Y AB
Y is 1 if
A and B are 1
Y is 1 if
A or B is 1
Y is 1 if A is 0
Y is 0 if A is 1
Symbol
•

Meaning
and
or
not
invert
result in
AB
AB
Y AB C
A B
Y (A B) C
A B
Y (A B) C
A
B
C
AND
AND
AND
NAND
OR
OR
NOR
Y AB C
A
B
C
OR
A
B
C
A
B
C
Figure 4-17 Logic operators used in combination to form Boolean equations.
are based on the basic Boolean operators of AND, OR,
and NOT. Although these instructions are programmed
in list format similar to BASIC and other text lan-
guages, they implement the same logic as relay ladder
logic.
Figure 4-15 summarizes the basic operators of Bool-
ean algebra as they relate to the basic AND, OR, and
A
B
A
B
A
B
A
B
AB
AB
A B
A + B
NOT A
A
AND
NAND
OR
NOR
Figure 4-16 Logic operators used singly to form logical
statements.
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Figure 4-19 shows a logic gate circuit developed from
the Boolean expression Y 5 A(BC 1 D). The procedure
is as follows:
Boolean expression: Y 5 A(BC 1 D)
Gates required: (by inspection)
1 - AND gate with input B and C
1 - OR gate with inputs B, C, and D
1 - AND gate with inputs A and the output from
the OR gate
4.5 Producing the Boolean Equation
for a Given Logic Gate Circuit
A simple logic gate is quite straightforward in its opera-
tion. However, by grouping these gates into combinations,
it becomes more difficult to determine which combina-
tions of inputs will produce an output. The Boolean equa-
tion for the logic circuit of Figure 4-20 is determined as
follows:
• The output of the OR gate is A 1 B
• The output of the inverter is D
–
• Based on the input combination applied to the AND
gate the Boolean equation for the circuit is Y 5 C
D
–
(A 1B)
4.4 Developing Logic Gate Circuits
from Boolean Expressions
As logic gate circuits become more complex, the need to
express these circuits in Boolean form becomes greater.
Figure 4-18 shows a logic gate circuit developed from the
Boolean expression Y 5 AB 1 C. The procedure is as
follows:
Boolean expression: Y 5 AB 1 C
Gates required: (by inspection)
1 - AND gate with input A and B
1 - OR gate with input C and output from previous
AND gate
A
B
C
Inputs
Output
Y
Figure 4-18 Logic gate circuit developed from the Boolean
expression Y 5 AB 1 C.
B
C
D
Output
Y
A
Inputs
Figure 4-19 Logic gate circuit developed from the Boolean
expression Y 5 A(BC 1 D).
COMMUTATIVE LAW
A 1 B 5 B 1 A
A ? B 5 B ? A
ASSOCIATIVE LAW
(A 1 B) 1 C 5 A 1 (B 1 C)
(A ? B) ? C 5 A ? (B ? C)
DISTRIBUTIVE LAW
A ? (B 1 C) 5 (A ? B) 1 (A ? C)
A 1 (B ? C) 5 (A 1 B) ? (A 1 C)
This law holds true only in
Boolean algebra.
ordinary algebra, as well as one major difference be-
tween the two:
Output
Inputs
C
D
Y
A
B
C
D
A B
Figure 4-20 Determining the Boolean equation for a logic
circuit.
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called rails or legs, connecting the two power lines with
what look like rungs of a ladder—thus the name, relay
ladder schematic.
Hardwired logic is fixed; it is changeable only by al-
tering the way devices are electrically interconnected.
In contrast, programmable control is based on the basic
logic functions, which are programmable and easily
changed. These functions (AND, OR, NOT) are used ei-
ther singly or in combinations to form instructions that
will determine if a device is to be switched on or off.
The form in which these instructions are implemented to
convey commands to the PLC is called the language. The
most common PLC language is ladder logic. Figure 4-23
shows a typical ladder logic program for the motor start/
stop circuit. The instructions used are the relay equiva-
lent of normally open (NO) and normally closed (NC)
contacts and coils.
PLC contact symbolism is a simple way of expressing
the control logic in terms of symbols. These symbols are
The Boolean equation for the logic circuit of Fig-
ure 4-21 is determined as follows:
• The output of AND gate 1 is A
–
B
• The output of AND gate 2 is AB
–
• Based on the combination of inputs applied to the
OR gate the Boolean equation for the circuit is
Y 5 A
–
B 1 AB
–
4.6 Hardwired Logic versus
Programmed Logic
The term hardwired logic refers to logic control functions
that are determined by the way devices are electrically in-
terconnected. Hardwired logic can be implemented using
relays and relay ladder schematics. Relay ladder sche-
matics are universally used and understood in industry.
Figure 4-22 shows a typical relay ladder schematic of a
motor stop/start control station with pilot lights. The con-
trol scheme is drawn between two vertical supply lines.
All the components are placed between these two lines,
Inputs
Output
B
Y
A
AB
AB
B
A
1
2
Figure 4-21 Determining the Boolean equation for a logic
circuit.
Figure 4-22 Motor stop/start relay ladder schematic.
STOP
START
M
M
M
L2
L1
Rung
Rail
OL
R
G
M
Input
module
Output
module L2
STOP
STOP
START
PROGRAM
START
Relay contact
equivalent
Rung 1
Rung 2
Rung 3
Relay coil
equivalent
OL
OL
M
M
M
M
G
R
L1
R
G
M
Figure 4-23 Motor stop/start ladder logic program.
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Because the PLC uses ladder logic diagrams, the con-
version from any existing relay logic to programmed
logic is simplified. Each rung is a combination of input
conditions (symbols) connected from left to right, with
the symbol that represents the output at the far right. The
symbols that represent the inputs are connected in series,
parallel, or some combination of the two to obtain the de-
sired logic. The following group of examples illustrates
the relationship between the relay ladder schematic, the
ladder logic program, and the equivalent logic gate circuit.
basically the same as those used for representing hard-
wired relay control circuits. A rung is the contact sym-
bolism required to control an output. Some PLCs allow
a rung to have multiple outputs while others allow only
one output per rung. A complete ladder logic program
then consists of several rungs, each of which controls an
output. In programmed logic all mechanical switch con-
tacts are represented by a software contact symbol and
all electromagnetic coils are represented by a software
coil symbol.
SOL
LS1 LS2 B Y
A
Relay schematic Ladder logic program
Inputs
Output
A
B
Y
Gate logic
Boolean equation: AB Y
Example 4-1 Two limit switches connected in series and used to control a solenoid valve.
SOL
LS1
Boolean equation: A B Y
A Y
Gate logic
LS2 B
Inputs
Output
A
Y
B
Example 4-2 Two limit switches connected in parallel and used to control a solenoid valve.
PS
LS1
Boolean equation: (A B)C Y
PL
A C
Relay schematic Ladder logic program Gate logic
LS2 B
G
Y
Inputs
Output
A B
A
C
Y
B
Example 4-3 Two limit switches connected in parallel with each other and in series with a pressure switch.
FS2
LS1
Boolean equation: (A B) (C D) Y
PL
FS1
Relay schematic Gate logic
LS2
R
Inputs
Output
C D
A B
A
B
C
D
Y
A
B
C
D
Y
Ladder logic program
Example 4-4 Two limit switches connected in parallel with each other and in series with two sets of ﬂow
switches (that are connected in parallel with each other), and used to control a pilot light.
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Boolean equation: (AB) C Y
H
Gate logic
LS1 LS2
LS3
A B
C
Y
Inputs
Y
Output
AB
A
C
B
Example 4-5 Two limit switches connected in series with each other and in parallel with a third limit
switch, and used to control a warning horn.
Boolean equation: (AB) (CD) Y
PL
Relay schematic
Gate logic
LS1
LS3
LS2
LS4
R
A
C
B
D
Y
Inputs
Output
A
B
Y
C
D
Example 4-6 Two limit switches connected in series with each other and in parallel with two other limit
switches (that are connected in series with each other), and used to control a pilot light.
PB SOL
Boolean equation: AB Y
LS1 A B Y
Inputs
Output
A
B
B
Y
Example 4-7 One limit switch connected in series with a normally closed pushbutton and used to control
a solenoid valve. This circuit is programmed so that the output solenoid will be turned on when the limit switch
is closed and the pushbutton is not pushed.
A B
A
B
A B
Boolean equation: AB AB Y
A B Y
Y
Y
Inputs
Output
A
B
Y
Example 4-8 Exclusive-OR circuit. The output lamp of this circuit is ON only when pushbutton A or B is pressed, but not both.
This circuit has been programmed using only the normally open A and B pushbutton contacts as the inputs to the program.
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Relay schematic
Stops Starts
Ladder logic program Gate logic
C D A
B
C D A
B
M
M
Stops
A
B
M
Starts
C
D
M
M
Example 4-9 A motor control circuit with two start/stop buttons. When either start button is depressed, the motor runs.
By use of a seal-in contact, it continues to run when the start button is released. Either stop button stops the motor when it is
depressed.
Figure 4-24 Word-level AND instruction.
Source A
B3:5
Source B
B3:7
Destination
B3:10
1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0
1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0
Input A
1100110000000000
Destination
1111111100000000
Source B
1100110011001100
BITWISE AND
Source A
AND
B3:10
B3:7
B3:5
Figure 4-25 Word-level OR instruction.
Source A
B3:1
OR
BITWISE INCLUS OR
Source A B3:1
Source B
Destination
Source B
B3:2
Destination
B3:20
1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0
1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 0 0 1 1 0 0
B3:2
B3:20
4.7 Programming Word Level Logic
Instructions
Most PLCs provide word-level logic instructions as part
of their instruction set. Table 4-2 shows how to select the
correct word logic instruction for different situations.
Figure 4-24 illustrates the operation of the AND in-
struction to perform a word-level AND operation using
the bits in the two source addresses. This instruction tells
the processor to perform an AND operation on B3:5 and
B3:7 and to store the result in destination B3:10 when
input device A is true. The destination bits are a result of
the logical AND operation.
Figure 4-25 illustrates the operation of a word-level
OR instruction, which ORs the data in Source A, bit by
bit, with the data in Source B and stores the result at the
destination address. The address of Source A is B3:1,
the address of Source B is B3:2, and the destination
address is B3:20. The instruction may be programmed
conditionally, with input instruction(s) preceding it, or
unconditionally, as shown, without any input instruc-
tions preceding it.
XOR instruction. In this example, data from input I:1.0
are compared, bit by bit, with data from input I:3.0. Any
mismatches energize the corresponding bit in word O:4.0.
As you can see, there is a 1 in every bit location in the
destination corresponding to the bit locations where
Source A and Source B are different, and a 0 in the des-
tination where Source A and Source B are the same. The
XOR is often used in diagnostics, where real-world in-
puts, such as rotary cam limit switches, are compared
with their desired states.
If you want to . . .
. . . use this
instruction.
Know when matching bits in two different
words are both ON
AND
Know when one or both matching bits in two
different words are ON
OR
Know when one or the other bit of matching
bits in two different words is ON
XOR
Reverse the state of bits in a word NOT
Table 4-2 Selecting Logic Instructions
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NOT instruction. This instruction inverts the bits from the
source word to the destination word. The bit pattern in
B3:10 is the result of the instruction being true and is the
inverse of the bit pattern in B3:9.
For 32-bit PLCs, such as the Allen-Bradley Control-
Logix controller, the source and destination may be a
SINT (one-byte integer), INT (two-byte integer), DINT
(four-byte integer), or REAL (four-byte floating decimal
point) value.
Figure 4-27 Word-level NOT operation.
NOT
NOT
Source
Destination B3:10
Source
B3:9
Destination
B3:10
Input A
0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0
1 1 1 1 1 1 1 1 0 1 0 1 0 1 0 1
B3:9
Source A
Ι:1.0
Rotating cam limit switch
XOR
Ι:1.0
O:4.0
Ι:3.0
Destination
Source B
Source A
BITWISE EXCLUS OR
Source B
Ι:3.0
Destination
O:4.0
0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0
0 0 0 0 0 0 0 0 1 1 1 0 1 0 1 1
0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1
Figure 4-26 Word-level XOR instruction.
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1. Explain the binary principle.
2. What is a logic gate?
3. Draw the logic symbol, construct a truth table, and
state the Boolean equation for each of the
following:
a. Two-input AND gate
b. NOT function
c. Three-input OR gate
d. XOR function
4. Express each of the following equations as a ladder
logic program:
a. Y 5 (A 1 B)CD
b. Y 5 A
__
BC 1
__
D 1 E
c. Y 5 [(
__
A 1
__
B)C] 1 DE
d. Y 5 (
__
AB
__
C) 1 (D
__
EF)
5. Write the ladder logic program, draw the logic gate
circuit, and state the Boolean equation for the two
relay ladder diagrams in Figure 4-28.
6. Develop a logic gate circuit for each of the follow-
ing Boolean expressions using AND, OR, and NOT
gates:
a. Y 5 ABC 1 D
b. Y 5 AB 1 CD
c. Y 5 (A 1 B)(
__
C 1 D)
d. Y 5
__
A(B 1 CD)
e. Y 5
__
AB 1 C
f. Y 5(ABC 1 D)(E
__
F)
7. State the logic instruction you would use when you
want to:
a. Know when one or both matching bits in two dif-
ferent words are 1.
b. Reverse the state of bits in a word.
c. Know when matching bits in two different words
are both 1.
d. Know when one or the other bit of matching bits,
but not both, in two different words is 1.
Figure 4-28 Question 5 relay ladder diagrams.
(a) (b)
LS1 LS2
LS3
R
PB1 PB2
H
LS1
PS1
1. It is required to have a pilot light come on when all
of the following circuit requirements are met:
• All four circuit pressure switches must be
closed.
• At least two out of three circuit limit switches
must be closed.
• The reset switch must not be closed.
Using AND, OR, and NOT gates, design a logic
circuit that will solve this hypothetical problem.
2. Write the Boolean equation for each of the logic gate
circuits in Figure 4-29a–f.
3. The logic circuit of Figure 4-30 is used to acti-
vate an alarm when its output Y is logic HIGH or
1. Draw a truth table for the circuit showing the
resulting output for all 16 of the possible input
conditions.
CHAPTER 4 PROBLEMS
Figure 4-29 Logic gate circuits for Problem 2.
(a)
A
C
D
Y
B
(b)
A
C Y
B
D
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(c)
A
C
Y
B
A
C
E
(d)
B
D Y
(e)
A
Y
B
C
D
(f )
A
C
E
Y
B
D
F
Figure 4-29 (Continued)
4. What will be the data stored in the destination ad-
dress of Figure 4-31 for each of the following logical
operations?
a. AND operation
b. OR operation
c. XOR operation
5. Write the Boolean expression and draw the gate
logic diagram and typical PLC ladder logic diagram
for a control system wherein a fan is to run only
when all of the following conditions are met:
• Input A is OFF
• Input B is ON or input C is ON, or both B and C
are ON
• Inputs D and E are both ON
• One or more of inputs F, G, or H are ON
Figure 4-30 Logic circuit for Problem 3.
Alarm
A
Y
C
D
B
Figure 4-31 Data for Problem 4.
Source A
Source B
Destination
0 0
0 0 0 0 0 0 0 1 1 0 1 0 1
0
0 1
0 0 0 0 0 0 0 1 1 1 0 1 0 1
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71
Each input and output PLC module terminal is
identiﬁed by a unique address. In PLCs, the in-
ternal symbol for any input is a contact. Similarly,
in most cases, the internal PLC symbol for all
outputs is a coil. This chapter shows how these
contact/coil functions are used to program a
PLC for circuit operation. This chapter covers
only the basic set of instructions that perform
functions similar to relay functions. You will also
learn more about the program scan cycle and
the scan time of a PLC.
Chapter Objectives
5.1 Define and identify the functions of a PLC memory
map
5.2 Describe input and output image table files and types
of data files
5.3 Describe the PLC program scan sequence
5.4 Understand how ladder diagram language, Boolean
language, and function chart programming language
are used to communicate information to the PLC
5.5 Define and identify the function of internal relay
instructions
5.6 Identify the common operating modes found in PLCs
5.7 Write and enter ladder logic programs
5
Basics of PLC Programming
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72 Chapter 5 Basics of PLC Programming
5.
1 Processor Memory Organization
While the fundamental concepts of PLC programming are
common to all manufacturers, differences in memory or-
ganization, I/O addressing, and instruction set mean that
PLC programs are never perfectly interchangeable among
different makers. Even within the same product line of a
single manufacturer, different models may not be directly
compatible.
The memory map or structure for a PLC processor con-
sists of several areas, some of these having specific roles.
Allen-Bradley PLCs have two different memory struc-
tures identified by the terms rack-based systems and tag-
based systems. The memory organization for rack-based
systems will be covered in this chapter and that for tag-
based systems in a later chapter.
Memory organization takes into account the way a
PLC divides the available memory into different sections.
The memory space can be divided into two broad catego-
ries: program files and data files. Individual sections, their
order, and the sections’ length will vary and may be fixed
or variable, depending on the manufacturer and model.
Program files are the part of the processor memory
that stores the user ladder logic program. The program
accounts for most of the total memory of a given PLC
system. It contains the ladder logic that controls the ma-
chine operation. This logic consists of instructions that
are programmed in a ladder logic format. Most instruc-
tions require one word of memory.
The data files store the information needed to carry out
the user program. This includes information such as the
status of input and output devices, timer and counter val-
ues, data storage, and so on. Contents of the data table can
be divided into two categories: status data and numbers or
codes. Status is ON/OFF type of information represented
by 1s and 0s, stored in unique bit locations. Number or
code information is represented by groups of bits that are
stored in unique byte or word locations.
The memory organizations of the rack-based Allen-
Bradley PLC-5 and SLC 500 controllers are very similar.
Figure 5-1 shows the program and data file organization
for the SLC 500 controller. The contents of each file are
as follows.
Program Files
Program files are the areas of processor memory where
ladder logic programming is stored. They may include:
• System functions (file 0)—This file is always
included and contains various system-related in-
formation and user-programmed information such
as processor type, I/O configuration, processor file
name, and password.
• Reserved (file 1)—This file is reserved by the pro-
cessor and is not accessible to the user.
• Main ladder program (file 2)—This file is always
included and contains user-programmed instructions
that define how the controller is to operate.
• Subroutine ladder program (files 3–255)—These
files are user-created and are activated according to
subroutine instructions residing in the main ladder
program file.
Data Files
The data file portion of the processor’s memory stores input
and output status, processor status, the status of various
bits, and numerical data. All this information is accessed
via the ladder logic program. These files are organized by
the type of data they contain and may include:
• Output (file 0)—This file stores the state of the out-
put terminals for the controller.
• Input (file 1)—This file stores the status of the
input terminals for the controller.
• Status (file 2)—This file stores controller operation
information and is useful for troubleshooting con-
troller and program operation.
• Bit (file 3)—This file is used for internal relay logic
storage.
• Timer (file 4)—This file stores the timer accumu-
lated and preset values and status bits.
Figure 5-1 Program and data ﬁle organization for the
SLC 500 controller.
Designated
User
defined
System functions
Reserved
User program
Subroutine programs
0
1
2
3–255
0
1
2
3
4
5
6
7
8
9
Program
files
Data
files
10–255
Output image
Input image
Status
Bit
Timer
Counter
Control
Integer
Reserved
Network
Bit
Timer
Counter
Control
Integer
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Basics of PLC Programming Chapter 5 73
processor set up for standard ladder logic, the main pro-
gram will always be in program file 2, and program files
3 through 999 will be subroutines. In either case, the pro-
cessor can store and execute only one program at a time.
Figure 5-3 shows a typical data file memory organiza-
tion for an Allen-Bradley PLC-5 controller. Each data file
is made up of numerous elements. Each element may be
one, two, or three words in length. Timer, counter, and
control elements are three words in length; floating-point
elements are two words in length; and all other elements
are a single word in length. A word consists of 16 bits, or
binary digits. The processor operates on two different data
types: integer and floating point. All data types, except the
floating-point files, are treated as integers or whole num-
bers. All element and bit addresses in the output and input
data files are numbered octally. Element and bit addresses
in all other data files are numbered decimally.
The PLC-5 and SLC 500 store all data in global data
tables and are based on 16-bit operations. You access these
data by specifying the address of the data you want. Typical
addressing formats for the PLC-5 controller are as follows:
• The addresses in the output data file and the input
data file are potential locations for either input mod-
ules or output modules mounted in the I/O chassis:
- The address O:012/15 is in the output image
table file, rack 1, I/O group 2, bit 15.
- The address I:013/17 is in the input image table
file, rack 1, I/O group 3, bit 17.
• The status data file contains information about the
processor status:
- The address S:015 addresses word 15 of the
status file.
- The address S:027/09 addresses bit 9 in word 27
of the status file.
• Counter (file 5)—This file stores the counter accu-
mulated and preset values and status bits.
• Control (file 6)—This file stores the length, pointer
position, and status bit for specific instructions such
as shift registers and sequencers.
• Integer (file 7)—This file is used to store numerical
values or bit information.
• Reserved (file 8)—This file is not accessible to the
user.
• Network communications (file 9)—This file is
used for network communications if installed or
used like files 10–255.
• User-defined (files 10–255)—These files are user-
defined as bit, timer, counter, control, and/or integer
data storage.
The I/O address format for the SLC family of PLCs is
shown in Figure 5-2. The format consists of the following
three parts:
Part 1: I for input, and a colon to separate the module
type from the slot.
O for output and a colon to separate the module type
from the slot.
Part 2: The module slot number and a forward slash
to separate the slot from the terminal screw.
Part 3: The screw terminal number.
There are about 1000 program files for an Allen-
Bradley PLC-5 controller. These program files may be set
up in two ways: either (1) standard ladder logic program-
ming, with the main program in program file 2 and program
files 3 through 999 assigned, as needed, to subroutines; or
(2) in sequential function charts in which files 2 through
999 are assigned steps or transitions, as required. With the
Figure 5-2 I/O address format for the SLC family of PLCs.
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
00
01
02
03
04
05
06
07
08
09
10
12
13
14
15
I:1 O:2
0 1 2 3
I : 1/2
O : 2/11
Separator
Input or
output
Slot
number
Bit
number
Bit
designator
11
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• The bit data file stores bit status. It frequently serves
for storage when using internal outputs, sequencers,
bit-shift instructions, and logical instructions:
- The address B3:400 addresses word 400 of the
bit file. The file number (3) must be included as
part of the address. Note that the input, output,
and status data files are the only files that do not
require the file number designator because there
can only be one input data, one output data, and
one status data file.
- Word 2, bit 15 is addressed as B3/47 because bit
numbers are always measured from the beginning
of the file. Remember that here, bits are num-
bered decimally (not octally, as the word repre-
senting the rack and slot).
• The timer file stores the timer status and timer data.
A timer element consists of three words: the control
word, preset word, and accumulated word. The ad-
dressing of the timer control word is the assigned
timer number. Timers in file 4 are numbered starting
with T4:0 and running through T4:999. The addresses
for the three timer words in timer T4:0 are:
The enable-bit address in the control word is T4:0/
EN, the timer-timing-bit address is T4:0/TT, and the
done-bit address is T4:0/DN.
• The counter file stores the counter status and coun-
ter data. A counter element consists of three words:
the control word, preset word, and accumulated
word. The addressing of the counter control is the
assigned counter number. Counters in file 5 are
numbered beginning with C5:0 and running through
C5:999. The addresses for the three counter words
in counter C5:0 are:
Figure 5-3 Data ﬁle memory organization for an Allen-Bradley PLC-5 controller.
Size, in
elements
Address
range
O:000
O:037
Ι:000
Ι:037
S:000
S:031
B3:000
B3:999
T4:000
T4:999
C5:000
C5:999
R6:000
R6:999
N7:000
N7:999
F8:000
F8:999
Output image file
Input image file
Processor status
Bit file
Timer file
Counter file
Control file
Integer file
Floating-point file
Files to be assigned file nos. 9–999
32
32
32
1–1000
1–1000
1–1000
1–1000
1–1000
1–1000
1–1000
per file
Control word: T4:0
Preset word: T4:0.PRE
Accumulated word: T4:0.ACC
Control word: C5:0
Preset word: C5:0.PRE
Accumulated word: C5:0.ACC
The count-up-enable-bit address in the control word
is C5:0/CU, the count-down-enable-bit address
is C5:0/CD, the done-bit address is C5:0/DN, the
overflow address is C5:0/OV, and the underflow ad-
dress is C5:0/UN.
• The control file stores the control element’s sta-
tus and data, and it is used to control various file
instructions. The control element consists of three
words: the control word, length word, and position
word. The addressing of the control’s control word
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is the assigned control number. Control elements in
control file 6 are numbered beginning with R6:0 and
running through R6:999. The addresses for the three
words in control element R6:0 are:
through the input module. Its operation can be summa-
rized as follows.
• For the switch that is closed, the processor detects a
voltage at the input terminal and records that infor-
mation by storing a binary 1 in its bit location.
• For the switch that is open, the processor detects no
voltage at the input terminal and records that infor-
mation by storing a binary 0 in its bit location.
• Each connected input has a bit in the input image
table file that corresponds exactly to the terminal to
which the input is connected.
• The input image table file is changed to reflect the
current status of the switch during the I/O scan
phase of operation.
• If the input is on (switch closed), its corresponding
bit in the table is set to 1.
• If the input is off (switch open), the corresponding
bit is cleared, or reset to 0.
• The processor continually reads the current input
status and updates the input image table file.
The output image table file is that part of the program
memory allocated to storing the actual on/off status of
connected discrete outputs. Figure 5-5 shows a typical
Control word: R6:0
Length: R6:0.LEN
Position: R6:0.POS
There are numerous control bits in the control word,
and their function depends on the instruction in
which the control element is used.
• The integer file stores integer data values, with a
range from 232,768 through 32,767. Stored values
are displayed in decimal form. The integer element
is a single-word (16-bit) element. As many as 1000
integer elements, addressed from N7:000 through
N7:999, can be stored.
- The address N7:100 addresses word 100 of the
integer file.
- Bit addressing is decimal, from 0 through 15.
For example, bit 12 in word 15 is addressed
N7:015/12.
• The floating-point file element can store val-
ues in the range from 61.1754944e-38 to
63.4028237e138. The floating-point element is a
two-word (32-bit) element. As many as 1000 ele-
ments, addressed from F8:000 through F8:999, can
be stored. Individual words or bits cannot be ad-
dressed in the floating-point file.
• Data files 9 through 999 may be assigned to dif-
ferent data types, as required. When assigned to a
certain type, a file is then reserved for that type and
cannot be used for any other type. Additional input,
output, or status files cannot be created.
The bit file, integer file, or floating-point file can be
used to store status or data. Which of these you use de-
pends on the intended use of the data. If you are deal-
ing with status rather than data, the bit file is preferable.
If you are using very large or very small numbers and
require a decimal point, the floating-point file is prefer-
able. The floating-point data type may have a restriction,
however, because it may not interface well with external
devices or with internal instructions such as counters and
timers, which use only 16-bit words. In such a situation, it
may be necessary to use the integer file type.
The input image table file is that part of the program
memory allocated to storing the on/off status of con-
nected discrete inputs. Figure 5-4 shows the connection
of an open and closed switch to the input image table file
Figure 5-4 Connection of an open and closed switch to the
input image table ﬁle through the input module.
Input image
Word corresponding
to input module
ON
(closed)
(open)
Input module
OFF
L1
1 0
Data table
files
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connection of two pilot lights to the output image table
file through the output module. Its operation can be sum-
marized as follows.
• The status of each light (ON/OFF) is controlled by
the user program and is indicated by the presence of
1 (ON) and 0 (OFF).
• Each connected output has a bit in the output image
table file that corresponds exactly to the terminal to
which the output is connected.
• If the program calls for a specific output to be ON,
its corresponding bit in the table is set to 1.
• If the program calls for the output to be OFF, its
corresponding bit in the table is set to 0.
• The processor continually activates or deactivates the
output status according to the output table file status.
Typically, micro PLCs have a fixed number of inputs
and outputs. Figure 5-6 shows the MicroLogix control-
ler from the Allen-Bradley MicroLogix 1000 family of
controllers. The controller has 20 discrete inputs with
predefined addresses I/0 through I/19 and 12 discrete
outputs with predefined addresses O/1 through O/11.
Some units also contain analog inputs and outputs em-
bedded into the base unit or available through add-on
modules.
5.2 Program Scan
When a PLC executes a program, it must know—in real
time—when external devices controlling a process are
changing. During each operating cycle, the processor
reads all the inputs, takes these values, and energizes or
de-energizes the outputs according to the user program.
This process is known as a program scan cycle. Figure 5-7
illustrates a single PLC operating cycle consisting of the
input scan, program scan, output scan, and housekeep-
ing duties. Because the inputs can change at any time, it
constantly repeats this cycle as long as the PLC is in the
RUN mode.
Figure 5-5 Connections of pilot lights to the output image
table ﬁle through the output module.
Output module
Output image
Word corresponding
to output module
OFF
L2
1
0
Data table
files
ON
Figure 5-6 Typical micro PLC with predeﬁned addresses.
AC
COM
I/0 AC
COM
I/4 I/5 I/6 I/7 I/8 I/9 I/10 I/11 I/12 I/13 I/14 I/15 I/16 I/17 I/18 I/19
I/1 I/2 I/3
VAC
VDC
VAC
VDC
VAC
VDC
VAC
VDC
O/4 O/2 O/3 O/4 O/5 O/6 O/7 O/8 O/9 O/10 O/11
VAC
VDC
O/0
L2
L2
L1
L1
VAC 2 VDC 1 VDC 2
VAC 2
COM
VDC 1
COM
VDC 2
COM
VDC 3
COM
VDC 3
L2 Discrete Inputs
Discrete Outputs
L1
CR CR CR CR CR
CR CR CR CR CR
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The time it takes to complete a scan cycle is called the
scan cycle time and indicates how fast the controller can
react to changes in inputs. The time required to make a
single scan can vary from about 1 millisecond to 20 mil-
liseconds. If a controller has to react to an input signal that
changes states twice during the scan time, it is possible
that the PLC will never be able to detect this change. For
example, if it takes 8 ms for the CPU to scan a program,
and an input contact is opening and closing every 4 ms,
the program may not respond to the contact changing
state. The CPU will detect a change if it occurs during the
update of the input image table file, but the CPU will not
respond to every change. The scan time is a function of
the following:
• The speed of the processor module
• The length of the ladder program
• The type of instructions executed
• The actual ladder true/false conditions
The actual scan time is calculated and stored in the
PLC’s memory. The PLC computes the scan time each
time the END instruction is executed. Scan time data can
be monitored via the PLC programming. Typical scan
time data include the maximum scan time and the last
scan time.
The scan is normally a continuous and sequential pro-
cess of reading the status of inputs, evaluating the control
logic, and updating the outputs. Figure 5-8 shows an over-
view of the data flow during the scan process. For each
rung executed, the PLC processor will:
• Examine the status of the input image table bits.
• Solve the ladder logic in order to determine logical
continuity.
• Update the appropriate output image table bits, if
necessary.
• Copy the output image table status to all of the out-
put terminals. Power is applied to the output device
if the output image table bit has been previously set
to a 1.
• Copy the status of all of the input terminals to the
input image table. If an input is active (i.e., there is
electrical continuity), the corresponding bit in the
input image table will be set to a 1.
Figure 5-9 illustrates the scan process applied to a sim-
ple single rung program. The operation of the scan pro-
cess can be summarized as follows:
• If the input device connected to address I:3/6 is
closed, the input module circuitry senses electrical
continuity and a 1 (ON) condition is entered into the
input image table bit I:3/6.
Figure 5-7 PLC program scan cycle.
Internal checks
on memory, speed
and operation.
Service any
communication
requests.
The output image
date is transferred
to the external output
circuits, turning the
output device
ON or OFF.
Each ladder rung
is scanned and solved
using the date in the
input file. The resulting
logic is written to the
output image table
(file or register).
The status of
external inputs
is written to the
input image table
(file or register).
HOUSE-
KEEPING
START INPUT
SCAN
PROGRAM
SCAN
OUTPUT
SCAN
Figure 5-8 Overview of the data ﬂow during the scan
process.
Input
modules
Input
data
Output
data
Input
image
table
file
Output
image
table
file
Return
result
Take some action
Examine
data
Check/compare/examine
specific conditions
Output
modules
Program
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• During the program scan, the processor examines
bit I:3/6 for a 1 (ON) condition.
• In this case, because input I:3/6 is 1, the rung is said
to be TRUE or have logic continuity.
• The processor then sets the output image table bit
O:4/7 to 1.
• The processor turns on output O:4/7 during the next
I/O scan, and the output device (light) wired to this
terminal becomes energized.
• This process is repeated as long as the processor is
in the RUN mode.
• If the input device opens, electrical continuity is
lost, and a 0 would be placed in the input image
table. As a result, the rung is said to be FALSE due
to loss of logic continuity.
• The processor would then set the output image table
bit O:4/7 to 0, causing the output device to turn off.
Ladder programs process inputs at the beginning of a
scan and outputs at the end of a scan, as illustrated in Fig-
ure 5-10. For each rung executed, the PLC processor will:
Step 1 Update the input image table by sensing the
voltage of the input terminals. Based on the
absence or presence of a voltage, a 0 or a 1 is
stored into the memory bit location designated
for a particular input terminal.
Step 2 Solve the ladder logic in order to determine
logical continuity. The processor scans the lad-
der program and evaluates the logical continu-
ity of each rung by referring to the input image
table to see if the input conditions are met. If
the conditions controlling an output are met, the
processor immediately writes a 1 in its memory
location, indicating that the output will be
turned ON; conversely, if the conditions are not
met a 0 indicating that the device will be turned
OFF is written into its memory location.
Step 3 The final step of the scan process is to update
the actual states of the output devices by trans-
ferring the output table results to the output
module, thereby switching the connected out-
put devices ON (1) or OFF (0). If the status of
any input devices changes when the processor
is in step 2 or 3, the output condition will not
react to them until the next processor scan.
Each instruction entered into a program requires a cer-
tain amount of time for the instruction to be executed. The
amount of time required depends on the instruction. For
example, it takes less time for a processor to read the sta-
tus of an input contact than it does to read the accumu-
lated value of a timer or counter. The time taken to scan
Figure 5-9 Scan process applied to a single rung program.
Input
module
Input
device
Output
device
Input
image
table
file
Output
image
table
file
Output
module
Program
Data
Processor memory
O:4/7 O:4/7
Ι:3/6 O:4/7
Ι:3/6 Ι:3/6
Field-device
power supply
Field-device
power supply
Figure 5-10 Scan process applied to a multiple rung
program.
START
Input image table
0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 0
Output image table
END
0 0 0 0 0 0 0 0 1 0 0
0 0 0 1 0
Step 3
Transfer
to output
module
Step 1
Read
input
module
Step 2
Solve the
ladder program
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the user program is also dependent on the clock frequency
of the microprocessor system. The higher the clock fre-
quency, the faster is the scan rate.
There are two basic scan patterns that different PLC
manufacturers use to accomplish the scan function
(Figure 5-11). Allen-Bradley PLCs use the horizontal
scan by rung method. In this system, the processor exam-
ines input and output instructions from the first com-
mand, top left in the program, horizontally, rung by rung.
Modicon PLCs use the vertical scan by column method.
In this system, the processor examines input and output
instructions from the top left command entered in the lad-
der diagram, vertically, column by column and page by
page. Pages are executed in sequence. Both methods are
appropriate; however, misunderstanding the way the PLC
scans a program can cause programming bugs.
5.3 PLC Programming Languages
The term PLC programming language refers to the
method by which the user communicates information to
the PLC. The standard IEC 61131 (Figure 5-12) was es-
tablished to standardize the multiple languages associated
with PLC programming by defining the following five
standard languages:
• Ladder Diagram (LD)—a graphical depiction of a
process with rungs of logic, similar to the relay lad-
der logic schemes that were replaced by PLCs.
• Function Block Diagram (FBD)—a graphical de-
piction of process flow using simple and complex
interconnecting blocks.
• Sequential Function Chart (SFC)—a graphical
depiction of interconnecting steps, actions, and
transitions.
• Instruction List (IL)—a low-level, text-based
language that uses mnemonic instructions.
• Structured Text (ST)—a high-level, text-based lan-
guage such as BASIC, C, or PASCAL specifically
developed for industrial control applications.
Ladder diagram language is the most commonly used
PLC language and is designed to mimic relay logic. The
ladder diagram is popular for those who prefer to de-
fine control actions in terms of relay contacts and coils,
and other functions as block instructions. Figure 5-13
shows a comparison of ladder diagram programming
and instruction list programming. Figure 5-13a shows
Figure 5-11 Scanning can be vertical or horizontal.
Horizontal scanning order
End of ladder
Vertical
scanning
order Return
for next
scan
Figure 5-12 Standard IEC 61131 languages associated with PLC programming.
PLC programming languages
Textural language
Instruction
list
Structured
text
Sequential
function chart
Functional
block diagram
Ladder
diagram
Graphical language
Figure 5-13 Comparison of ladder diagram and instruction list programming.
PB1 CR CR SOL
LS1
1 2
(a) Hardwired relay control circuit
SOL
A B D Y
(LS1)
SOL
PB1
CR1
CR2
START
AND
OR
AND NOT
OUT
LS1
(CR2)
(CR1)
(PB1)
(b) Equivalent ladder diagram (LD) program (c) Equivalent instruction
list (IL) program
C
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the original relay hardwired control circuit. Figure 5-13b
shows the equivalent logic ladder diagram programmed
into a controller. Note how closely the ladder diagram
program closely resembles the hardwired relay circuit.
The input/output addressing is generally different for
each PLC manufacturer. Figure 5-13c show how the
original hardwired circuit could be programmed using
the instruction list programming language. Note that
the instructional list consists of a series of instructions
that refer to the basic AND, OR, and NOT logic gate
functions.
Functional block diagram programming uses instruc-
tions that are programmed as blocks wired together on
screen to accomplish certain functions. Typical types of
function blocks include logic, timers, and counters. Func-
tional block diagrams are similar in layout to electrical/
electronic block diagrams used to simplify complex sys-
tems by showing blocks of functionality. The primary
concept behind a functional block diagram is data flow.
Function blocks are linked together to complete a circuit
that satisfies a control requirement. Data flow on a path
from inputs, through function blocks or instructions, and
then to outputs.
The use of function blocks for programming of pro-
grammable logic controllers (PLCs) is gaining wider
acceptance. Rather than the classic contact and coil repre-
sentation of ladder diagram or relay ladder logic program-
ming, function blocks present a graphical image to the
programmer with underlying algorithms already defined.
The programmer simply completes needed information
within the block to complete that phase of the program.
Figure 5-14 shows function block diagram equivalents to
ladder logic contacts.
Figure 5-15 illustrates how ladder diagram and func-
tional block diagram programming could be used to pro-
duce the same logical output. For this application, the
objective is to turn on caution pilot light PL 1 whenever
both sensor switch 1 and sensor switch 2 are closed. The
ladder logic consists of a single rung across the power
rails. This rung contains the two input sensor instructions
programmed in series with the pilot light output instruc-
tion. The function block solution consists of a logic Bool-
ean And function block with two input references tags for
the sensors and a single output reference tag for the pilot
light. Note there are no power rails in the function block
diagram.
Sequential function chart programming language is
similar to a flowchart of your process. SFC programming
is designed to accommodate the programming of more
advanced processes. This type of program can be split
into steps with multiple operations happening in paral-
lel branches. The basic elements of a sequential function
chart program are shown in Figure 5-16.
Structured text is a high level text language primarily
used to implement complex procedures that cannot be
easily expressed with graphical languages. Structured text
uses statements to define what to execute. Figure 5-17 il-
lustrates how structured text and ladder diagram program-
ming could be used to produce the same logical output.
For this application, the objective is to energize SOL 1
whenever either one of the two following circuit condi-
tions exists:
• Sensor 1 and Sensor 2 switches are both closed.
• Sensor 3 and Sensor 4 switches are both closed and
Sensor 5 switch is open.
Figure 5-14 Function block diagram equivalents to ladder
logic contacts.
AND_BOOL
Functional block
diagram equivalent
Ladder logic
A
A B
B
AND_BOOL
A
B
OR_BOOL
A
B
B
A
A
B
Figure 5-15 PLC ladder and equivalent function block
diagram.
Caution
Sensor 1 Sensor 2
Caution
PL 1
Ladder diagram
Function block diagram
BAND_01
BAND
0
Boolean And
Out PL 1
0
Sensor 1 In1
0
Sensor 2 In2
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5.4 Relay-Type Instructions
The ladder diagram language is basically a symbolic set of
instructions used to create the controller program. These
ladder instruction symbols are arranged to obtain the de-
sired control logic that is to be entered into the memory
of the PLC. Because the instruction set is composed of
contact symbols, ladder diagram language is also referred
to as contact symbology.
Representations of contacts and coils are the basic sym-
bols of the logic ladder diagram instruction set. The three
fundamental symbols that are used to translate relay control
logictocontactsymboliclogicareExamineIfClosed(XIC),
Examine If Open (XIO), and Output Energize (OTE). Each
of these instructions relates to a single bit of PLC memory
that is specified by the instruction’s address.
The symbol for the Examine If Closed (XIC) instruc-
tion is shown in Figure 5-18. The XIC instruction, which
is also called the Examine-on instruction, looks and oper-
ates like a normally open relay contact. Associated with
each XIC instruction is a memory bit linked to the status
of an input device or an internal logical condition in a
rung. This instruction asks the PLC’s processor to exam-
ine if the contact is closed. It does this by examining the
bit at the memory location specified by the address in the
following manner:
• The memory bit is set to 1 or 0 depending on the
status of the input (physical) device or internal
(logical) relay address associated with that bit.
• A 1 corresponds to a true status or on condition.
• A 0 corresponds to a false status or off condition.
• When the Examine-on instruction is associated
with a physical input, the instruction will be set to 1
when a physical input is present (voltage is applied
to the input terminal), and 0 when there is no physi-
cal input present (no voltage applied to the input
terminal).
• When the Examine-on instruction is associated by
address with an internal relay, then the status of the
Figure 5-16 Major elements of a sequential function chart
program.
Initial
Step 1
Step 2 Action
Step 3 Action
Action
Transition
Wire
Wire
loop
Transition
Transition
Stop
Figure 5-17 PLC ladder and equivalent structured text
program.
Structured text (ST) program
IF Sensor_1 AND Sensor_2 THEN
SOL_1 := 1;
ELSEIF Sensor_3 AND Sensor_4 AND NOT Sensor_5 THEN
SOL_1 := 1;
END_IF;
Sensor 1
Sensor 3 Sensor 4 Sensor 5
Sensor 2
Ladder diagram (LD) program
SOL 1
Figure 5-18 Examine If Closed (XIC) instruction.
Bit
number
Ι:1/4
Ι:1/4
Ι:1/4
Ι:1/4
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0
Status
Instruction interpreted
as true
as false
1
Symbol
Examine if closed (XIC)
Examine-on
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bit is dependent on the logical status of the internal
bit with the same address as the instruction.
• If the instruction memory bit is a 1 (true) this in-
struction will allow rung continuity through itself,
like a closed relay contact.
• If the instruction memory bit is a 0 (false) this
instruction will not allow rung continuity through
itself and will assume a normally open state just like
an open relay contact.
The symbol for the Examine If Open (XIO) instruction
is shown in Figure 5-19. The XIO instruction, which is
also called the Examine-off instruction, looks and oper-
ates like a normally closed relay contact. Associated with
each XIO instruction is a memory bit linked to the status
of an input device or an internal logical condition in a
rung. This instruction asks the PLC’s processor to exam-
ine if the contact is open. It does this by examining the
bit at the memory location specified by the address in the
following manner:
• As with any other input the memory bit is set to 1
or 0 depending on the status of the input (physical)
device or internal (logical) relay address associated
with that bit.
• A 1 corresponds to a true status or on condition.
• A 0 corresponds to a false status or off condition.
• When the Examine-off instruction is used to ex-
amine a physical input, then the instruction will be
interpreted as false when there is a physical input
(voltage) present (the bit is 1) and will be inter-
preted as true when there is no physical input pres-
ent (the bit is 0).
• If the Examine-off instruction were associated by
address with an internal relay, then the status of the
bit would be dependent on the logical status of the
internal bit with the same address as the instruction.
• Like the Examine-on instruction, the status of the
instruction (true or false) determines if the instruc-
tion will allow rung continuity through itself, like a
closed relay contact.
• The memory bit always follows the status (true = 1
or false = 0) of the input address or internal address
assigned to it. The interpretation of that bit, how-
ever, is determined by which instruction is used to
examine it.
• Examine-on instructions always interpret a 1 status
as true and a 0 status as false, while Examine-off in-
structions interpret a 1 status as false and a 0 status
as true.
The symbol for the Output Energize (OTE) instruc-
tion is shown in Figure 5-20. The OTE instruction looks
and operates like a relay coil and is associated with a
memory bit. This instruction signals the PLC to ener-
gize (switch on) or de-energize (switch off) the output.
The processor makes this instruction true (analogous to
energizing a coil) when there is a logical path of true
XIC and XIO instructions in the rung. The operation of
the Output Energize instruction can be summarized as
follows:
• The status bit of the addressed Output Energize in-
struction is set to 1 to energize the output and to 0 to
de-energize the output.
• If a true logic path is established with the input
instructions in the rung, the OTE instruction is ener-
gized and the output device wired to its terminal is
energized.
• If a true logic path cannot be established or
rung conditions go false, the OTE instruction is
de-energized and the output device wired to it is
switched off.
Sometimes beginner programmers used to thinking in
terms of hardwired relay control circuits tend to use the
same type of contact (NO or NC) in the ladder logic pro-
gram that corresponds to the type of field switch wired to
the discrete input. While this is true in many instances, it
is not the best way to think of the concept. A better ap-
proach is to separate the action of the field device from
Figure 5-19 Examine If Open (XIO) instruction.
Symbol
Bit
number
Ι:1/4
Ι:1/4
Ι:1/4
Ι:1/4
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1
0
Examine if open (XIO)
Examine-off
Status
as false
as true
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the action of the PLC bits as illustrated in Figure 5-21. A
signal present makes the NO bit (1) true; a signal absent
makes the NO bit (0) false. The reverse is true for an NC
bit. A signal present makes the NC bit (1) false; a signal
absent makes the NO bit (0) true.
The main function of the ladder logic diagram pro-
gram is to control outputs based on input conditions, as
illustrated in Figure 5-22. This control is accomplished
through the use of what is referred to as a ladder rung. In
general, a rung consists of a set of input conditions, rep-
resented by contact instructions, and an output instruction
at the end of the rung, represented by the coil symbol.
Each contact or coil symbol is referenced with an address
that identifies what is being evaluated and what is being
controlled. The same contact instruction can be used
throughout the program whenever that condition needs to
be evaluated. The number of ladder logic relays and input
and output instructions is limited only by memory size.
Most PLCs allow more than one output per rung.
Figure 5-20 Output Energize (OTE) instruction.
Symbol
Output energize
Output
data
Input
data
Program
Ι:1/1
Ι:1/4
Ι:1/1
Ο:2/1
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
ON
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1
1 1
Output
module
Input
module
Ι:1/4 Ο:2/1
Figure 5-21 Separating the action of the ﬁeld device and
PLC bit.
A
A A
FALSE
Button not actuated
TRUE
OUTPUT
Input
module
Bit
status
OFF
A
ON
0
A
FALSE
Button actuated
TRUE
OUTPUT
Input
module
Bit
status
ON
1
OFF
A
Figure 5-22 Ladder logic diagram rungs.
I/3
I/2
I/1 O/1
Inputs
Rung 0
Rung 1
Outputs
I/4
I/2 O/2
O/1
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The addressing of real inputs and outputs, as well as
internals, depends on the PLC model used. Addressing
formats can vary from one PLC family to another as well
as for different manufacturers. These addresses can be
represented in decimal, octal, or hexadecimal depend-
ing on the number system used by the PLC. The address
identifies the function of an instruction and links it to a
particular bit in the data table portion of the memory.
Figure 5-24 shows the addressing format for an Allen-
Bradley SLC 500 controller. Addresses contain the slot
number of the module where input or output devices
are connected. Addresses are formatted as file type, slot
number, and bit.
The assignment of an I/O address can be included in
the I/O connection diagram, as shown in Figure 5-25. In-
puts and outputs are typically represented by squares and
diamonds, respectively.
For an output to be activated or energized, at least one
left-to-right true logical path must exist, as illustrated in
Figure 5-23. A complete closed path is referred to as hav-
ing logical continuity. When logical continuity exists in
at least one path, the rung condition and Output Energize
instruction are said to be true. The rung condition and
OTE instruction are false if no logical continuity path has
been established. During controller operation, the proces-
sor evaluates the rung logic and changes the state of the
outputs according to the logical continuity of rungs.
5.5 Instruction Addressing
To complete the entry of a relay-type instruction, you
must assign an address to each instruction. This address
indicates what PLC input is connected to what input de-
vice and what PLC output will drive what output device.
Figure 5-23 Logical continuity.
Rung 0
Rung 1
Rung 2
F
T
T
F
T
T
T
F
T
T
T
T
Figure 5-24 Addressing format for an Allen-Bradley SLC 500 controller.
Closed
switch
Energized
output
Output image table
file 0
User-programmed rung
O:4/6
Bit address
6
12
Ι:3 O:4
Ι:3/12
Input file (I)
Slot (3)
Bit (12)
L1
Input image table
file 1
0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0
Ι:3/12
Bit address
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ο:4/6
Output file (O)
Slot (4)
Bit (6)
L2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 5-25 I/O connection diagram.
O:3/6
Outputs
L
L
Inputs
LS1
L
L
PB1
SOL1
O:2/3
1 2 1
PL1
2
I:4/5
I:4/6 R
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Additional input logic instructions (conditions) can
be programmed in the output branches to enhance con-
ditional control of the outputs. When there is a true
logic path, including extra input conditions on an output
branch, that branch becomes true. In the example shown
in Figure 5-29, either A and D or B and D provide a true
logic path to E.
Input and output branches can be nested to avoid
redundant instructions and to speed up processor scan
time. Figure 5-30 illustrates nested input and output
branches. A nested branch starts or ends within another
branch.
In some PLC models, the programming of a branch
circuit within a branch circuit or a nested branch can-
not be done directly. It is possible, however, to program
a logically equivalent branching condition. Figure 5-31
shows an example of a circuit that contains a nested
contact D. To obtain the required logic, the circuit
would be programmed as shown in Figure 5-32. The
duplication of contact C eliminates the nested con-
tact D. Nested branching can be converted into non-
nested branches by repeating instructions to make par-
allel equivalents.
5.6 Branch Instructions
Branch instructions are used to create parallel paths of
input condition instructions. This allows more than one
combination of input conditions (OR logic) to establish
logic continuity in a rung. Figure 5-26 illustrates a typical
branch instruction. The rung will be true if either instruc-
tion A or B is true.
Input branching by formation of parallel branches can
be used in your application program to allow more than
one combination of input conditions. If at least one of these
parallel branches forms a true logic path, the rung logic is
true and the output will be energized. If none of the paral-
lel branches complete a logical path, logic rung continuity
is not established and the output will not be de-energized.
In the example shown in Figure 5-27, either A and B, or C
provides logical continuity and energizes output D.
On most PLC models, branches can be established
at both input and output portions of a rung. With output
branching, you can program parallel outputs on a rung to
allow a true logic path to control multiple outputs, as il-
lustrated in Figure 5-28. When there is a true logic rung
path, all parallel outputs become true. In the example
shown, either A or B provides a true logical path to all
three output instructions: C, D, and E.
Figure 5-26 Typical branch instruction.
A
B
C
Figure 5-27 Parallel input branches.
B D
C
A
Figure 5-28 Parallel output branches.
B
D
E
C
A
Figure 5-29 Parallel output branching with conditions.
B D E
C
A
Figure 5-30 Nested input and output branches.
Figure 5-31 Nested contact program.
A branch within
a branch
Y
A B C
D
E
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Figure 5-33 PLC matrix limitation diagram.
Maximum 7
parallel lines
Maximum 10 contacts
Figure 5-34 Program with vertical contact.
Boolean equation: Y ⫽ (AD) ⫹ (BCD) ⫹ (BE) ⫹ (ACE)
A D
B E
C
Y
Figure 5-35 Reprogrammed to eliminate vertical contact.
Y
D
A
C D
B
C E
A
E
B
Some PLC manufacturers have virtually no limitations
on allowable series elements, parallel branches, or out-
puts. For others, there may be limitations to the number of
series contact instructions that can be included in one rung
of a ladder diagram as well as limitations to the number
of parallel branches. Also, there is an additional limitation
with some PLCs: only one output per rung and the output
must be located at the end of the rung. The only limita-
tion on the number of rungs is memory size. Figure 5-33
shows the matrix limitation diagram for a typical PLC. A
maximum of seven parallel lines and 10 series contacts
per rung is possible.
Another limitation to branch circuit programming is
that the PLC will not allow for programming of vertical
contacts. A typical example of this limitation is contact
C of the user program drawn in Figure 5-34. To obtain
the required logic, the circuit would be reprogrammed as
shown in Figure 5-35.
The processor examines the ladder logic rung for logic
continuity from left to right only. The processor never al-
lows for flow from right to left. This situation presents a
problem for user program circuits similar to that shown in
Figure 5-36. If programmed as shown, contact combination
FDBC would be ignored. To obtain the required logic, the
circuit would be reprogrammed as shown in Figure 5-37. Figure 5-36 Original circuit.
Boolean equation: Y ⫽ (ABC) ⫹ (ADE) ⫹ (FE) ⫹ (FDBC)
A
E
B
D
F
C Y
Figure 5-32 Program required to eliminate nested
contact.
A B C Y
C
D
E
Contact
instruction C
repeated
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Internal outputs are single-bit storage locations in
memory and are addressed as such. SLC 500 controllers
use bit file B3 for storage and addressing of internal out-
put bits. The addressing for bit B3:1/3 illustrated in Fig-
ure 5-38 consists of the file number followed by word and
bit numbers.
An internal control relay can be used when a program
requires more series contacts than the rung allows. Fig-
ure 5-39 shows a circuit that allows for only 7 series con-
tacts when 12 are actually required for the programmed
logic. To solve this problem, the contacts are split into
two rungs. Rung 1 contains seven of the required con-
tacts and is programmed to control internal relay coil
B3:1/3. The address of the first programmed contact on
Rung 2 is B3:1/3 followed by the remaining five contacts
and the discrete output. When the logic controlling the
internal output is true, the referenced bit B3:1/3 is turned
on or set to 1. The advantage of an internal storage bit in
this manner is that it does not waste space in a physical
output.
5.7 Internal Relay Instructions
Most PLCs have an area of the memory allocated for what
are known as internal storage bits. These storage bits are
also called internal outputs, internal coils, internal con-
trol relays, or simply internal bits. Internal outputs are on/
off signals generated by programmed logic. Unlike a dis-
crete output, an internal output does not directly control
an output field device. The internal output operates just
like any output that is controlled by programmed logic;
however, the output is used strictly for internal purposes.
The advantage of using internal outputs is that there
are many situations in which an output instruction is re-
quired in a program but no physical connection to a field
device is needed. If there are no physical outputs wired
to a bit address, the address can be used as an internal
storage point. Internal storage bits or points can be pro-
grammed by the user to perform relay functions without
occupying a physical output. In this way internal outputs
can minimize output module point requirements when-
ever practical.
Figure 5-37 Reprogrammed circuit.
C
B
A D E
C
A
E
F
F D B
Y
Figure 5-38 SLC 500 controllers use bit ﬁle B3 for internal bit
addressing.
0
Output
File
1
Input
File
2
Status
File
File (B3)
Word (1)
Bit (3)
B3:0 1 1 1 1 0 0 1 0 0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0
0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
B3:1
B3:2
B3:3
B3:4
B3:5
B3:6
Bit
File
3
Address
B3:1/3
Data File B3 (bin) -- BINARY
Offset 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
3
Bit
File
4
Timer
File
5
Counter
File
6
Control
Files
7
Integer
Files
Figure 5-39 Programmed internal relay control.
2
1 3 4 5
9
8 10 11 12
6 7
Internal
relay
coil
B3:1/3
Discrete
output
Discrete inputs
Discrete inputs
Rung 1
Rung 2
Internal
relay
contact
B3:1/3
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symbol. This is because the normal state of an input (NO
or NC) does not matter to the controller. What does matter
is that if contacts need to close to energize the output, then
the Examine If Closed instruction is used. Since both PB1
and PB2 must be closed to energize the pilot light, the
Examine If Closed instruction is used for both.
A simple program using the Examine If Open (XIO)
instruction is shown in Figure 5-41. Both the hardwired
circuit and user program are shown. In the hardwired cir-
cuit, when the pushbutton is open relay coil CR is de-
energized and its NO contact closes to switch the pilot
light on. When the pushbutton is closed, relay coil CR
is energized and its NC contact opens to switch the pilot
light off. The pushbutton is represented in the user pro-
gram by an Examine If Open instruction. This is because
the rung must be true when the external pushbutton is
open and false when the pushbutton is closed. Using an
Examine If Open instruction to represent the pushbutton
satisfies these requirements. The NO or NC mechanical
action of the pushbutton is not a consideration. It is im-
portant to remember that the user program is not an elec-
trical circuit but a logic circuit. In effect, we are interested
in logic continuity when establishing an output.
Figure 5-42 shows a simple program using both the
XIC and XIO instructions. The logic states (0 or 1) indi-
cate whether an instruction is true or false and is the basis
5.8 Programming Examine If Closed
and Examine If Open Instructions
A simple program using the Examine If Closed (XIC)
instruction is shown in Figure 5-40. This figure shows a
hardwired circuit and a user program that provides the
same results. You will note that both the NO and the NC
pushbuttons are represented by the Examine If Closed
Figure 5-40 Simple program that uses the Examine If
Closed (XIC) instruction.
Hardwired circuit
PL
PL
PB_2
PB_1
PB2
PB1
User program providing
the same results
Figure 5-41 Simple program that uses the Examine If
Open (XIO) instruction.
Hardwired circuit
CR
PB1
CR
PL
User program providing
the same results
PB_1 PL
Figure 5-42 Simple program using both the XIC and XIO instructions.
The status of the instruction is
If the data table bit
is
XIC
EXAMINE IF CLOSED
False
False
True
False False
False
False
False
True
True
False
True
True
Logic 0
Logic 1
XIO
EXAMINE IF OPEN
OTE
OUTPUT ENERGIZE
XIC
Instruction outcome
Time
t1 (initial)
t2
t3
t4
Input bit status
XIO OTE XIC XIO OTE
0 0 0
1 0 1
1 1 0
0 1 0
True
True Goes true
Goes false
Remains false
XIC XIO OTE
Input instructions Output instruction
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from other rungs in the project. There are several different
methods that you can use to address instructions. You can
enter an address by manually typing it in or by dragging
the address from data files or other instructions.
Some of the windows you will need to use when work-
ing with RSLogix 500 software include:
• Main Window—This window opens each time you
create a new project or open an existing one. Some
of the features associated with this window include
the following:
- Window Title Bar—The title bar is located at
the topmost strip of the window and displays
the name of the program as well as that of the
opened file.
- Menu Bar—The menu bar is located below the
title bar. The menu contains key words associated
with menus that are opened by clicking on the
key word.
- Windows Toolbar—The Windows toolbar buttons
execute standard Windows commands when you
click on them.
- Program/Processor Status Toolbar—This toolbar
contains four drop-down lists that identify the
current processor operating mode, current online
edit status, and whether forces are present and
enabled.
- Project Window—This window displays the file
folders listed in the project tree.
of controller operation. The figure summarizes the on/off
state of the output as determined by the changing states
of the inputs in the rung. The time aspect relates to the
repeated scans of the program, wherein the input table is
updated with the most current status bits.
5.9 Entering the Ladder Diagram
Most of today’s PLC programming packages operate in
the Windows environment. For example, Allen-Bradley’s
RSLogix software packages are Windows programming
packages used to develop ladder logic programs. This
software, in various versions, can be used to program the
PLC-5, SLC 500, ControlLogix, and MicroLogic family
of processors. An added feature is that RSLogix programs
are compatible with programs that have been previously
created with DOS-based programming packages.You can
import projects that were developed with DOS products
or export to them from RSLogix.
Entering the ladder diagram, or actual programming,
is usually accomplished with a computer keyboard or
hand-held programming device. Because hardware and
programming techniques vary with each manufacturer, it
is necessary to refer to the programming manual for a spe-
cific PLC to determine how the instructions are entered.
One method of entering a program is through a hand-
held keyboard. Keyboards usually have relay symbol
and special function keys along with numeric keys for
addressing. Some also have alphanumeric keys (letters
and numbers) for other special programming functions.
In hand-held units, the keyboard is small and the keys
have multiple functions. Multiple-function keys work like
second-function keys on calculators.
A personal computer is most often used today as the
programmer. The computer is adapted to the particular
PLC model through the use of the relevant programmable
controller software.
Figure 5-43 shows the RSLogix SLC 500 main win-
dow. Different screens, toolbars, and dialog boxes are
used to navigate through the Windows environment. It is
important that you understand the purpose of the various
screens, toolbars, and windows to make the most effective
use of the software. This information is available from the
software reference manual for the particular PLC family
and will become more familiar to you as you develop pro-
grams using the software.
Figure 5-44 shows a typical instruction toolbar with bit
instructions selected. To place an instruction on a rung,
click its icon on the toolbar and simply drag the instruc-
tion straight off the toolbar onto the rung of the ladder.
Drop points are shown on the ladder to help position the
instruction. In addition, instructions can also be dragged
Figure 5-43 RSLogix SLC 500 main window.
Figure 5-44 Typical instruction toolbar with bit
instructions selected.
OSR
Bit
User Timer/Counter Input/Output Compare
L U
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• I/O Configuration—The I/O Configuration screen
(Figure 5-46) lets you click or drag-and-drop a
module from an all-inclusive list to assign it to a slot
in your configuration.
• Data Files—Data File screens contain data that are
used in conjunction with ladder program instruc-
tions and include input and output files as well as
timer, counter, integer, and bit files. Figure 5-47
shows an example of the bit file B3, which is used
for internal relays. Note that all the addresses from
this file start with B3.
Relay ladder logic is a graphical programming language
designed to closely represent the appearance of a wired
relay system. It offers considerable advantages for PLC
control. Not only is it reasonably intuitive, especially for
users with relay experience, but it is also particularly effec-
tive in an online mode when the PLC is actually perform-
ing control. Operation of the logic is apparent from the
highlighting of rungs of the various instructions on-screen,
- Project Tree—The project tree is a visual repre-
sentation of all folders and their associated files
contained in the current project. From the project
tree, you can open files, create files, modify file
parameters, copy files, hide or unhide files, delete
files, and rename files.
- Result Window—This window displays the re-
sults of either a search or a verify operation. The
verify operation is used to check the ladder pro-
gram for errors.
- Active Tab—This tab identifies which program is
currently active.
- Status Bar—This bar contains information rel-
evant to the current file.
- Split Bar—The split bar is used to split the ladder
window to display two different program files or
groups of ladder rungs.
- Tabbed Instruction Toolbar—This toolbar dis-
plays the instruction set as a group of tabbed
categories.
- Instruction Palette—This tool contains all the
available instructions displayed in one table to
make the selection of instructions easier.
- Ladder Window—This window displays the cur-
rently open ladder program file and is used to
develop and edit ladder programs.
- Ladder Window Properties—This window allows
you to change the display of your ladder program
and its associated addressing and documentation.
• Select Processor Type—The programming soft-
ware needs to know what processor is being used in
conjunction with the user program. The Select Pro-
cessor Type screen (Figure 5-45) contains a list of
the different processors that the RSLogix software
can program. You simply scroll down the list until
you find the processor you are using and select it.
Figure 5-45 Select processor type screen.
Figure 5-46 I/O conﬁguration screen.
Figure 5-47 Data bit ﬁle B3 screen.
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Some common operating modes are explained in the
following paragraphs.
Program Mode The program mode is used to enter
a new program, edit or update an existing program,
upload files, download files, document (print out) pro-
grams, or change any software configuration file in the
program. When the PLC is switched into the program
mode, all outputs from the PLC are forced off regard-
less of their rung logic status, and the ladder I/O scan
sequence is halted.
Run Mode The run mode is used to execute the user
program. Input devices are monitored and output de-
vices are energized accordingly. After all instructions
have been entered in a new program or all changes
made to an existing program, the processor is put in
the run mode.
Test Mode The test mode is used to operate or
monitor the user program without energizing any
outputs. The processor still reads inputs, executes
the ladder program, and updates the output status
table files, but without energizing the output cir-
cuits. This feature is often used after developing
or editing a program to test the program execu-
tion before allowing the PLC to operate real-world
outputs. Variations of the test mode can include
the single-step test mode, which directs the proces-
sor to execute a selected single rung or group of
rungs; the single-scan test mode, which executes
a single processor operating scan or cycle; and the
continuous-scan test mode, which directs the pro-
cessor to continuously run the program for checking
or troubleshooting.
Remote Mode Some processors have a three-
position switch to change the processor operating
mode. In the Run position, all logic is solved and the
I/O is enabled. In the Program position, all logic solv-
ing is stopped and the I/O is disabled. The Remote
position allows the PLC to be remotely changed
between program and run mode by a personal com-
puter connected to the PLC processor. The remote
mode may be beneficial when the controller is in a
location that is not easily accessible.
which identifies the logic state of contacts in real time (Fig-
ure 5-48) and which rungs have logic continuity.
For most PLC systems, each Examine If Closed and
Examine If Open contact, each output, and each branch
Start/End instruction requires one word of user memory.
You can refer to the SLC 500 Controller Properties to see
the number of instruction words used and the number left
as the program is being developed.
5.
10 Modes of Operation
A processor has basically two modes of operation: the
program mode and some variation of the run mode. The
number of different operating modes and the method of
accessing them varies with the manufacturer. Figure 5-49
shows a typical three-position keyswitch used to select
different processor modes of operation.
Figure 5-48 Monitoring a ladder logic program.
Highlighted rungs indicate
the instruction is true.
Figure 5-49 Three-position keyswitch used to select
different processor modes of operation.
PROG
REM
RUN
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1. What does the memory map for a typical PLC pro-
cessor consist of?
2. Compare the function of the PLC program and data
files.
3. In what manner are data files organized?
4. List eight different types of data files use by an
SLC 500 controller.
5. a. What information is stored in the input image
table file?
b. In what form is this information stored?
6. a. What information is stored in the output image
table file?
b. In what form is this information stored?
7. Outline the sequence of events involved in a PLC
scan cycle.
8. List four factors that enter into the length of the
scan time.
9. Compare the way horizontal and vertical scan pat-
terns examine input and output instructions.
10. List the five standard PLC languages as defined by
the International Standard for Programmable Con-
trollers, and give a brief description of each.
11. Draw the symbol and state the equivalent instruc-
tion for each of the following: NO contact, NC
contact, and coil.
12. Answer the following with regard to the Examine If
Closed instruction:
a. What is another common name for this instruction?
b. What is this instruction asking the processor to
examine?
c. Under what condition is the status bit associated
with this instruction 0?
d. Under what condition is the status bit associated
e. Under what condition is this instruction logically
true?
f. What state does this instruction assume when it
is false?
13. Answer the following with regard to the Examine If
Open instruction:
a. What is another common name for this
instruction?
b. What is this instruction asking the processor to
examine?
e. Under what condition is this instruction logically
true?
f. What state does this instruction assume when it
is false?
14. Answer the following with regard to the Output En-
ergize instruction:
a. What part of an electromagnetic relay does this
instruction look and act like?
b. What is this instruction asking the processor to do?
15. A normally closed pushbutton is connected to a
PLC discrete input. Does this mean it must be rep-
resented by a normally closed contact in the ladder
logic program? Explain why or why not.
16. Answer the following with regard to a ladder logic
rung:
a. Describe the basic makeup of a ladder logic
rung.
b. How are the contacts and coil of a rung
identified?
c. When is the ladder rung considered as having
logic continuity?
17. What does the address assigned to an instruction
indicate?
18. When are input branch instructions used as part of
a ladder logic program?
19. Identify two matrix limitations that may apply to
certain PLCs.
20. In what way does an internal output differ from a
discrete output.
21. A normally open limit switch is to be programmed
to control a solenoid. What determines whether an
Examine-on or Examine-off contact instruction is
used?
22. Explain the purpose of Windows based program-
ming software such as RSLogix.
23. Briefly describe each of the following PLC modes
of operation:
a. Program
b. Test
c. Run
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Figure 5-52 Program for Problem 4.
A Y
C D
E
B
A C D E
B Y
Figure 5-54 Hardwired circuit for Problem 6.
L2
SOL B
SOL A
LS1
L1
1. Assign each of the following discrete input and out-
put addresses based on the SLC 500 format.
a. Limit switch connected to terminal screw 4 of the
module in slot 1 of the chassis.
b. Pressure switch connected to terminal screw 2 of
the module in slot 3 of the chassis.
c. Pushbutton connected to terminal screw 0 of the
d. Pilot light connected to terminal screw 13 of the
e. Motor starter coil connected to terminal screw 6
of the module in slot 4 of the chassis.
f. Solenoid connected to terminal screw 8 of the
2. Redraw the program shown in Figure 5-50 corrected
to solve the problem of a nested contact.
3. Redraw the program shown in Figure 5-51 cor-
rected to solve the problem of a nested vertical pro-
grammed contact.
to solve the problem of some logic ignored.
to solve the problem of too many series contacts
(only four allowed).
6. Draw the equivalent ladder logic program used
to implement the hardwired circuit drawn in
Figure 5-54, wired using:
a. A limit switch with a single NO contact con-
nected to the PLC discrete input module
b. A limit switch with a single NC contact connected
to the PLC discrete input module
7. Assuming the hardwired circuit drawn in Fig-
ure 5-55 is to be implemented using a PLC pro-
gram, identify
a. All input field devices
b. All output field devices
c. All devices that could be programmed using inter-
nal relay instructions
8. What instruction would you select for each of the
following discrete input field devices to accomplish
the desired task? (State the reason for your answer.)
a. Turn on a light when a conveyor motor is run-
ning in reverse. The input field device is a set of
CHAPTER 5 PROBLEMS
A
C D
E
B Y
A
C D
B
Y
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contacts on the conveyor start relay that close
when the motor is running forward and open
when it is running in reverse.
b. When a pushbutton is pressed, it operates a so-
lenoid. The input field device is a normally open
pushbutton.
c. Stop a motor from running when a pushbutton
is pressed. The input field device is a normally
closed pushbutton.
d. When a limit switch is closed, it triggers an in-
struction ON. The input field device is a limit
switch that stores a 1 in a data table bit when
closed.
9. Write the ladder logic program needed to implement
each of the following (assume inputs A, B, and C are
all normally open toggle switches):
a. When input A is closed, turn ON and hold ON
outputs X and Y until A opens.
b. When input A is closed and either input B or C is
open, turn ON output Y; otherwise, it should be
OFF.
c. When input A is closed or open, turn ON
output Y.
d. When input A is closed, turn ON output X and
turn OFF output Y.
Figure 5-55 Hardwired circuit for Problem 7.
L2
LS1
CR1
CR1 PL1
SOL A
SS1
CR1
PS1
LS2 LS3 CR2
CR2
SOL B
SOL C
PB2
PB3 CR2
CR3
PL2
LS4 CR3
PB1
L1
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95
For ease of understanding, ladder logic pro-
grams can be compared to relay schematics.
This chapter gives examples of how traditional
relay schematics are converted into PLC ladder
logic programs. You will learn more about the
wide variety of ﬁeld devices commonly used in
connection with the I/O modules.
Chapter Objectives
6.1 Identify the functions of electromagnetic control
relays, contactors, and motor starters
6.2 Identify switches commonly found in PLC installations
6.3 Explain the operation of sensors commonly found in
PLC installations
6.4 Explain the operation of output control devices
commonly found in PLC installations
6.5 Describe the operation of an electromagnetic latching
relay and the PLC-programmed LATCH/UNLATCH
instruction
6.6 Compare sequential and combination control processes
6.7 Convert fundamental relay ladder diagrams to PLC
ladder logic programs
6.8 Write PLC programs directly from a narrative description
6
Developing Fundamental
PLC Wiring Diagrams and
Ladder Logic Programs
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96 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs
control relay. With no current flow through the coil (de-
energized), the armature is held away from the core of
the coil by spring tension. When the coil is energized, it
produces an electromagnetic field. Action of this field,
in turn, causes the physical movement of the armature.
Movement of the armature causes the contact points of
the relay to open or close. The coil and contacts are insu-
lated from each other; therefore, under normal conditions,
no electric circuit will exist between them.
The symbol used to represent a control relay is shown
in Figure 6-3. The contacts are represented by a pair of
short parallel lines and are identified with the coil by
means of the letters. The letter M frequently indicates a
motor starter, while CR is used for control relays. Nor-
mally open (NO) contacts are defined as those contacts
that are open when no current flows through the coil but
that close as soon as the coil conducts a current or is ener-
gized. Normally closed (NC) contacts are closed when the
coil is de-energized and open when the coil is energized.
Each contact is usually drawn as it would appear with the
coil de-energized.
A typical control relay used to control two pilot lights
is shown in Figure 6-4. The operation of the circuit can be
summarized as follows:
• With the switch open, coil CR is de-energized.
• The circuit to the green pilot light is completed
through the normally closed contact, so this light
will be on.
• At the same time, the circuit to the red pilot light is
opened through the normally open contact, so this
light will be off.
6.
1 Electromagnetic Control Relays
The PLC’s original purpose was the replacement of elec-
tromagnetic relays with a solid-state switching system
that could be programmed. Although the PLC has re-
placed much of the relay control logic, electromagnetic
relays are still used as auxiliary devices to switch I/O field
devices. The programmable controller is designed to re-
place the physically small control relays that make logic
decisions but are not designed to handle heavy current or
high voltage (Figure 6-1). In addition, an understanding
of electromagnetic relay operation and terminology is im-
portant for correctly converting relay schematic diagrams
to ladder logic programs.
An electrical relay is a magnetic switch. It uses elec-
tromagnetism to switch contacts. A relay will usually
have only one coil but may have any number of different
contacts. Figure 6-2 illustrates the operation of a typical
Figure 6-1 Electromechanical control relay.
Relay
Output circuit
Contact
Coil
Load
Switch
Input circuit
⫹
⫺
Figure 6-2 Relay operation.
⫹
⫺
Coil energized
Fixed contacts
Spring
Armature
Movable contact
Coil de-energized
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Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 97
Control relay coils and contacts have separate ratings.
Coils are rated for the type of operating current (DC or
AC) and normal operating voltage. Contacts are rated in
terms of the maximum amount of current the contacts are
capable of handling at a specified voltage level and type
(AC or DC). Control relay contacts generally are not nor-
mally designed to carry heavy currents or high voltages.
The contacts are usually rated between 5 and 10 amperes,
with the most common rating for the coil voltage being
120 VAC.
6.2 Contactors
A contactor is a special type of relay designed to handle
heavy power loads that are beyond the capability of con-
trol relays. Figure 6-5 shows a three-pole magnetic con-
tactor. Unlike relays, contactors are designed to make and
break higher powered circuits without being damaged.
Such loads include lights, heaters, transformers, capaci-
tors, and electric motors for which overload protection is
provided separately or not required.
Programmable controllers normally have an output ca-
pacity capable of operating a contactor coil, but not that
needed to operate heavy power loads directly. Figure 6-6
illustrates the application of a PLC used in conjunction
with a contactor to switch power on and off to a pump.
The output module is connected in series with the coil to
form a low-current switching circuit. The contacts of the
contactor are connected in series with the pump motor to
form a high-current switching circuit.
• With the switch closed, the coil is energized.
• The normally open contact closes to switch the red
pilot light on.
• At the same time, the normally closed contact opens
to switch the green pilot light off.
Figure 6-3 Relay normally open and normally closed contacts.
Source: Photo courtesy Eaton Corporation, www.eaton.com.
Armature Coil
Movable contacts
Stationary
contact
Normally closed
(NC) contacts
Normally open
(NO) contacts
Coil de-energized
Coil
Energized
NC NO
Armature
Coil energized
Coil
M
Associated
contacts
M
Coil
CR
Associated
contacts
CR
Figure 6-4 Control relay used to control two pilot lights.
Source: Photo courtesy Digi-Key Corporation, www.digikey.com.
Single-pole
double-throw relay
Switch open—coil de-energized
G
CR
CR
R
S
L1 L2
On
Off
Switch closed—coil energized
G
CR
CR
R
S
L1 L2
Off
On
CR
CR
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Figure 6-5 Three-pole magnetic contactor.
Line side
Line side
Coil
Coil
Contacts
Load side
Load side
Load side
Wiring symbol
Stationary
contacts
Movable
contacts
Movable iron
armature or
plunger
Line side
6.3 Motor Starters
A motor starter is designed to provide power to motors.
The motor starter is made up of a contactor with an over-
load relay attached physically and electrically to it as il-
lustrated in Figure 6-7. The function of the overload relay
can be summarized as follows:
• Overload relays are designed to meet the special
protective needs of motor control circuits.
• They allow harmless temporary overloads that occur
when a motor starts.
• The overload relay will trip and disconnect power to
the motor if an overload condition persists.
• Overload relays can be reset after the overload con-
dition has been corrected.
Figure 6-8 shows the diagram for a typical three-phase,
magnetic motor starter. The operation of the circuit can be
• When the START button is pressed coil M is ener-
gized closing all normally open M contacts.
• The M contacts in series with the motor close to
complete the current path to the motor. These con-
tacts are part of the power circuit and must be de-
signed to handle the full load current of the motor.
Figure 6-6 Contactor used in conjunction with a PLC output.
Pump
Contactor power
contacts
L2
High-current
wiring
Coil
terminals
Low-current
wiring
PLC output
module
L2 L1
L1
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Figure 6-7 Motor starter is a contactor with an attached
overload relay.
Contactor
Overload
relay
Figure 6-8 Three-phase magnetic motor starter.
Stop
Start
M OL
M
M
M
M
OL
L3
T3
High-current
power circuit
Low-current
control circuit
Magnetic starter
L1
L2
T1
T2 Three-
phase
motor
OL
OL
Figure 6-9 PLC control of a motor.
Motor
Stop
OL
Start
L1 Inputs
PLC Program
L2
Output
Starter
coil
Start Stop OL
M
M
Pushbuttons
PLC
Magnetic
starter
M
an overload current is sensed to de-energize the M
coil and stop the motor.
Motor starters are available in various standard Na-
tional Electric Manufacturers Association (NEMA) sizes
and ratings. When a PLC needs to control a large motor,
it must work in conjunction with a starter as illustrated in
Figure 6-9. The power requirements for the starter coil
must be within the power rating of the output module of
the PLC. Note that the control logic is determined and
executed by the program within the PLC and not by the
hardwired arrangement of the input control devices.
6.4 Manually Operated Switches
Manually operated switches are controlled by hand.
These include toggle switches, pushbutton switches, knife
switches, and selector switches.
Pushbutton switches are the most common form of
manual control.A pushbutton operates by opening or clos-
ing contacts when pressed. Figure 6-10 shows commonly
used types of pushbutton switches, which include:
• Normally open (NO) pushbutton, which makes a
circuit when it is pressed and returns to its open po-
sition when the button is released.
• Normally closed (NC) pushbutton, which opens the
circuit when it is pressed and returns to the closed
position when the button is released.
• Break-before-make pushbutton in which the top
section contacts are NC and the bottom section con-
tacts are NO. When the button is pressed, the top
contacts open before the bottom contacts are closed.
• Control contact M (across START button) closes
to seal in the coil circuit when the START button is
released. This contact is part of the control circuit
and, as such, is only required to handle the small
amount of current needed to energize the coil.
• An overload (OL) relay is provided to protect the
motor against current overloads. The normally
closed relay contact OL opens automatically when
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Figure 6-11 Three-position selector switch.
Hand
1
Auto
3
A
B
Off
2
Symbol
Position
A B
Contacts
X
1
2
X
3
Figure 6-12 DIP switch.
ON
OFF
1 2 3 4 5 6 7
on a particular input module may be selected by means of
DIP switches located on the back of the module.
6.5 Mechanically Operated Switches
A mechanically operated switch is controlled automati-
cally by factors such as pressure, position, or tempera-
ture. The limit switch, shown in Figure 6-13, is a very
common industrial control device. Limit switches are
designed to operate only when a predetermined limit is
reached, and they are usually actuated by contact with
an object such as a cam. These devices take the place
of a human operator. They are often used in the control
circuits of machine processes to govern the starting, stop-
ping, or reversal of motors.
The temperature switch, or thermostat, shown in Fig-
ure 6-14 is used to sense temperature changes. Although
there are many types available, they are all actuated
by some specific environmental temperature change.
The selector switch is another common manually op-
erated switch. The main difference between a pushbut-
ton and selector switch is the operator mechanism. A
selector switch operator is rotated (instead of pushed)
to open and close contacts of the attached contact block.
Figure 6-11 shows a three-position selector switch.
Switch positions are established by turning the opera-
tor knob right or left. Selector switches may have two
or more selector positions, with either maintained con-
tact position or spring return to give momentary contact
operation.
Dual in-line package (DIP) switches are small switch
assemblies designed for mounting on printed circuit board
modules (Figure 6-12). The pins or terminals on the bot-
tom of the DIP switch are the same size and spacing as
an integrated circuit (IC) chip. The individual switches
may be of the toggle, rocker, or slide kind. DIP switches
use binary (on/off) settings to set the parameters for a
particular module. For example, the input voltage range
Figure 6-13 Mechanically operated limit switch.
Source: Photo courtesy Eaton Corporation.
Cam (on machine)
Operator
Operating
force
Enclosure
containing
contact
mechanism
Normally open
limit switch
Normally closed
limit switch
NEMA symbols
IEC symbols
Normally
open
Normally
closed
Figure 6-10 Commonly used types of pushbutton switches.
Normally open Normally closed Break-make
NEMA
symbol
IEC
symbol
NEMA
symbol
IEC
symbol
NEMA symbol
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or close piping solenoid valves to control fluids. The float
switch shown in Figure 6-16 is a type of level switch. This
switch is weighted so that as the liquid rises the switch float
and turns upside down, actuating its internal contacts.
6.6 Sensors
Sensors are used for detecting, and often measuring, the
magnitude of something. They convert mechanical, mag-
netic, thermal, optical, and chemical variations into elec-
tric voltages and currents. Sensors are usually categorized
by what they measure, and they play an important role in
modern manufacturing process control.
Proximity Sensor
Proximity sensors or switches, such as that shown in Fig-
ure 6-17, are pilot devices that detect the presence of an
object (usually called the target) without physical contact.
These solid-state electronic devices are completely en-
capsulated to protect against excessive vibration, liquids,
chemicals, and corrosive agents found in the industrial
environment. Proximity sensors are used when:
• The object being detected is too small, lightweight,
or soft to operate a mechanical switch.
• Rapid response and high switching rates are required,
as in counting or ejection control applications.
• An object has to be sensed through nonmetallic bar-
riers such as glass, plastic, and paper cartons.
• Hostile environments demand improved sealing
properties, preventing proper operation of mechani-
cal switches.
Figure 6-14 Temperature switch.
Source: Photo courtesy Honeywell, www.honeywell.com.
NEMA symbols
IEC symbols
NO contact NC contact
NO contact NC contact
Programmable thermostat
Figure 6-15 Pressure switch.
Source: Photo courtesy Honeywell, www.honeywell.com.
NO
contact
NC
contact
NEMA symbols for
pressure switch contacts
IEC symbols for
pressure switch contacts
␳
␳
NO
contact
NC
contact
Figure 6-16 Float type level switch.
Source: Courtesy Dwyer Instruments.
Symbols
NC contact
NO contact
Temperature switches open or close when a designated
temperature is reached. Industrial applications for these
devices include maintaining the desired temperature
range of air, gases, liquids, or solids.
Pressure switches, such as that shown in Figure 6-15,
are used to control the pressure of liquids and gases.
Although many different types are available, they are
all basically designed to actuate (open or close) their
contacts when a specified pressure is reached. Pres-
sure switches can be pneumatically (air) or hydrauli-
cally (liquid) operated switches. Generally, bellows or
a diaphragm presses up against a small microswitch and
causes it to open or close.
Level switches are used to sense liquid levels in vessels
and provide automatic control for motors that transfer liq-
uids from sumps or into tanks. They are also used to open
Figure 6-17 Proximity sensor.
Source: Photo courtesy Turck, Inc., www.turck.com.
Normally open (NO)
sensor symbols
NEMA
IEC
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• Long life and reliable service are required.
• A fast electronic control system requires a bounce-
free input signal.
Proximity sensors operate on different principles, de-
pending on the type of matter being detected. When an
application calls for noncontact metallic target sensing,
an inductive-type proximity sensor is used. Inductive
proximity sensors are used to detect both ferrous metals
(containing iron) and nonferrous metals (such as copper,
aluminum, and brass).
Inductive proximity sensors operate under the electri-
cal principle of inductance, where a fluctuating current
induces an electromotive force (emf) in a target object.
The block diagram for an inductive proximity sensor is
shown in Figure 6-18 and its operation can be summa-
rized as follows:
• The oscillator circuit generates a high-frequency
electromagnetic field that radiates from the end of
the sensor.
• When a metal object enters the field, eddy currents
are induced in the surface of the object.
• The eddy currents on the object absorb some of
the radiated energy from the sensor, resulting in
a loss of energy and change of strength of the
oscillator.
• The sensor’s detection circuit monitors the oscilla-
tor’s strength and triggers a solid-state output at a
specific level.
• Once the metal object leaves the sensing area, the
oscillator returns to its initial value.
Most sensor applications operate either at 24V DC
or at 120V AC. The method of connecting a proximity
Figure 6-18 Inductive proximity sensor.
Coil Oscillator Detector Output
Metal
object
Output
L1 L2
OFF
Output
Target
ON
Target
Figure 6-19 Typical three-wire DC sensor connection.
⫹
⫺
(⫹)
(⫺)
NO
Signal
Load
Load
Figure 6-20 Typical two-wire proximity sensor connection.
L1
L2
NO
Load
Load
L1
L2
sensor varies with the type of sensor and its applica-
tion. Figure 6-19 shows a typical three-wire DC sensor
connection. The three-wire DC proximity sensor has the
positive and negative line leads connected directly to it.
When the sensor is actuated, the circuit will connect the
signal wire to the positive side of the line if operating
normally open. If operating normally closed, the circuit
will disconnect the signal wire from the positive side of
the line.
Figure 6-20 shows a typical two-wire proximity sen-
sor connection intended to be connected in series with the
load. They are manufactured for either AC or DC sup-
ply voltages. In the off state, enough current must flow
through the circuit to keep the sensor active. This off state
current is called leakage current and typically may range
from 1 to 2 mA. When the switch is actuated, it will con-
duct the normal load circuit current.
Figure 6-21 shows the proximity sensor sensing range.
Hysteresis is the distance between the operating point
when the target approaches the proximity sensor face and
the release point when the target is moving away from the
sensor face. The object must be closer to turn the sensor
on rather than to turn it off. If the target is moving toward
the sensor, it will have to move to a closer point. Once the
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Figure 6-21 Proximity sensor sensing range.
Source: Photo courtesy Eaton Corporation, www.eaton.com.
Hysteresis
zone
Cube sensor
with status LED
Target
Operating
point
Release
point
Figure 6-22 Bleeder resistor connected to continuously
power a proximity sensor.
Proximity
sensor
L1 L2
Input
module
Bleeder
resistor
sensor turns on, it will remain on until the target moves to
the release point. Hysteresis is needed to keep proximity
sensors from chattering when subjected to shock and vi-
bration, slow-moving targets, or minor disturbances such
as electrical noise and temperature drift. Most proximity
sensors come equipped with an LED status indicator to
verify the output switching action.
As a result of solid-state switching of the output, a
small leakage current flows through the sensor even
when the output is turned off. Similarly, when the sensor
is on, a small voltage drop is lost across its output termi-
nals. To operate properly, a proximity sensor should be
powered continuously. Figure 6-22 illustrates the use of
a bleeder resistor connected to allow enough current for
the sensor to operate but not enough to turn on the input
of the PLC.
Capacitive proximity sensors are similar to inductive
proximity sensors. The main differences between the
two types are that capacitive proximity sensors produce
an electrostatic field instead of an electromagnetic field
and are actuated by both conductive and nonconductive
materials.
Figure 6-23 illustrates the operation of a capacitive
sensor. A capacitive sensor contains a high-frequency
oscillator along with a sensing surface formed by two
metal electrodes. When the target nears the sensing sur-
face, it enters the electrostatic field of the electrodes and
changes the capacitance of the oscillator. As a result, the
oscillator circuit begins oscillating and changes the out-
put state of the sensor when it reaches certain amplitude.
As the target moves away from the sensor, the oscilla-
tor’s amplitude decreases, switching the sensor back to
its original state.
Capacitive proximity sensors will sense metal objects
as well as nonmetallic materials such as paper, glass, liq-
uids, and cloth. They typically have a short sensing range
of about 1 inch, regardless of the type of material being
sensed. The larger the dielectric constant of a target, the
easier it is for the capacitive sensor to detect. This makes
possible the detection of materials inside nonmetallic
containers as illustrated in Figure 6-24. In this example,
the liquid has a much higher dielectric constant than the
cardboard container, which gives the sensor the ability to
Figure 6-23 Capacitive proximity sensor.
Sensor
electrodes
Electrostatic
field
Oscillator waveform
Target
present
Target
absent
Target
absent
Metallic or
nonmetallic
target
MILK
MILK
MILK
Figure 6-24 Capacitive proximity sensor liquid detection.
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see through the container and detect the liquid. In the pro-
cess shown, detected empty containers are automatically
diverted via the push rod.
Inductive proximity switches may be actuated only by
a metal and are insensitive to humidity, dust, dirt, and the
like. Capacitive proximity switches, however, can be ac-
tuated by any dirt in their environment. For general appli-
cations, the capacitive proximity switches are not really
an alternative but a supplement to the inductive proximity
switches. They are a supplement when there is no metal
available for the actuation (e.g., for woodworking ma-
chines and for determining the exact level of liquids or
powders).
Magnetic Reed Switch
A magnetic reed switch is composed of two flat contact
tabs that are hermetically sealed (airtight) in a glass tube
filled with protective gas, as illustrated in Figure 6-25.
When a magnetic force is generated parallel to the reed
switch, the reeds become flux carriers in the magnetic cir-
cuit. The overlapping ends of the reeds become opposite
Figure 6-25 Magnetic reed switch.
Source: Courtesy of Reed Switch Developments Corp., used with permission.
Reed switch sensor
N S
Figure 6-26 Photovoltaic and photoconductive light cells.
ⴙ ⴚ
DCV
(a) Photovoltaic solar cell (b) Photoconductive cell
Light changes resistance
Light produces voltage
Ohms
magnetic poles, which attract each other. If the magnetic
force between the poles is strong enough to overcome
the restoring force of the reeds, the reeds will be drawn
together to actuate the switch. Because the contacts are
sealed, they are unaffected by dust, humidity, and fumes;
thus, their life expectancy is quite high.
Light Sensors
The photovoltaic cell and the photoconductive cell,
illustrated in Figure 6-26, are two examples of light
sensors. The photovoltaic or solar cell reacts to light
by converting the light energy directly into electric en-
ergy. The photoconductive cell (also called a photore-
sistive cell) reacts to light by change in the resistance
of the cell.
A photoelectric sensor is an optical control device
that operates by detecting a visible or invisible beam of
light and responding to a change in the received light in-
tensity. Photoelectric sensors are composed of two basic
components: a transmitter (light source) and a receiver
(sensor), as shown in Figure 6-27. These two compo-
nents may or may not be housed in separate units. The
Figure 6-27 Photoelectric sensor.
Source: Photo courtesy SICK, Inc., www.sick.com.
Modulated
light
beam
Load
Transmitter
Receiver
Object to
be sensed
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Figure 6-28 Through-beam scan.
Source: Photo courtesy SICK, Inc., www.sick.com.
Transmitter
Receiver
basic operation of a photoelectric sensor can be sum-
marized as follows:
• The transmitter contains a light source, usually an
LED along with an oscillator.
• The oscillator modulates or turns the LED on and
off at a high rate of speed.
• The transmitter sends this modulated light beam to
the receiver.
• The receiver decodes the light beam and switches
the output device, which interfaces with the load.
• The receiver is tuned to its emitter’s modulation
frequency and will only amplify the light signal that
pulses at the specific frequency.
• Most sensors allow adjustment of how much light
will cause the output of the sensor to change state.
• Response time is related to the frequency of the
light pulses. Response times may become impor-
tant when an application calls for the detection of
very small objects, objects moving at a high rate of
speed, or both.
The scan technique refers to the method used by pho-
toelectric sensors to detect an object. The through-beam
scan technique (also called direct scan) places the trans-
mitter and receiver in direct line with each other, as il-
lustrated in Figure 6-28. Because the light beam travels
in only one direction, through-beam scanning provides
long-range sensing. Quite often, a garage door opener has
a through-beam photoelectric sensor mounted near the
floor, across the width of the door. For this application
the sensor senses that nothing is in the path of the door
when it is closing.
In a retroreflective scan, the transmitter and receiver
are housed in the same enclosure. This arrangement re-
quires the use of a separate reflector or reflective tape
mounted across from the sensor to return light back to
the receiver. The retroreflective scan is designed to re-
spond to objects that interrupt the beam normally main-
tained between the transmitter and receiver, as illustrated
Figure 6-29 Retroreﬂective scan.
Source: Photo courtesy ifm efector, www.ifm.com/us.
Reflector
Transmitter
Receiver
Figure 6-30 Fiber optic sensors.
Through-beam
To receiver
To receiver
From transmitter
From transmitter
Retroreflective
in Figure 6-29. In contrast to a through-beam applica-
tion, retroreflective sensors are used for medium-range
applications.
Fiber optics is not a scan technique, but another method
for transmitting light. Fiber optic sensors use a flexible
cable containing tiny fibers that channel light from emitter
to receiver, as illustrated in Figure 6-30. Fiber optic sensor
systems are completely immune to all forms of electrical
interference. The fact that an optical fiber does not contain
any moving parts and carries only light means that there is
no possibility of a spark. This means that it can be safely
used even in the most hazardous sensing environments
such as a refinery for producing gases, grain bins, mining,
pharmaceutical manufacturing, and chemical processing.
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Bar code technology is widely implemented in in-
dustry to enter data quickly and accurately. Bar code
scanners are the eyes of the data collection system. A
light source within the scanner illuminates the bar code
symbol; those bars absorb light, and spaces reflect light.
A photodetector collects this light in the form of an
electronic-signal pattern representing the printed sym-
bol. The decoder receives the signal from the scanner
and converts these data into the character data represen-
tation of the symbol’s code. Figure 6-31 illustrates a typ-
ical PLC application which involves a bar code module
reading the bar code on boxes as they move along a con-
veyor line. The PLC is then used to divert the boxes to
the appropriate product lines according to the data read
from the bar code.
Ultrasonic Sensors
An ultrasonic sensor operates by sending high-frequency
sound waves toward the target and measuring the time it
takes for the pulses to bounce back. The time taken for
this echo to return to the sensor is directly proportional to
the distance or height of the object because sound has a
constant velocity.
Figure 6-32 illustrates a practical application in which
the returning echo signal is electronically converted to a
4- to 20-mA output, which supplies a monitored flow rate
to external control devices. The operation of this process
• The 4-20 mA represents the sensor’s measurement
span.
• The 4-mA set point is typically placed near the bot-
tom of the empty tank, or the greatest measurement
distance from the sensor.
• The 20-mA set point is typically placed near the top
of the full tank, or the shortest measurement dis-
tance from the sensor.
• The sensor will proportionately generate a 4-mA
signal when the tank is empty and a 20-mA signal
when the tank is full.
• Ultrasonic sensors can detect solids, fluids, granular
objects, and textiles. In addition, they enable the de-
tection of different objects irrespective of color and
transparency and therefore are ideal for monitoring
transparent objects.
Strain/Weight Sensors
A strain gauge converts a mechanical strain into an elec-
tric signal. Strain gauges are based on the principle that
Figure 6-31 PLC bar code application.
Source: Courtesy Keyence Canada, Inc.
Programmable
controller
Scanner/decoder
Diverter
Figure 6-32 Ultrasonic sensor.
Source: Courtesy Keyence Canada, Inc.
Detecting transparent bottles
Detecting the level
of chocolate
4
5
5
10
15
20
25
30
30
20
Level detection
Output
(mA)
Inches
Inches
4- to 20-mA
output
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the resistance of a conductor varies with length and cross-
sectional area. The force applied to the gauge causes the
gauge to bend. This bending action also distorts the physical
size of the gauge, which in turn changes its resistance. This
resistance change is fed to a bridge circuit that detects small
changes in the gauge’s resistance. Strain gauge load cells
are usually made with steel and sensitive strain gauges. As
the load cell is loaded, the metal elongates or compresses
very slightly. The strain gauge detects this movement and
translates it to a varying voltage signal. Many sizes and
shapes of load cells are available, and they range in sensitiv-
ity from grams to millions of pounds. Strain gauge–based
load cells are used extensively for industrial weighing ap-
plications similar to the one illustrated in Figure 6-33.
Temperature Sensors
The thermocouple is the most widely used temperature
sensor. Thermocouples operate on the principle that when
two dissimilar metals are joined, a predictable DC volt-
age will be generated that relates to the difference in tem-
perature between the hot junction and the cold junction
(Figure 6-34). The hot junction (measuring junction) is
the joined end of a thermocouple that is exposed to the
process where the temperature measurement is desired.
The cold junction (reference junction) is the end of a ther-
mocouple that is kept at a constant temperature to provide
a reference point. For example, a K-type thermocouple,
when heated to a temperature of 300°C at the hot junc-
tion, will produce 12.2 mV at the cold junction. Because
of their ruggedness and wide temperature range, thermo-
couples are used in industry to monitor and control oven
and furnace temperatures.
Flow Measurement
Many industrial processes depend on accurate measure-
ment of fluid flow. Although there are several ways to
measure fluid flow, the usual approach is to convert the
kinetic energy that the fluid has into some other measur-
able form.
Turbine-type flowmeters are a popular means of mea-
surement and control of liquid products in industrial,
chemical, and petroleum operations. Turbine flowme-
ters, like windmills, utilize their angular velocity (rota-
tion speed) to indicate the flow velocity. The operation
of a turbine flowmeter is illustrated in Figure 6-35. Its
basic construction consists of a bladed turbine rotor in-
stalled in a flow tube. The bladed rotor rotates on its axis
in proportion to the rate of the liquid flow through the
tube. A magnetic pickup sensor is positioned as close
to the rotor as practical. Fluid passing through the flow
tube causes the rotor to rotate, which generates pulses
in the pickup coil. The frequency of the pulses is then
transmitted to readout electronics and displayed as gal-
lons per minute.
Programmable controller
Hopper
ON/OFF
control
Drum
Load cell
PLC sensor
input
560 lbs.
Figure 6-33 Strain gauge load cell.
Source: Courtesy RDP Group.
Figure 6-34 Thermocouple temperature sensor.
Chromel
(nickel-chromium)
⫹
⫺
12.2 mV
300⬚C
Alumel (nickel-aluminum)
type K thermocouple
Metal A
Metal B
Cold
junction
Hot
junction
HEAT
Leads
Figure 6-35 Turbine type ﬂowmeter.
Gallons/minute
Generated pulses
Readout
electronics
Magnetic pickup
Flow
Rotating
turbine
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Velocity and Position Sensors
Tachometer generators provide a convenient means of
converting rotational speed into an analog voltage sig-
nal that can be used for motor speed indication and con-
trol applications. A tachometer generator is a small AC
or DC generator that develops an output voltage (pro-
portional to its rpm) whose phase or polarity depends
on the rotor’s direction of rotation. The DC tachometer
generator usually has permanent magnetic field excita-
tion. The AC tachometer generator field is excited by
a constant AC supply. In either case, the rotor of the
tachometer is mechanically connected, directly or indi-
rectly, to the load.
Figure 6-36 illustrates motor speed control applications
in which a tachometer generator is used to provide a feed-
back voltage to the motor controller that is proportional to
motor speed. The control motor and tachometer generator
may be contained in the same or separate housings.
Figure 6-36 Tachometer generator feedback.
Source: Courtesy ATC Digitec.
Load
Tach
Motor
1450 rpm
CONTROLLER
Separate
tachometer
generator
Figure 6-37 Optical encoder.
Source: Photo courtesy Avtron, www.avtron.com.
Optical
encoder
Optical
sensor
Light
source
Optical
disk
Lines
An encoder is used to convert linear or rotary motion
into a binary digital signal. Encoders are used in appli-
cations where positions have to be precisely determined.
The optical encoder illustrated in Figure 6-37 uses a light
source shining on an optical disk with lines or slots that
interrupt the beam of light to an optical sensor. An elec-
tronic circuit counts the interruptions of the beam and
generates the encoder’s digital output pulses.
6.7 Output Control Devices
A variety of output control devices can be operated
by the PLC output to control traditional industrial
processes. These devices include pilot lights, control
relays, motor starters, alarms, heaters, solenoids, sole-
noid valves, small motors, and horns. Similar electri-
cal symbols are used to represent these devices both on
relay schematics and PLC output connection diagrams.
Figure 6-38 shows common electrical symbols used
Figure 6-38 Symbols for output control devices.
Pilot light
Relay
Motor starter coil
Motor overload relay contact
Heater
Solenoid
Solenoid valve
Motor
Horn
Alarm
CR1-1 CR1-2
NO NC
OL
ALARM
PL
M
SV
CR1
MTR
HTR
SOL
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Figure 6-39 Solenoid construction and operation.
Source: Photos courtesy Guardian Electric, www.guardian-electric.com.
Air
Frame
Coil Plunger
Symbol
DC solenoid
AC solenoid
Figure 6-40 Solenoid valve construction and operation.
Source: Photo courtesy ASCO Valve Inc., www.ascovalve.com.
L1 L2
Control circuit
Solenoid coil
energized
L1 L2
Control circuit
Solenoid coil
de-energized
Valve orifice opened
Outlet
Inlet
Coil
de-energized
Valve orifice closed
Coil
energized
Valve
Solenoid operator
Figure 6-40 illustrates the construction and principle
of operation of a typical fluid solenoid valve. Its operation
• The valve body contains an orifice in which a disk
or plug is positioned to restrict or allow flow.
• Flow through the orifice is either restricted or al-
lowed depending on whether the solenoid coil is
energized or de-energized.
• When the coil is energized, the core is drawn into
the solenoid coil to open the valve.
• The spring returns the valve to its original closed
position when the current coil is de-energized.
• A valve must be installed with direction of flow in
accordance with the arrow cast on the side of the
valve body.
Stepper motors operate differently than standard types,
which rotate continuously when voltage is applied to their
terminals. The shaft of a stepper motor rotates in discrete
increments when electrical command pulses are applied
to it in the proper sequence. Every revolution is divided
into a number of steps, and the motor must be sent a volt-
age pulse for each step. The amount of rotation is directly
for various output devices. Although these symbols are
generally acceptable, some differences among manu-
facturers do exist.
An actuator, in the electrical sense, is any device
that converts an electrical signal into mechanical move-
ment. An electromechanical solenoid is an actuator that
uses electrical energy to magnetically cause mechani-
cal control action. A solenoid consists of a coil, frame,
and plunger (or armature, as it is sometimes called).
Figure 6-39 shows the basic construction and opera-
tion of a solenoid. Its operation can be summarized as
follows:
• The coil and frame form the fixed part.
• When the coil is energized, it produces a magnetic
field that attracts the plunger, pulling it into the
frame and thus creating mechanical motion.
• When the coil is de-energized the plunger returns
to its normal position through gravity or assistance
from spring assemblies within the solenoid.
• The frame and plunger of an AC-operated solenoid
are constructed with laminated pieces instead of a
solid piece of iron to limit eddy currents induced by
the magnetic field.
Solenoid valves are electromechanical devices that
work by passing an electrical current through a solenoid,
thereby changing the state of the valve. Normally, there is
a mechanical element, which is often a spring, that holds
the valve in its default position.A solenoid valve is a com-
bination of a solenoid coil operator and valve, which con-
trols the flow of liquids, gases, steam, and other media.
When electrically energized, they open, shut off, or direct
the flow of media.
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proportional to the number of pulses, and the speed of
rotation is relative to the frequency of those pulses. A
1-degree-per-step motor will require 360 pulses to move
through one revolution; the degrees per step are known as
the resolution. When stopped, a stepper motor inherently
holds its position. Stepper systems are used most often
in “open-loop” control systems, where the controller tells
the motor only how many steps to move and how fast to
move, but does not have any way of knowing what posi-
tion the motor is at.
The movement created by each pulse is precise
and repeatable, which is why stepper motors are so
effective for load-positioning applications. Conver-
sion of rotary to linear motion inside a linear actua-
tor is accomplished through a threaded nut and lead
screw. Generally, stepper motors produce less than 1 hp
and are therefore frequently used in low-power posi-
tion control applications. Figure 6-41 shows a stepper
motor/drive unit along with typical rotary and linear
applications.
All servo motors operate in closed-loop mode,
whereas most stepper motors operate in open-loop
mode. Closed-loop and open-loop control schemes are
illustrated in Figure 6-42. Open loop is control with-
out feedback, for example, when the controller tells the
stepper motor how many steps to move and how fast to
move, but does not verify where the motor is. Closed-
loop control compares speed or position feedback with
the commanded speed or position and generates a modi-
fied command to make the error smaller. The error is the
difference between the required speed or position and
the actual speed or position.
Figure 6-43 illustrates a closed-loop servo motor sys-
tem. The motor controller directs operation of the servo
motor by sending speed or position command signals to
the amplifier, which drives the servo motor. A feedback
device such as an encoder for position and a tachometer
for speed are either incorporated within the servo motor
or are remotely mounted, often on the load itself. These
provide the servo motor’s position and speed feedback
information that the controller compares to its pro-
grammed motion profile and uses to alter its position
or speed.
6.8 Seal-In Circuits
Seal-in, or holding, circuits are very common in both
relay logic and PLC logic. Essentially, a seal-in circuit is
a method of maintaining current flow after a momentary
switch has been pressed and released. In these types of
circuits, the seal-in contact is usually in parallel with the
momentary device.
Rotary application
Linear application
Stepper motor
Motor drive
Figure 6-41 Stepper motor/drive unit.
Source: Photos courtesy Oriental Motor, www.orientalmotor.com.
Figure 6-42 Open- and closed-loop motor control systems.
Output
shaft
To load
Open-loop
control
Motor
CONTROLLER
Speed
setting
Motor
CONTROLLER
Output
shaft
To load
Closed-loop
control
Tachometer
Feedback signal
Speed
setting
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Motor/controller
Controller
Servo
amplifier
Feedback
device
Tachometer: speed
Encoder: position
Load
Position
feedback
Speed
feedback
Servo
motor
Figure 6-43 Closed-loop servo motor system.
Source: Photos courtesy Omron Industrial Automation, www.ia.omron.com.
Figure 6-44 Hardwired and programmed seal-in circuit.
Programmed
Motor
starter
coil
M
Start Stop
L1
L1
Inputs
L2
L2
Hardwired
M
Motor
starter coil
M
Seal-in contact
Start
Stop
M
Motor
starter coil
Stop
Start
Figure 6-45 Motor seal-in circuit implemented using an
Allen-Bradley Pico controller.
M
Motor
starter coil
Q1
Q1
Q1
L1
L1
L2
L2
I1 I2
I2 I1
Q2 Q3
Outputs
Inputs
Q3
Stop Start
The motor stop/start circuit shown in Figure 6-44 is a
typical example of a seal-in circuit. The hardwired circuit
consists of a normally closed stop button in series with a
normally open start button. The seal-in auxiliary contact
of the starter is connected in parallel with the start button
to keep the starter coil energized when the start button is
released. When this circuit is programmed into a PLC,
both the start and stop buttons are examined for a closed
condition because both buttons must be closed to cause
the motor starter to operate.
Figure 6-45 shows a PLC wiring diagram of the motor
seal-in circuit implemented using an Allen-Bradley Pico
controller. The controller is programmed using ladder
logic. Each programming element can be entered directly
via the Pico display. This controller also lets you program
the circuit from a personal computer using PicoSoft pro-
gramming software.
6.9 Latching Relays
Electromagnetic latching relays are designed to hold the
relay closed after power has been removed from the coil.
Latching relays are used where it is necessary for contacts
to stay open and/or closed even though the coil is ener-
gized only momentarily. Figure 6-46 shows a latching
relay that uses two coils. The latch coil is momentarily
energized to set the latch and hold the relay in the latched
position. The unlatch or release coil is momentarily
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OFF
ON
Latch coil
Unlatch coil
L L
In unlatch position
Relay contact
2
1
PL
L
U
Figure 6-47 Hardwired control circuit for an
electromagnetic latching relay.
Command Name Symbol Description
OTL
OTU Output
unlatch
Output latch
OTL sets the bit to
1 when the rung
becomes true and
retains its state when
the rung loses
continuity or a power
cycle occurs.
OTU resets the bit to
0 when the rung
becomes true and
retains it.
Latch coil
Unlatch coil
XXX
XXX
L
Same
address
U
U
L
Figure 6-48 Output latch and output unlatch instruction.
• When the ON button is momentarily actuated, the
latch coil is energized to set the relay to its latched
position.
• The contacts close, completing the circuit to the
pilot light, and so the light is switched on.
• The relay coil does not have to be continuously
energized to hold the contacts closed and keep the
light on.
• The only way to switch the lamp off is to actu-
ate the OFF button, which will energize the
unlatch coil and return the contacts to their open,
unlatched state.
• In cases of power loss, the relay will remain in its
original latched or unlatched state when power is
restored.
An electromagnetic latching relay function can be
programmed on a PLC to work like its real-world coun-
terparts. The instruction set for the SLC 500 includes
a set of output instructions that duplicates the opera-
tion of the mechanical latch. A description of the out-
put latch (OTL) and output unlatch (OTU) instruction
is given in Figure 6-48. The OTL and OTU instructions
differ from the OTE instruction in that they must be
used together. Both the latch and unlatch outputs must
have the same address. The OTL (latch) instruction can
only turn a bit on and the OTU (unlatch) instruction can
only turn a bit off.
The operation of the output latch and output unlatch
coil instruction is illustrated in the ladder program of Fig-
ure 6-49. The operation of the program can be summa-
rized as follows:
• Both the latch (L) and the unlatch (U) coil have the
same address (O:2/5).
• When the on pushbutton (I:1/0) is momentarily
actuated, the latch rung becomes true and the latch
status bit (O:2/5) is set to 1, and so the light output
is switched on.
energized to disengage the mechanical latch and return
the relay to the unlatched position.
Figure 6-47 shows a hardwired control circuit for an
electromagnetic latching relay. The operation of the cir-
cuit can be summarized as follows:
• The contact is shown with the relay in the unlatched
position.
• In this state the circuit to the pilot light is open and
so the light is off.
Figure 6-46 Two-coil mechanical latching relay.
Source: Courtesy Relay Service Company.
Latch
mechanism
L
U
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Status Indicating Lights—Water pump running light
(green)
- Low water level status light (red)
- High water level status light (yellow)
Figure 6-51 shows a program that can be used to imple-
ment control of the water level in the storage tank. The
latch and unlatch instructions form part of the program.
The operation of the program can be summarized as
follows:
• An internal storage bit is used for the latch and ad-
dress rather than an actual discrete output address.
Both have the same addresses.
• The rung 1 Examine-on instruction addressed
to the off/on switch prevents the pump motor
from starting under any condition when in the off
(open) state.
• In the MAN mode, the rung 1 Examine-on instruc-
tion addressed to the low sensor switch allows the
pump motor to operate only when the low level sen-
sor switch is closed.
• In the AUTO mode, whenever the high sensor
switch is momentarily closed the Examine-on in-
struction of rung 1 addressed to it will energize the
latch coil. The pump will begin running and con-
tinue to operate until the unlatch coil is energized by
the rung 3 Examine-off instruction addressed to the
low sensor switch.
• The pump running status light is controlled by the
rung 4 Examine-on instruction addressed to the
motor output.
• The status bit will remain set to 1 when the pushbut-
ton is released and logical continuity of the latch
rung is lost.
• When the off pushbutton (I:1/1) is momentarily ac-
tuated, the unlatch rung becomes true and the latch
status bit (O:2/5) is reset back to 0 and so the light
is switched off.
• The status bit will remain reset to 0 when the push-
button is released and logical continuity of the latch
rung is lost.
Output latch is an output instruction with a bit-level
address. When the instruction is true, it sets a bit in the
output image file. It is a retentive instruction because the
bit remains set when the latch instruction goes false. In
most applications it is used with an unlatch instruction.
The output unlatch instruction is also an output instruc-
tion with a bit-level address. When the instruction is true,
it resets a bit in the output image file. It, too, is a retentive
instruction because the bit remains reset when the instruc-
tion goes false.
The process shown in Figure 6-50 is to be used to con-
trol the level of water in a storage tank by turning a dis-
charge pump on or off. The modes of operation are to be
programmed as follows:
OFF Position—The water pump will stop if it is run-
ning and will not start if it is stopped.
Manual Mode—The pump will start if the water in
the tank is at any level except low.
Automatic Mode—If the level of water in the tank
reaches a high point, the water pump will start so that
water can be removed from the tank, thus lowering
the level.
- When the water level reaches a low point, the pump
will stop.
Figure 6-49 Output latch and output unlatch operation.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Latch
rung
I:1/0
Unlatch
rung
I:1/1
I:1/0
O:2/5
O:2/
O:2/5
O:2/5
I:1/1
ON
Inputs Output
Program
L1 L2
OFF
PL
L
U
Status bit
Figure 6-50 Process used to control the level of water in a
storage tank.
High sensor switch
Low sensor switch
MAN/AUTO
Pump motor
ON/OFF
Pump
running
G
Low
level
R
High
level
Y
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• The low level status light is controlled by the rung
5 Examine-off instruction addressed to the low
sensor switch.
• The high level status light is controlled by the rung
6 Examine-on instruction addressed to the high
sensor switch.
Figure 6-52 shows a typical I/O module wiring dia-
gram and addressing format for the water level control
program implemented using an Allen-Bradley modu-
lar SLC 500 controller. The chassis power supply has a
relatively small power rating and is used to supply DC
power to all devices physically mounted in the backplane
of the PLC rack. In this application a 24 VDC field power
supply is used for the input devices and a 120 VAC field
power supply for the output devices. This allows a low-
voltage 24-volt control signal to control 240-volt output
devices. SLC 500 controllers use a rack/slot-based ad-
dress system where the slot location of the I/O modules
in the rack establishes the PLC address. The addresses
for the field devices of this particular application are
shown below:
L1
1
2
3
4
5
6
Input module
wiring connections
OFF ON
Auto
Man
Low sensor switch
L2
High sensor switch
Man/Auto Low sensor switch
Man/Auto
Man/Auto
Latch/Unlatch
OFF/ON Motor
Man/Auto High sensor switch
Latch coil
L
Man/Auto Low sensor switch
Unlatch coil
U
Motor G
Low sensor switch R
High sensor switch Y
Motor
Output module
wiring connection
Pump running
Low level
High level
M
G
R
Y
Figure 6-51 Program used to implement control of the water level in the storage tank.
FIELD DEVICE ADDRESS Signiﬁes
OFF/ON Switch I:2/0 The input module in slot 2 and screw terminal 0
MAN/AUTO Switch I:2/4 The input module in slot 2 and screw terminal 4
LOW SENSOR SWITCH I:2/8 The input module in slot 2 and screw terminal 8
HIGH SENSOR SWITCH I:2/12 The input module in slot 2 and screw terminal 12
MOTOR O:3/1 The output module in slot 3 and screw terminal 1
PUMP RUNNING Light O:3/5 The output module in slot 3 and screw terminal 5
LOW LEVEL Light O:3/9 The output module in slot 3 and screw terminal 9
HIGH LEVEL Light O:3/13 The output module in slot 3 and screw terminal 13
B3:0/0 Internal retentive bit instruction that does not drive a real-word device
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Low sensor switch OFF/ON Motor
Man/Auto
Man/Auto
Man/Auto
Man/Auto
Man/Auto
Motor G
R
Y
Low sensor switch
Low sensor switch
Unlatch coil
Latch/Unlatch
High sensor switch Latch coil
I:2/4 I:2/8
I:2/4 B3:0/0
I:2/4
I:2/4 I:2/8
I:2/4
O:3/1
I:2/8
O:3/5
O:3/9
B3:0/0
I:2/12 B3:0/0
I:2/0 O:3/1
I:2/12 O:3/13
Slots
0
High sensor switch
Input module
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
IN 0
IN 2
IN 4
IN 6
IN 8
IN 10
IN 12
IN 14
DC
COM
IN 1
IN 3
IN 5
IN 7
IN 9
IN 11
IN 13
IN 15
DC
COM
Output module
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
VAC
OUT 1
OUT 3
OUT 5
OUT 7
OUT 9
OUT 11
OUT 13
OUT 15
OUT 0
OUT 2
OUT 4
OUT 6
OUT 8
OUT 10
OUT 12
OUT 14
AC
COM
24 VDC
16 point discrete
input module
240 VAC
16 point discrete
output module
24 VDC
Field device
power supply
⫹DC ⫺DC
240 VAC
M
Field device
power supply
L2 L1
Power
supply
1 2 3 4 5 6
OFF ON
Man
Motor
Pump running
Low level
High level
Auto
Low sensor
switch
High sensor
switch
R
G
Y
L
U
Figure 6-52 Water level control program implemented using an Allen-Bradley modular SLC 500 controller.
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Automatic control involves maintaining a desired set
point at an output. One example is maintaining a certain
set-point temperature in a furnace as illustrated in Fig-
ure 6-55. If there is deviation from that set point, an error
is determined by comparing the output against the set point
and using this error to make a correction. This requires
feedback from the output to the control for the input.
The converting of a simple sequential process can be
examined with reference to the process flow diagram illus-
trated in Figure 6-56. The sequential task is as follows:
1. Start button is pressed.
2. Table motor is started.
Figure 6-53 Sequential control process.
1 - Filling operation 2 - Capping operation
Figure 6-54 Combination control process.
Label solenoid 1
Label
solenoid 2
6.
10 Converting Relay Schematics
into PLC Ladder Programs
The best approach to developing a PLC program from a
relay schematic is to understand first the operation of each
relay ladder rung.As each relay ladder rung is understood,
an equivalent PLC rung can be generated. This process
will require access to the relay schematic, documentation
of the various input and output devices used, and possibly
a process flow diagram of the operation.
Most control processes require the completion of several
operations to produce the required output. Manufacturing,
machining, assembling, packaging, finishing, or transport-
ing of products requires the precise coordination of tasks.
A sequential control process is required for processes
that demand that certain operations be performed in a spe-
cific order. Figure 6-53 illustrates part of a bottle filling
process. In the filling and capping operations, the tasks
are (1) fill bottle and (2) press on cap. These tasks must
be performed in the proper order. Obviously we could not
fill the bottle after the cap is pressed on. This process,
therefore, requires sequential control.
Combination controls require that certain operations
be performed without regard to the order in which they
are performed. Figure 6-54 illustrates another part of the
same bottle filling process. Here, the tasks are (1) place
label 1 on bottle and (2) place label 2 on bottle. The order
in which the tasks are performed does not really matter.
In fact, however, many industrial processes that are not in-
herently sequential in nature are performed in a sequential
manner for the most efficient order of operations.
Figure 6-55 Automatic control process.
PLC
Furnace
Feedback
Controller
Figure 6-56 Sequential process ﬂow diagram.
Package
Limit
switch
Motor
Pilot lights
Stop
Start
R
G
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Figure 6-58 shows an I/O connection diagram for a
programmed version of the sequential process. Each input
and output device is represented by its symbol and asso-
ciated address. These addresses will indicate what PLC
input is connected to what input device and what PLC
output will drive what output device. The address code, of
course, will depend on the PLC model used. This example
uses SLC 500 addressing for the process. Note that the
electromagnetic control relay CR is not needed because
its function is replaced by an internal PLC control relay.
The hardwired relay schematic for the sequential pro-
cess can be converted to the PLC ladder logic program
shown in Figure 6-59. In converting the process to a pro-
gram the operation of each rung must be understood. The
pushbuttons PB1, PB2 as well as limit switch LS are all
programmed using the examine-closed (–] [–) instruction
to produce the desired logic control. Also, internal relay
B3:1/0 is used to replace control relay CR. To obtain the
desired control logic, all internal relay contacts are pro-
grammed using the PLC contact instruction that matches
the coil de-energized state. The internal relay imple-
mented in software requires one coil address the contacts
of which can be examined for an ON or OFF condition as
many times as you like.
There is more than one method to correctly design the
ladder logic program for a given control process. In some
cases one arrangement may be more efficient in terms of
the amount of memory used and the time required to scan
the program. Figure 6-60 illustrates an example of an ar-
rangement of series instructions of a rung programmed
for optimum scan time. The series instructions are pro-
grammed from the most likely to be false (far left) to
the least likely to be false (far right). Once the processor
sees a false input instruction in series, the processor stops
checking the rung at the false condition and sets the out-
put false.
Figure 6-61 illustrates an example of an arrangement
of parallel instructions of a rung programmed for opti-
mum scan time. The parallel path that is most often true is
3. Package moves to the position of the limit switch
and automatically stops.
Other auxiliary features include:
• A stop button that will stop the table, for any reason,
before the package reaches the limit switch position
• A red pilot light to indicate the table is stopped
• A green pilot light to indicate the table is running
A relay schematic for the sequential process is shown
in Figure 6-57. The operation of this hardwired circuit can
be summarized as follows:
• Start button is actuated; CR is energized if stop but-
ton and limit switch are not actuated.
• Contact CR-1 closes, sealing in CR when the start
button is released.
• Contact CR-2 opens, switching the red pilot light
from on to off.
• Contact CR-3 closes, switching the green pilot light
from off to on.
• Contact CR-4 closes to energize the motor starter
coil, starting the motor and moving the package to-
ward the limit switch.
• Limit switch is actuated, de-energizing relay
coil CR.
• Contact CR-1 opens, opening the seal-in circuit.
• Contact CR-2 closes, switching the red pilot light
from off to on.
• Contact CR-3 opens, switching the green pilot light
from on to off.
• Contact CR-4 opens, de-energizing the motor starter
coil to stop the motor and end the sequence.
Figure 6-57 Relay schematic for the sequential process.
PB1
Start
L1 L2
PB2
Stop
LS
Limit
switch
Control
relay
CR-2
CR-3
CR-4
CR-1
OL
PL1
Stop
PL2
Run
Motor
starter coil
R
G
CR
M
O:4/2
Outputs
Inputs
LS - Limit switch
L2
L1 L2
L1
PB1 - Start
Motor starter
coil
O:4/1
PL1 - Stop
I:3/0
I:3/2
I:3/1
PB1 - Stop
R
M
O:4/3
PL2 - Run
G
Figure 6-58 I/O connection diagram.
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placed on the top of the rung. The processor will not look
at the others unless the top path is false.
Figure 6-62 shows a hardwired jog control circuit that
incorporates a jog control relay. The operation of the cir-
cuit can be summarized as follows:
• Pressing the start pushbutton completes a circuit for
the CR coil, closing the CR1 and CR2 contacts.
• The CR1 contact completes the circuit for the M
coil, starting the motor.
• The M maintaining contact closes; this maintains
the circuit for the M coil.
• Pressing the jog button energizes the M coil only,
starting the motor. Both CR contacts remain open,
and the CR coil is de-energized. The M coil will
not remain energized when the jog push button is
released.
Figure 6-63 shows a PLC program equivalent of the
hardwired relay jog circuit. Note that the function of the
control relay is now accomplished using an internal PLC
instruction (B3:1/0).
Figure 6-59 Sequential process PLC ladder logic program.
L1
Input module
wiring connections
PB1
PB2
L2
LS
I:3/2
I:3/1
I:3/0
I:3/0
B3:1/0
B3:1/0
B3:1/0
B3:1/0
I:3/1
I:3/2 B3:1/0
PB1 PB2 LS
Internal
relay
O:4/2
PL1
O:4/3
PL2
O:4/1
M
O:4/1
O:4/2
Output module
wiring connection
PL1
O:4/3
PL2
OL
R
M
G
Instruction most
likely to be FALSE
Instruction least
likely to be FALSE
Figure 6-60 Series instructions programmed for optimum
scan time.
Path most likely to be TRUE
Less likely
Least likely
Figure 6-61 Parallel instructions programmed for
optimum scan time.
OL
Control relay
A B
H1 H3 H2 H4
X1 X2
120 V
Start
Jog
CR2
CR1
M
Stop
CR
M
Figure 6-62 Jog circuit with control relay.
Source: Photo courtesy IDEC Corporation, www.IDEC.com/usa, RR Relay.
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I:3/0
B3:1/0
I:3/1 B3:1/0
I:3/2
B3:1/0
I:3/1 O:2/2
O:2/2
Start
L1 L2
Inputs Output
Stop
Internal
relay
Jog
Jog
Stop M
I:3/2
Start
Stop
I:3/0
I:3/1
M
OL
Figure 6-63 PLC program equivalent of the hardwired relay jog circuit.
6.
11 Writing a Ladder Logic Program
Directly from a Narrative Description
In most cases, it is possible to prepare a ladder logic program
directly from the narrative description of a control process.
Some of the steps in planning a program are as follows:
• Define the process to be controlled.
• Draw a sketch of the process, including all sensors
and manual controls needed to carry out the control
sequence.
• List the sequence of operational steps in as much
detail as possible.
• Write the ladder logic program to be used as a basis
for the PLC program.
• Consider different scenarios where the process se-
quence may go astray and make adjustments as needed.
• Consider the safety of operating personnel and
make adjustments as needed.
The following are examples of ladder logic programs
derived from narrative descriptions of control processes.
Figure 6-64 shows the sketch of a drilling process that
requires the drill press to turn on only if there is a part
present and the operator has one hand on each of the
start switches. This precaution will ensure that the opera-
tor’s hands are not in the way of the drill.
The sequence of operation requires that switches 1
and 2 and the part sensor all be activated to make the
drill motor operate. Figure 6-65 shows the ladder logic
program required for the process implemented using an
SLC 500 controller.
E X A M P L E 6 - 1
L1
Inputs
Ι:3/5
Sensor
Ι:3/6
Ι:3/4
Motor
contactor
PB1 PB1 Sensor
Motor
contactor
PB1
PB2
Ι:3/4 Ι:3/5 Ι:3/6 O:4/0
O:4/0
L2
Output
M
Figure 6-65 Drilling process PLC program.
PB1
Drill
motor
Switches
PB2
Part sensor
Figure 6-64 Sketch of the drilling process.
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A motorized overhead garage door is to be operated au-
tomatically to preset open and closed positions. The ﬁeld
devices include one of each of the following:
• Reversing motor contactor for the up and down
directions.
• Normally closed down limit switch to sense when the
door is fully closed.
• Normally closed up limit switch to sense when the
door is fully opened.
• Normally open door up button for the up direction.
• Normally open door down button for the down
direction.
• Normally closed door stop button for stopping the
door.
• Red door ajar light to signal when the door is partially
open.
• Green door open light to signal when the door is fully
open.
• Yellow door closed light to signal when the door is fully
closed.
The sequence of operation requires that:
• When the up button is pushed, the up motor contac-
tor energizes and the door travels upward until the up
limit switch is actuated.
• When the down button is pushed, the down motor
contactor energizes and the door travels down until
the down limit switch is actuated.
• When the stop button is pushed, the motor stops.
The motor must be stopped before it can change
direction.
Figure 6-66 shows the ladder logic program required for
the operation implemented using an SLC 500 controller.
E X A M P L E 6 - 2
Figure 6-66 Motorized overhead garage door PLC program.
L1
Input devices
(shown in unactuated
condition)
Output devices
Up limit
L2
Door up
Ι:3/0 Ι:3/7
Program
O:4/3 O:4/0
Door ajar
Door down
Stop door
Ι:3/4 Ι:3/5
Down limit
Ι:3/1
Ι:3/4
Ι:3/5
Ι:3/7
O:4/4 Ι:3/0
Stop Motor up
Up
Ι:3/0 O:4/0
Ι:3/1
Up limit Door ajar
Down limit
Ι:3/0 O:4/1
Up limit Door open
O:4/3
Motor up
Down
Down
Intrlock Up limit
O:4/4
Ι:3/1 O:4/2
Down limit Door closed
Motor down
O:4/4
Motor down
Ι:3/7 Ι:3/5 Ι:3/4 O:4/3 Ι:3/1
Stop Down Up
Up
intrlock Down limit
O:4/1
Door open
O:4/2
Door
closed
O:4/3
Motor
contactor
O:4/4
Motor
contactor
R
G
Y
UP
DN
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Figure 6-67 shows the sketch of a continuous filling opera-
tion. This process requires that boxes moving on a con-
veyor be automatically positioned and filled.
The sequence of operation for the continuous filling op-
eration is as follows:
• Start the conveyor when the start button is momen-
tarily pressed.
• Stop the conveyor when the stop button is momen-
tarily pressed.
• Energize the run status light when the process is
operating.
• Energize the standby status light when the process is
stopped.
• Stop the conveyor when the right edge of the box is
first sensed by the photosensor.
• With the box in position and the conveyor stopped,
open the solenoid valve and allow the box to fill. Filling
should stop when the level sensor goes true.
• Energize the full light when the box is full. The full light
should remain energized until the box is moved clear
of the photosensor.
Figure 6-68 shows the ladder logic program required for
the operation.
E X A M P L E 6 - 3
Figure 6-67 Sketch of the continuous filling operation.
Run
Standby
PL
PL
Full
Level
switch
Photo
switch
Motor
Start
Solenoid
Hopper
Stop
PL
L1
Stop
Start
Photo
L2
Level
Stop Start
Run
Run
Run Standby
Outputs
Inputs
Motor
Solenoid
Run
Standby
Full
Level Photo
Full
Full
Photo Run
Level Photo
Motor
Run Solenoid
Full
Full
Figure 6-68 Continuous filling operation PLC program.
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1. Explain the basic operating principle of an electro-
magnetic control relay.
2. What is the operating difference between a nor-
mally open and a normally closed relay contact?
3. In what ways are control relay coils and contacts
rated?
4. How do contactors differ from relays?
5. What is the main difference between a contactor
and a magnetic motor starter?
6. a. Draw the schematic for an across-the-line AC
magnetic motor starter.
b. With reference to this schematic, explain the
function of each of the following parts:
i. Main contact M
ii. Control contact M
iii. Starter coil M
iv. OL relay coils
v. OL relay contact
7. The current requirement for the control circuit of a
magnetic starter is normally much smaller than that
required by the power circuit. Why?
8. Compare the method of operation of each of the
following types of switches:
a. Manually operated switch
b. Mechanically operated switch
c. Proximity switch
9. What do the abbreviations NO and NC represent
when used to describe switch contacts?
10. Draw the electrical symbol used to represent each
of the following switches:
a. NO pushbutton switch
b. NC pushbutton switch
c. Break-make pushbutton switch
d. Three-position selector switch
e. NO limit switch
f. NC temperature switch
g. NO pressure switch
h. NC level switch
i. NO proximity switch
11. Outline the method used to actuate inductive and
capacitive proximity sensors.
12. How are reed switch sensors actuated?
13. Compare the operation of a photovoltaic solar cell
with that of a photoconductive cell.
14. What are the two basic components of a photoelec-
tric sensor?
15. Compare the operation of the reflective-type and
through-beam photoelectric sensors.
16. Give an explanation of how a scanner and a de-
coder act in conjunction with each other to read a
bar code.
17. How does an ultrasonic sensor operate?
18. Explain the principle of operation of a strain gauge.
19. Explain the principle of operation of a thermocouple.
20. What is the most common approach taken with re-
gard to the measurement of fluid flow?
21. Explain how a tachometer is used to measure rota-
tional speed.
22. How does an optical encoder work?
23. Draw an electrical symbol used to represent each of
the following PLC output control devices:
a. Pilot light
b. Relay
c. Motor starter coil
d. OL relay contact
e. Alarm
f. Heater
g. Solenoid
h. Solenoid valve
i. Motor
j. Horn
24. Explain the function of each of the following
actuators:
a. Solenoid
b. Solenoid valve
c. Stepper motor
25. Compare the operation of open-loop and closed-
loop control.
26. What is a seal-in circuit?
27. In what is the construction and operation of an
electromechanical latching relay different from a
standard relay?
28. Give a short description of each of the following
control processes:
a. Sequential
b. Combination
c. Automatic
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1. Design and draw the schematic for a conventional
hardwired relay circuit that will perform each of
the following circuit functions when a normally
closed pushbutton is pressed:
• Switch a pilot light on
• De-energize a solenoid
• Start a motor running
• Sound a horn
2. Design and draw the schematic for a conven-
tional hardwired circuit that will perform the fol-
lowing circuit functions using two break-make
pushbuttons:
• Turn on light L1 when pushbutton PB1 is pressed.
• Turn on light L2 when pushbutton PB2 is pressed.
• Electrically interlock the pushbuttons so that L1
and L2 cannot both be turned on at the same time.
3. Study the ladder logic program in Figure 6-69, and
answer the questions that follow:
a. Under what condition will the latch rung 1 be true?
b. Under what conditions will the unlatch rung 2 be
true?
c. Under what condition will rung 3 be true?
d. When PL1 is on, the relay is in what state
(latched or unlatched)?
e. When PL2 is on, the relay is in what state
(latched or unlatched)?
f. If AC power is removed and then restored to the
circuit, what pilot light will automatically come
on when the power is restored?
g. Assume the relay is in its latched state and all three
inputs are false. What input change(s) must occur
for the relay to switch into its unlatched state?
h. If the examine if closed instructions at addresses
I/1, I/2, and I/3 are all true, what state will the
relay remain in (latched or unlatched)?
4. Design a PLC program and prepare a typical I/O
connection diagram and ladder logic program that
will correctly execute the hardwired control circuit
in Figure 6-70.
Assume: Stop pushbutton used is an NO type.
Run pushbutton used is an NO type.
Jog pushbutton used has one set of NO
contacts.
OL contact is hardwired.
in Figure 6-71.
CHAPTER 6 PROBLEMS
Figure 6-69 Ladder logic program for Problem 3.
I/1
L1
Inputs Ladder logic program Outputs
L2
I/2
I/3
I/1 I/2 O/9
L
I/3 O/9
U
O/9 O/10
Rung 1
Rung 2
Rung 3
PL2
PL1
O/10
O/9
Figure 6-70 Hardwired control circuit for Problem 4.
L
L
Stop
Run
OL
Jog M
2
1
M
Start
CR1
SOL
CR1-2
CR1-1
PS1
LS1
SS1
CR2-2
CR2-1
CR2
SOL
SOL
L
L
1
2
3
2
1
2
1
Stop
PB
PB
PL1
PL2
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Assume: PB1 pushbutton used is an NO type.
PB2 pushbutton used is an NC type.
PS1 pressure switch used is an NO type.
LS1 limit switch used has only one set of
NC contacts.
in Figure 6-72.
Assume: PB1 pushbutton used is an NC type.
PB2 and PB3 are each wired using one set
of NO contacts.
OL contact is hardwired.
connection diagram and ladder logic program for
the following motor control specifications:
• A motor must be started and stopped from any
one of three start/stop pushbutton stations.
• Each start/stop station contains one NO start
pushbutton and one NC stop pushbutton.
• Motor OL contacts are to be hardwired.
the following motor control specifications:
• Three starters are to be wired so that each starter
is operated from its own start/stop pushbutton
station.
• A master stop station is to be included that will
trip out all starters when pushed.
• Overload relay contacts are to be programmed so
that an overload on any one of the starters will
automatically drop all of the starters.
• All pushbuttons are to be wired using one set of
NO contacts.
9. A temperature control system consists of four
thermostats controlling three heating units. The
thermostat contacts are set to close at 50°, 60°,
70°, and 80°F, respectively. The PLC ladder logic
program is to be designed so that at a temperature
below 50°F, three heaters are to be ON. Between
50° to 60°F, two heaters are to be ON. For 60° to
70°F, one heater is to be ON. Above 80°F, there is a
safety shutoff for all three heaters in case one stays
on because of a malfunction. A master switch is to
be used to turn the system ON and OFF. Prepare a
typical PLC program for this control process.
10. A pump is to be used to fill two storage tanks. The
pump is manually started by the operator from a
start/stop station. When the first tank is full, the
control logic must be able to automatically stop
flow to the first tank and direct flow to the second
tank through the use of sensors and electric sole-
noid valves. When the second tank is full, the pump
must shut down automatically. Indicator lamps are
to be included to signal when each tank is full.
a. Draw a sketch of the process.
b. Prepare a typical PLC program for this control
process.
11. Write the optimum ladder logic rung for each of the
following scenarios, and arrange the instructions
for optimum performance:
a. If limit switches LS1 or LS2 or LS3 are on, or if
LS5 and LS7 are on, turn on; otherwise, turn off.
(Commonly, if LS5 and LS7 are on, the other
conditions rarely occur.)
b. Turn on an output when switches SW6, SW7,
and SW8 are all on, or when SW55 is on.
(SW55 is an indication of an alarm state, so it
is rarely on; SW7 is on most often, then SW8,
then SW6.)
Stop
PB
REV
FWD
PB
PB
R-1
F
OL
PL1
F-2
R-2
PL2
L L
F-1
R
2
3
2
1
1
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125
7
Programming Timers
The most commonly used PLC instruction, after
coils and contacts, is the timer. This chapter
deals with how timers time intervals and the way
in which they can control outputs. We discuss
the basic PLC on-delay timer function, as well as
other timing functions derived from it, and typical
industrial timing tasks.
Chapter Objectives
7.1 Describe the operation of pneumatic on-delay and
off-delay timers
7.2 Describe PLC timer instruction and differentiate
between a nonretentive and retentive timer
7.3 Convert fundamental timer relay schematic diagrams
to PLC ladder logic programs
7.4 Analyze and interpret typical PLC timer ladder logic
programs
7.5 Program the control of outputs using the timer
instruction control bits
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126 Chapter 7 Programming Timers
Figure 7-2 shows the construction of an on-delay pneu-
matic (air) timer. The time-delay function depends on
the transfer of air through a restricted orifice. The time-
delay period is adjusted by positioning the needle valve
to vary the amount of orifice restriction. When the coil
is energized, the timed contacts are delayed from open-
ing or closing. However, when the coil is de-energized,
the timed contacts return instantaneously to their normal
state. This particular pneumatic timer has instantaneous
contacts in addition to timed contacts. The instantaneous
contacts change state as soon as the timer coil is pow-
ered while the delayed contacts change state at the end
of the time delay. Instantaneous contacts are often used
as holding or sealing contacts in a control circuit.
Mechanical timing relays provide time delay through
two arrangements. The first arrangement, on delay, pro-
vides time delay when the relay coil is energized. The
second arrangement, off delay, provides time delay when
the relay coil is de-energized. Figure 7-3 illustrates the
different relay symbols used for timed contacts.
The on-delay timer is sometimes referred to as DOE,
which stands for delay on energize. The time delay of the
contacts begins once the timer is switched on; hence the term
on-delay timing. Figure 7-4 shows an on-delay timer circuit
that uses a normally open, timed closed (NOTC) contact.
The operation of the circuit can be summarized as follows:
• With S1 initially open, TD coil is de-energized so
TD1 contacts are open and light L1 will be off.
7.
1 Mechanical Timing Relays
There are very few industrial control systems that do not
need at least one or two timed functions. Mechanical tim-
ing relays are used to delay the opening or closing of con-
tacts for circuit control. The operation of a mechanical
timing relay is similar to that of a control relay, except that
certain of its contacts are designed to operate at a preset
time interval, after the coil is energized or de-energized.
Typical types of mechanical and electronic timing relays
are shown in Figure 7-1. Timers allow a multitude of op-
erations in a control circuit to be automatically started and
stopped at different time intervals.
Figure 7-1 Timing relays.
Solid-state timing relay Plug-in timing relay
Pneumatic timing relay
Figure 7-2 Pneumatic on-delay timer.
Time adjustment
Normally closed terminals
Normally closed terminals
Instantaneous contacts
Normally open terminals
Time control contacts
Normally open terminals
Operating coil
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Programming Timers Chapter 7 127
Figure 7-6 shows an off-delay timer circuit that uses a
normally open, timed open (NOTO) contact. The opera-
tion of the circuit can be summarized as follows:
TD1 contacts are open and light L1 will be off.
• When S1 is closed, TD coil is energized and TD1
contacts close instantly to switch light L1 on.
• When S1 is opened, TD coil is de-energized and the
timing period starts.
• After the 10 s time-delay period has elapsed,
TD1contacts open to switch the light off.
• When S1 is closed TD coil is energized and the tim-
ing period starts. TD1 contacts are delayed from
closing so L1 remains off.
• After the 10 s time-delay period has elapsed, TD1
contacts close and L1 is switched on.
• When S1 is opened, TD coil is de-energized and
TD1 contacts open instantly to switch L1 off.
Figure 7-5 shows an on-delay timer circuit that uses a
normally closed, timed open (NCTO) contact. The opera-
tion of the circuit can be summarized as follows:
• With S1 initially open, TD coil is de-energized
so TD1 contacts are closed and light L1 will
be on.
• When S1 is closed, TD coil is energized and the
timing period starts. TD1 contacts are delayed from
opening so L1 remains on.
contacts open and L1 is switched off.
• When S1 is opened, TD coil is de-energized and
TD1 contacts close instantly to switch L1 on.
Figure 7-3 Timed contact symbols.
On-delay symbols
Normally open, timed
closed contact (NOTC).
Contact is open when
relay coil is de-energized.
When relay is energized,
there is a time delay in
closing.
Normally closed, timed
open contact (NCTO).
Contact is closed when
relay coil is de-energized.
When relay is energized,
there is a time delay in
opening.
or
Normally closed, timed
closed contact (NCTC).
Contact is normally
closed when relay coil
is de-energized.
When relay coil is
energized, contact
opens instantly.
When relay coil is
de-energized, there is
a time delay before the
contact closes.
Off-delay symbols
Normally open, timed
open contact (NOTO).
Contact is normally
open when relay coil
is de-energized.
When relay coil is
energized, contact
closes instantly.
When relay coil is
de-energized, there is
a time delay before the
contact opens.
or
or or
Figure 7-4 On-delay timer circuit that uses a normally
open, timed closed (NOTC) contact.
L2
L1
TD1
L1
S1
10 s
Timing diagram
Input
(S1)
Output
(L1)
Off
On
TD
Figure 7-5 On-delay timer circuit that uses a normally
closed, timed open (NCTO) contact.
L1
TD1
L1
S1
TD
10 s
10 s
Timing diagram
Input
(S1)
Output
(L1) Off
On
L2
Figure 7-6 Off-delay timer circuit that uses a normally
open, timed open (NOTO) contact.
L2
L1
TD1
L1
S1
TD
10 s
Timing diagram
Input
(S1)
Output
(L1)
10 s
Off
On
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RTO (Retentive Timer On)—Counts time-based
intervals when the instruction is true and retains the
accumulated value when the instruction goes false or
when power cycle occurs.
RES (Reset)—Resets a retentive timer’s accumulated
value to zero.
Several quantities are associated with the timer
instruction:
• The preset time represents the time duration for
the timing circuit. For example, if a time delay
of 10 s is required, the timer will have a preset of
10 s.
• The accumulated time represents the amount of
time that has elapsed from the moment the timing
coil became energized.
• Every timer has a time base. Once the timing rung
has continuity, the timer counts in time-based in-
tervals and times until the preset value and accu-
mulated value are equal or, depending on the type
of controller, up to the maximum time interval of
the timer. The intervals that the timers time out at
are generally referred to as the time bases of the
timer. Timers can be programmed with several dif-
ferent time bases: 1 s, 0.1 s, and 0.01 s are typical
time bases. If a programmer entered 0.1 for the time
base and 50 for the number of delay increments, the
timer would have a 5-s delay (50 3 0.1 s 5 5 s).
The smaller the time base selected, the better the ac-
curacy of the timer.
Although each manufacturer may represent timers
differently on the ladder logic program, most timers
operate in a similar manner. One of the first methods used
depicts the timer instruction as a relay coil similar to that
of a mechanical timing relay. Figure 7-9 shows a coil-
formatted timer instruction. Its operation can be summa-
rized as follows:
• The timer is assigned an address and is identified as
a timer.
• Also included as part of the timer instruction is the
time base of the timer, the timer’s preset value or
time-delay period, and the accumulated value or
current time-delay period for the timer.
Figure 7-7 shows an off-delay timer circuit that uses a
normally closed, timed closed (NCTC) contact. The op-
eration of the circuit can be summarized as follows:
TD1 contacts are closed and light L1 will be on.
• When S1 is closed, TD coil is energized and TD1
contacts open instantly to switch light L1 off.
• When S1 is opened, TD coil is de-energized and the
timing period starts. TD1 contacts are delayed from
closing so L1 remains off.
contacts close to switch the light on.
7.2 Timer Instructions
PLC timers are instructions that provide the same func-
tions as on-delay and off-delay mechanical and electronic
timing relays. PLC timers offer several advantages over
their mechanical and electronic counterparts. These in-
clude the fact that:
• Time settings can be easily changed.
• The number of them used in a circuit can be in-
creased or decreased through the use of program-
ming changes rather than wiring changes.
• Timer accuracy and repeatability are extremely high
because its time delays are generated in the PLC
processor.
In general, there are three different PLC timer types: the
on-delay timer (TON), off-delay timer (TOF), and retentive
timer on (RTO). The most common is the on-delay timer,
which is the basic function. There are also many other tim-
ing configurations, all of which can be derived from one or
more of the basic time-delay functions. Figure 7-8 shows
the timer selection toolbar for the Allen-Bradley SLC 500
PLC and its associated RSLogix software. These timer
commands can be summarized as follows:
TON (Timer On Delay)—Counts time-based inter-
vals when the instruction is true.
TOF (Timer Off Delay)—Counts time-based inter-
vals when the instruction is false.
Figure 7-7 Off-delay timer circuit that uses a normally
closed, timed closed (NCTC) contact.
L2
L1
TD1
L1
S1
TD
10 s
Timing diagram
10 s
Input
Output
Off
On Figure 7-8 Timer selection toolbar.
TON TOF RTO CTU CTD RES HSC
User Bit Timer/Counter Input/Output Compare
HSC
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• The timer instruction block contains information
pertaining to the operation of the timer, including
the preset time, the time base of the timer, and the
current or accumulated time.
• All block-formatted timers provide at least one out-
put signal from the timer. The timer continuously
compares its current time with its preset time, and
its output is false (logic 0) as long as the current
time is less than the preset time. When the current
time equals the preset time, the output changes to
true (logic 1).
7.3 On-Delay Timer Instruction
Most timers are output instructions that are conditioned
by input instructions. An on-delay timer is used when
you want to program a time delay before an instruction
becomes true. Figure 7-11 illustrates the principle of op-
eration of an on-delay timer. Its operation can be summa-
rized as follows:
• The on-delay timer operates such that when the
rung containing the timer is true, the timer time-out
period commences.
• At the end of the timer time-out period, an output is
made true.
• The timed output becomes true sometime after the
timer rung becomes true; hence, the timer is said to
have an on-delay.
• The length of the time delay can be adjusted by
changing the preset value.
• In addition, some PLCs allow the option of chang-
ing the time base, or resolution, of the timer. As the
time base you select becomes smaller, the accuracy
of the timer increases.
• When the timer rung has logic continuity, the timer
begins counting time-based intervals and times until
the accumulated value equals the preset value.
• When the accumulated time equals the preset time,
the output is energized and the timed output contact
associated with the output is closed. The timed con-
tact can be used as many times as you wish through-
out the program as an NO or NC contact.
Timers are most often represented by boxes in ladder
logic. Figure 7-10 illustrates a generic block format for a
retentive timer that requires two input lines. Its operation
• The timer block has two input conditions associated
with it, namely, the control and reset.
• The control line controls the actual timing operation
of the timer. Whenever this line is true or power is
supplied to this input, the timer will time. Removal
of power from the control line input halts the further
timing of the timer.
• The reset line resets the timer’s accumulated value
to zero.
• Some manufacturers require that both the control and
reset lines be true for the timer to time; removal of
power from the reset input resets the timer to zero.
• Other manufacturers’ PLCs require power flow for
the control input only and no power flow on the
reset input for the timer to operate. For this type of
timer operation, the timer is reset whenever the reset
input is true.
Figure 7-9 Coil-formatted timer instruction.
TON
XXX
Timer address
Type of timer
Determines
rung continuity
Timer preset value
Time accumulated
or current value
YYY
0.1 s
000
PR:
TB:
AC:
Time
base of
timer
Figure 7-10 Block-formatted timer instruction.
Preset time
Time base
Accumulated time
Control line
Reset line
Output line
Figure 7-11 Principle of operation of an on-delay timer.
Preset value = accumulated value
Timed output bit
Timed period
Rung condition
Off (logic 0)
False
False
On (logic 1)
True
True
Timer
Input
On-delay
timed duration
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Figure 7-13 shows an example of the on-delay timer
instruction used as part of the Allen-Bradley PLC-5 and
SLC 500 controller instruction sets. The information to be
entered includes:
Timer number—This number must come from the
timer file. In the example shown, the timer number is
T4:0, which represents timer file 4, timer 0 in that file.
The timer address must be unique for this timer and
may not be used for any other timer.
Time base—The time base (which is always ex-
pressed in seconds) may be either 1.0 s or 0.01 s. In
the example shown, the time base is 1.0 s.
Preset value—In the example shown, the preset value
is 15. The timer preset value can range from 0 through
32,767.
Accumulated value—In the example shown, the ac-
cumulated value is 0. The timer’s accumulated value
normally is entered as 0, although it is possible to
enter a value from 0 through 32,767. Regardless of the
value that is preloaded, the timer value will become 0
whenever the timer is reset.
The on-delay timer (TON) is the most commonly used
timer. Figure 7-14 shows a PLC program that uses an on-
delay timer. The operation of the program can be sum-
marized as follows:
• The timer is activated by input switch A.
• The preset time for this timer is 10 s, at which time
output D will be energized.
• When input switch is A is closed, the timer becomes
true and the timer begins counting and counts until
the accumulated time equals the preset value; the
output D is then energized.
• If the switch is opened before the timer is timed out,
the accumulated time is automatically reset to 0.
• This timer configuration is termed nonretentive be-
cause any loss of continuity to the timer causes the
timer instruction to reset.
• This timing operation is that of an on-delay timer
because output D is switched on 10 s after the
switch has been actuated from the off to the on
position.
The Allen-Bradley SLC 500 timer file is file 4 (Fig-
ure 7-12). Each timer is composed of three 16-bit words,
collectively called a timer element. There can be up to
256 timer elements. Addresses for timer file 4, timer ele-
ment number 2 (T4:2), are listed below.
T4 5 timer file 4
:2 5 timer element number 2 (0–255 timer elements
per file)
T4:2/DN is the address for the done bit of the timer.
T4:2/TT is the address for the timer-timing bit of the
timer.
T4:2/EN is the address for the enable bit of the timer.
The control word uses the following three control bits:
Enable (EN) bit—The enable bit is true (has a status
of 1) whenever the timer instruction is true. When the
timer instruction is false, the enable bit is false (has a
status of 0).
Timer-timing (TT) bit—The timer-timing bit is true
whenever the accumulated value of the timer is chang-
ing, which means the timer is timing. When the timer
is not timing, the accumulated value is not changing,
so the timer-timing bit is false.
Done (DN) bit—The done bit changes state whenever
the accumulated value reaches the preset value. Its
state depends on the type of timer being used.
The preset value (PRE) word is the set point of the timer,
that is, the value up to which the timer will time. The preset
word has a range of 0 through 32,767 and is stored in binary
form. The preset will not store a negative number.
The accumulated value (ACC) word is the value that in-
crements as the timer is timing. The accumulated value will
stop incrementing when its value reaches the preset value.
The timer instruction also requires that you enter a time
base, which is either 1.0 s or 0.01 s. The actual preset time
interval is the time base multiplied by the value stored
in the timer’s preset word. The actual accumulated time
interval is the time base multiplied by the value stored in
the timer’s accumulated word.
Figure 7-12 SLC 500 timer ﬁle.
File number
Timers
4
File type Timer number
EN TT DN
Word 0
T4:2
15 14 13
Preset value
Word 1
Accumulated value
Word 2
T4:0
1.0
15
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
Figure 7-13 On-delay timer instruction.
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The timer uses three words per element. Each element
consists of a control word, a preset word, and an accumu-
lated word. Each word has 16 bits, which are numbered
from 0 to 15. When addressing to the bit level, the address
always refers to the bit within the word:
EN 5 Bit 15 enable
TT 5 Bit 14 timer timing
DN 5 Bit 13 done
Timers may or may not have an instantaneous output
(also known as the enable bit) signal associated with
them. If an instantaneous output signal is required from
a timer and it is not provided as part of the timer in-
struction, an equivalent instantaneous contact instruc-
tion can be programmed using an internally referenced
relay coil. Figure 7-17 shows an application of this tech-
nique. The operation of the program can be summarized
as follows:
• According to the hardwired relay circuit diagram,
coil M is to be energized 5 s after the start pushbut-
ton is pressed.
Figure 7-15 shows the timing diagram for the on-
delay timer’s control bits. The sequence of operation is
as follows:
• The first true period of the timer rung shows the
timer timing to 4 s and then going false.
• The timer resets, and both the timer-timing bit and
the enable bit go false. The accumulated value also
resets to 0.
• For the second true period input A remains true in
excess of 10 s.
• When the accumulated value reaches 10 s, the done
bit (DN) goes from false to true and the timer-
timing bit (TT) goes from true to false.
• When input A goes false, the timer instruction goes
false and also resets, at which time the control bits
are all reset and the accumulated value resets to 0.
The timer table for an Allen-Bradley SLC 500 is shown
in Figure 7-16. Addressing is done at three different lev-
els: the element level, the word level, and the bit level.
L1
Input A
Input A
T4:0
T4:0
T4:0
EN
TT
DN
Output C
TON
TIMER ON DELAY
Timer T4:0
Time base 1.0
Preset 10
Accumulated 0
L2
Output B
Output C
Output D
Output B
Output D
Ladder logic program Outputs
Input
G
R
Y
EN
DN
Figure 7-14 PLC on-delay timer program.
Figure 7-15 Timing diagram for an on-delay timer.
Input condition A
Timer-enable bit
Timer-done bit
On
On
On
On
Off
Off
Off
Off
Timer
accumulated
value
0
4 s 10 s
Timer-timing bit
Timer Table
/EN /TT /DN .PRE .ACC
T4:0 0 0 0 10 0
T4:1 0 0 0 0 0
T4:2 0 0 0 0 0
T4:3 0 0 0 0 0
T4:4 0 0 0 0 0
T4:5 0 0 0 0 0
Address T4:0 Table: T4: Timer
Figure 7-16 SLC 500 timer table.
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• The logic on the last rung is the same as the timer-
timing bit and as such can be used with timers that
do not have a timer-timing output.
Timers are often used as part of automatic sequential
control systems. Figure 7-19 shows how a series of mo-
tors can be started automatically with only one start/stop
control station. The operation of the circuit can be sum-
marized as follows:
• According to the relay ladder schematic, lube-oil
pump motor starter coil M1 is energized when the
start pushbutton PB2 is momentarily actuated.
• As a result, M1-1 control contact closes to seal in
M1, and the lube-oil pump motor starts.
• When the lube-oil pump builds up sufficient oil
pressure, the lube-oil pressure switch PS1 closes.
• This in turn energizes coil M2 to start the main
drive motor and energizes coil TD to begin the time-
delay period.
• After the preset time-delay period of 15 s, TD-1
contact closes to energize coil M3 and start the feed
motor.
• The ladder logic program shows how an equivalent
circuit could be programmed using a PLC.
• Contact TD-1 is the instantaneous contact, and con-
tact TD-2 is the timed contact.
• The ladder logic program shows that a contact in-
struction referenced to an internal relay is now used
to operate the timer.
• The instantaneous contact is referenced to the in-
ternal relay coil, whereas the time-delay contact is
referenced to the timer output coil.
Figure 7-18 shows an application for an on-delay timer
that uses an NCTO contact. This circuit is used as a warn-
ing signal when moving equipment, such as a conveyor
motor, is about to be started. The operation of the circuit
• According to the hardwired relay circuit diagram,
coil CR is energized when the start pushbutton PB1
is momentarily actuated.
• As a result, contact CR-1 closes to seal in CR coil,
contact CR-2 closes to energize timer coil TD,
and contact CR-3 closes to sound the horn.
• After a 10-s time-delay period, timer contact TD-1
opens to automatically switch the horn off.
• The ladder logic program shows how an equivalent
circuit could be programmed using a PLC.
Internal
relay
Output line
Motor
M
Internal
relay
Timer
PR: 5
TB: 1 s
L1
Inputs
L2
Output
Motor
Start
Start
M
Stop
Stop
Hardwired relay circuit
L2
L1
Stop
Start
TD-1
TD-2
(5 s)
M
TD
Figure 7-17 Instantaneous contact instruction can be programmed using an
internally referenced relay coil.
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this reason, this timer is also classified as
nonretentive.
Figure 7-21 illustrates the use of an off-delay timer
instruction used to switch motors off sequentially at
5 second intervals. The operation of the program can be
• Timer preset values for T4:1, T4:2, and T4:3 are set
for 5 s, 10s, and 15 s, respectively.
• Closing the input switch SW immediately sets the
done bit of each of the three off-delay timers to 1,
immediately turning on motors M1, M2, and M3.
• If SW is then opened, logic continuity to all three
timers is lost and each timer begins counting.
• Timer T4:1 times out after 5 s resetting its done bit
to zero to de-energize motor M1.
• Timer T4:2 times out 5 s later resetting its done bit
• Timer T4:3 times out 5 s later resetting its done bit
7.4 Off-Delay Timer Instruction
The off-delay timer (TOF) operation will keep the out-
put energized for a time period after the rung containing
the timer has gone false. Figure 7-20 illustrates the pro-
gramming of an off-delay timer that uses the SLC 500
TOF timer instruction. If logic continuity is lost, the
timer begins counting time-based intervals until the ac-
cumulated time equals the programmed preset value.
The operation of the circuit can be summarized as
follows:
• When the switch connected to input I:1/0 is first
closed, timed output O:2/1 is set to 1 immediately
and the lamp is switched on.
• If this switch is now opened, logic continuity is lost
and the timer begins counting.
• After 15 s, when the accumulated time equals the
preset time, the output is reset to 0 and the lamp
switches off.
• If logic continuity is gained before the timer is
timed out, the accumulated time is reset to 0. For
PB2
PB1
L2
Output
Horn
TON
TIMER ON DELAY
Timer T4:0
Time base 1.0
Preset 10
Accumulated 0
L1
Inputs
Start-up
Reset
PB1
PB2
T4:0
EN
Horn
T4:0 T4:0
DN EN
EN
DN
L2
L1
PB2
PB1
Reset
Start-up
CR-3
CR-1
CR-2
Horn
CR
TD
TD-1
(10 s)
Figure 7-18 Conveyor warning signal circuit.
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• As a result, timed contact TD-1 opens to de-
energize motor starter coil M1, timed contact
TD-2 closes to energize motor starter coil M2,
instantaneous contact TD-3 opens to switch the
green light off, and instantaneous contact TD-4
closes to switch the red light on. The circuit
remains in this state as long as limit switch LS1
is closed.
• When limit switch LS1 is opened, the off-delay
timer coil TD de-energizes and the time-delay
period is started.
Figure 7-22 shows how a hardwired off-delay timer
relay circuit with both instantaneous and timed con-
tacts. The operation of the circuit can be summarized as
follows:
• When power is first applied (limit switch LS open),
motor starter coil M1 is energized and the green
pilot light is on.
• At the same time, motor starter coil M2 is de-
energized, and the red pilot light is off.
• When limit switch LS closes, off-delay timer coil
TD energizes.
Figure 7-19 Automatic sequential control system.
OL
OL
Start
Stop
M1-1
(15 s)
Feed
motor
Main drive
motor
Lube oil
pump motor
(Lube oil
pressure switch)
TD-1
L2
L1
PB1
PB2
PS1
OL
M1
M2
TD
M3
PB2
PB1
PS1
L2
OL
Outputs
T4:0
1.0
15
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
M1
M1
M2
M3
T4:0
DN
PB2
PB1
PS1
L1
Inputs
OL
M3
OL
M2
M1
EN
DN
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Figure 7-20 Off-delay programmed timer.
O:2/1
L2
Output
I:1/0
L1
Input
S1
TOF
TIMER OFF DELAY
Timer T4:3
Time base 1.0
Preset 15
Accumulated 0
T4:3/DN O:2/1
I:1/0
EN
DN PL
S1 input
enable bit (EN)
True (logic 1)
True
False
False (logic 0)
Timed period timing bit (TT)
15 s
Off delay
timed duration
Timed output
done bit (DN)
Preset value = accumulated value
O:2/1
Figure 7-21 Program for switching motors off at 5 s intervals.
M1
M2
M3
L2
OL
Outputs
OL
OL
SW
SW
T4:1
5
0
TOF
TIMER OFF DELAY
Timer
Preset
Accumulated
T4:1/DN
T4:2/DN
T4:3/DN
M1
M2
M3
T4:2
10
0
TOF
TIMER OFF DELAY
Timer
Preset
Accumulated
T4:3
15
0
TOF
TIMER OFF DELAY
Timer
Preset
Accumulated
L1
Input
Switch EN
DN
EN
DN
EN
DN
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Figure 7-24 shows a program that uses both the on-
delay and the off-delay timer instruction. The process
involves pumping fluid from tank A to tank B. The opera-
tion of the process can be summarized as follows:
• Before starting, PS1 must be closed.
• When the start button is pushed, the pump starts.
The button can then be released and the pump con-
tinues to operate.
• When the stop button is pushed, the pump stops.
• PS2 and PS3 must be closed 5 s after the pump
starts. If either PS2 or PS3 opens, the pump will
shut off and will not be able to start again for an-
other 14 s.
7.5 Retentive Timer
A retentive timer accumulates time whenever the device
receives power, and it maintains the current time should
power be removed from the device. When the timer ac-
cumulates time equal to its preset value, the contacts of
the device change state. Loss of power to the timer after
reaching its preset value does not affect the state of the
contacts. The retentive timer must be intentionally reset
with a separate signal for the accumulated time to be reset
and for the contacts of the device to return to its nonener-
gized state.
Figure 7-25 illustrates the action of a motor-driven,
electromechanical retentive timer used in some appli-
ances. The shaft-mounted cam is driven by a motor. Once
power is applied, the motor starts turning the shaft and
• Instantaneous contact TD-3 closes to switch the
green light on, and instantaneous contact TD-4
opens to switch the red light off.
• After a 5-s time-delay period, timed contact TD-1
closes to energize motor starter M1, and timed con-
tact TD-2 opens to de-energize motor starter M2.
Figure 7-23 shows an equivalent PLC program of the
hardwired off-delay timer relay circuit containing both in-
stantaneous and timed contacts. The timer instruction car-
ries out all of the functions of the original physical timer.
Figure 7-22 Hardwired off-delay timer relay circuit with
both instantaneous and timed contacts.
TD-4
L2
5 s
TD-1 OL
TD-2 OL
TD-3
LS1
L1
R
G
M2
M1
TD
Figure 7-23 Equivalent PLC program of the hardwired off-delay timer relay circuit
containing both instantaneous and timed contacts.
L1 L2
LS1
M1
M2
G
R
Input Outputs
OL
OL
TOF
TIMER OFF DELAY
Timer T4:1
Time base 1.0
Preset 5
Accumulated 0
EN
DN
T4:/DN
T4:/DN
T4:/EN
T4:/EN
LS1 M1
M2
G
R R
G
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on-delay timer (TON), with one major exception—a re-
tentive timer reset (RES) instruction. Unlike the TON,
the RTO will hold its accumulated value when the timer
rung goes false and will continue timing where it left off
when the timer rung goes true again. This timer must be
accompanied by a timer reset instruction to reset the ac-
cumulated value of the timer to 0. The RES instruction
is the only automatic means of resetting the accumu-
lated value of a retentive timer. The RES instruction has
the same address as the timer it is to reset. Whenever
the RES instruction is true, both the timer accumulated
value and the timer done bit (DN) are reset to 0. Fig-
ure 7-26 shows a PLC program for a retentive on-delay
timer. The operation of the program can be summarized
as follows:
• The timer will start to time when time pushbutton
PB1 is closed.
• If the pushbutton is closed for 3 seconds and then
opened for 3 seconds, the timer accumulated value
will remain at 3 seconds.
• When the time pushbutton is closed again, the
timer picks up the time at 3 seconds and continues
timing.
cam. The positioning of the lobes of the cam and the gear
reduction of the motor determine the time it takes for the
motor to turn the cam far enough to activate the contacts.
If power is removed from the motor, the shaft stops but
does not reset.
A PLC retentive timer is used when you want to
retain accumulated time values through power loss
or the change in the rung state from true to false. The
PLC-programmed retentive on-delay timer (RTO) is
programmed in a manner similar to the nonretentive
Figure 7-24 Fluid pumping process.
OL
Start T4:5
Pump delay
PS1 Pump
Pump
Pump L2
Output
M
Stop
Pump
T4:6
1.0
5
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
Pump time
T4:6
DN T4:5
1.0
14
0
TOF
TIMER OFF DELAY
Timer
Time base
Preset
Accumulated
EN
DN
PS2
PS3
L1
Inputs DN
Stop
PS1
PS2
PS3
Start
Tank
A
Tank
B
PS1 PS2 PS3
Pump
Figure 7-25 Electromechanical retentive timer.
Cam-operated
contact
Motor-driven
cam
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• If the timing rung goes false the timer will stop tim-
ing but will recommence timing for the stored accu-
mulated value each time the rung goes true.
• When the reset PB2 is closed, the T4:2/DN bit is
reset to 0 and turns the pilot light output off. The ac-
cumulated value is also reset and held at zero until
the reset pushbutton is opened.
The program drawn in Figure 7-28 illustrates a prac-
tical application for an RTO. The purpose of the RTO
timer is to detect whenever a piping system has sustained
• When the accumulated value (9) equals the preset
value (9), the timer done bit T4:2/DN is set to 1 and
the pilot light output PL is switched on.
• Whenever the momentary reset pushbutton is closed
the timer accumulated value is reset to 0.
Figure 7-27 shows a timing chart for the retentive on-
delay timer program. The timing operation can be sum-
marized as follows:
• When the timing rung is true (PB1 closed) the timer
will commence timing.
Figure 7-27 Retentive on-delay timer timing chart.
Accumulated value
retained when rung
condition goes false
Accumulated value
DN (done) bit
PL output
Reset input PB2
Enable bit is reset
when input pushbutton
PB1 is opened.
0
1
2
3
4
5
6
7
8 9
10 11 12
Time in seconds
0 1 2 3 4 5 6 7 8 9
True
False
On
Off
On
Off
On
Off
On
Off
Accum ⴝ Preset
EN (enable) bit
Time input PB1
Figure 7-26 Retentive on-delay timer program.
PB2
PB1
L2
Output
T4:2
RES
DN
PB2
Time
PB1
L1
Inputs
Reset
RTO
RETENTIVE TIMER ON
Timer T4:2
Time Base 1.0
Preset 9
Accumulated 0
EN
DN
PL
PL
T4:2
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the problem has been corrected, the alarm system can be
reactivated by switching the key switch to open contact
position.
Figure 7-29 shows a practical application that
uses the on-delay, off-delay, and retentive on-delay
a cumulative overpressure condition for 60 s. At that
point, a horn is sounded automatically to call attention
to the malfunction. When they are alerted, maintenance
personnel can silence the alarm by switching the key
switch S1 to the reset (contact closed) position. After
Figure 7-28 Retentive on-delay timer alarm program.
Reset
Key switch
Pressure
switch
L1
Inputs
L2
Output
S1
S1
PS
PS
RTO
RETENTIVE TIMER ON
Timer T4:1
Preset 60
Accumulated 0
RES
EN
DN
Horn
Horn
T4:1
DN
T4:1
T4:2
L1
Inputs
Reset
Reset
SW
SW
T4:0
1.0
10
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
Pump
Pump
Pump
T4:2
1.0
10800
0
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
EN
DN
T4:1
DN
T4:1
1.0
15
0
TOF
TIMER OFF DELAY
Timer
Time base
Preset
Accumulated
EN
Motor
Motor
T4:0
DN
PL
OL
L2
Outputs
OL
PL
T4:2
DN
T4:2
RES
DN
Pump running time
Pump Off time delay
Motor starting time delay
DN
DN
EN
M2
M1
Figure 7-29 Bearing lubrication program.
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• After the preset time period of 20 s, TD2-1 contact
closes to energize motor starter coil M3, and so
motor 3 starts.
Figure 7-31 shows an equivalent PLC program of the
hardwired sequential time-delayed motor-starting circuit.
Two programmed on-delay timers are cascaded together
to obtain the same logic as the original hardwired timer
relay circuit. Note that the output of timer T4:1 is used to
control the input logic to timer T4:2.
Two timers can be interconnected to form an oscil-
lator circuit. The oscillator logic is basically a timing
circuit programmed to generate periodic output pulses
of any duration. Figure 7-32 shows the program for an
annunciator flasher circuit. Two internal timers form the
oscillator circuit, which generates a timed, pulsed output.
The oscillator circuit output is programmed in series with
the alarm condition. If the alarm condition (temperature,
pressure, or limit switch) is true, the appropriate output
indicating light will flash. Note that any number of alarm
conditions could be programmed using the same flasher
circuit.
At times you may require a time-delay period longer
than the maximum preset time allowed for the single timer
instruction of the PLC being used. When this is the case,
the problem can be solved by simply cascading timers, as
illustrated in Figure 7-33. The operation of the program
• The total time-delay period required is 42,000 s.
• The first timer, T4:1, is programmed for a preset
time of 30,000 s and begins timing when input SW
is closed.
instructions in the same program. In this industrial ap-
plication, there is a machine with a large steel shaft
supported by babbitted bearings. This shaft is coupled
to a large electric motor. The bearings need lubrication,
which is supplied by an oil pump driven by a small
electric motor. The operation of the program can be
• To start the machine, the operator turns SW on.
• Before the motor shaft starts to turn, the bearings
are supplied with oil by the pump for 10 seconds.
• The bearings also receive oil when the machine is
running.
• When the operator turns SW off to stop the
machine, the oil pump continues to supply oil for
15 seconds.
• A retentive timer is used to track the total running
time of the pump. When the total running time is
3 hours, the motor is shut down and a pilot light is
turned on to indicate that the filter and oil need to be
changed.
• A reset button is provided to reset the process after
the filter and oil have been changed.
Retentive timers do not have to be timed out com-
pletely to be reset. Rather, such a timer can be reset at
any time during its operation. Note that the reset input
to the timer will override the control input of the timer
even though the control input to the timer has logic
continuity.
7.6 Cascading Timers
The programming of two or more timers together is called
cascading. Timers can be interconnected, or cascaded, to
satisfy a number of logic control functions.
Figure 7-30 shows how three motors can be started
automatically in sequence with a 20 s time delay between
each using two hardwired on-delay timers. The operation
of the circuit can be summarized as follows:
• Motor starter coil M1 is energized when the
momentary start pushbutton PB2 is actuated.
• As a result, motor 1 starts, contact M1-1 closes to
seal in M1, and timer coil TD1 is energized to begin
the first time-delay period.
• After the preset time period of 20 s, TD1-1 contact
closes to energize motor starter coil M2.
• As a result, motor 2 starts and timer coil TD2
is energized to begin the second time-delay
period.
L2
(20 s)
L1
PB1
M1-1
TD2-1 OL
OL
TD1-1 OL
TD1
PB2
TD2
M1
M3
(20 s)
M2
Stop
Start
Figure 7-30 Hardwired sequential time-delayed motor-
starting circuit.
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• Once T4:2 reaches its preset time, the T4:2/DN bit
will be set to 1, which switches on the output PL,
the pilot light, to indicate the completion of the full
42,000-s time delay.
• When T4:1 completes its time-delay period 30,000 s
later, the T4:1/DN bit will be set to 1.
• This in turn activates the second timer, T4:2, which
is preset for the remaining 12,000 s of the total
42,000-s time delay.
L1 L2
PB1
PB1 PB2
PB2
Inputs Outputs
M1
M2
M3
M1
M1
OL
OL
OL
Start
Stop
EN
M1
DN
TON
TIMER ON DELAY
Timer
Preset
Accumulated
T4:1
20
0
EN
T4:1/DN
T4:1/DN
T4:2/DN
DN
T4:2
20
0
TON
TIMER ON DELAY
Timer
Preset
Accumulated
M2
M3
Figure 7-31 Equivalent PLC program of the sequential time-delayed motor-starting circuit.
L1 L2
Outputs
Inputs
T4:6
DN
T4:5
1.0
1
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
G
DN
T4:5
T4:6
1.0
1
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
R
T4:5
Y
Y
R
G
T4:5
DN
TS1
PS1
TS1
PS1
LS1
LS1
T4:5
DN
DN
DN
Figure 7-32 Annunciator ﬂasher program.
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• Opening input SW at any time will reset both timers
and switch output PL off.
A typical application for PLC timers is the control of
traffic lights. The ladder logic circuit of Figure 7-34 il-
lustrates a control of a set of traffic lights in one direc-
tion. The operation of the program can be summarized as
follows:
• Transition from red light to green light to amber
light is accomplished by the interconnection of the
three TON timer instructions.
• The input to timer T4:0 is controlled by the T4:2
done bit.
• The input to timer T4:1 is controlled by the T4:0
done bit.
• The input rung to timer T4:2 is controlled by the
T4:1 done bit.
• The timed sequence of the lights is:
Red—30 s on
Green—25 s on
Amber—5 s on
• The sequence then repeats itself.
The chart shown in Figure 7-35 shows the timed se-
quence of the lights for two-directional control of traffic
lights.
Figure 7-36 shows the original traffic light program
modified to include three more lights that control traffic
flow in two directions.
Figure 7-33 Cascading of timers for longer time delays.
L2
Output
T4:1
1.0
30000
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:2
1.0
12000
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
SW
T4:1
PL
DN
T4:2
DN
SW
L1
Input
PL
Figure 7-34 Control of trafﬁc lights in one direction.
T4:2
DN
T4:0
1.0
30
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
DN
T4:0
DN
T4:1
1.0
25
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:1
T4:2
1.0
5
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:0
EN
T4:0
T4:1
EN
T4:1
T4:2
EN
T4:2
L2
Outputs
Red
Red
Green
Amber
Red
Traffic lights
Red time
Green time
Amber time
EN
Green
Green
Amber
Amber
DN
DN
DN
DN
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Figure 7-35 Timing chart for two-directional control of trafﬁc lights.
Red ⫽ north/south
Red ⫽ east/west
Green ⫽ east/west
25 s
Amber ⫽ east/west
5 s
Green ⫽ north/south Amber ⫽ north/south
25 s 5 s
Figure 7-36 Control of trafﬁc lights in two directions.
North/south
North/south
North/south
Red
East/west
Amber
East/west
Green
East/west
Red
North/south
Amber
North/south
Green
North/south
North/south
traffic lights
East/west
traffic lights
Green
Amber
Red
East/west
East/west
East/west
T4:2
DN
T4:0
1.0
30
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
DN
T4:0
DN
T4:1
1.0
25
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:1
T4:2
1.0
5
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:0
EN
T4:0 Red
T4:1
EN
T4:1 Green
T4:2
EN
T4:2 Amber
L1
Outputs
Green
Amber
Red
Green
Amber
EN
DN
DN
DN
DN
T4:0
T4:3
1.0
25
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
EN
T4:3
EN
T4:3
T4:3
DN
T4:0
T4:0
DN
DN
DN
Red
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Figure 7-37 Relay schematic diagram for Problem 1.
Relay schematic diagram
L2
TD2-2
L1
TD1
TD2
S1
(5 s)
TD1-1
(5 s)
TD1-2
(5 s)
TD2-1
(5 s)
PL4
PL3
PL2
PL1
1. Explain the difference between the timed and
instantaneous contacts of a mechanical timing
relay.
2. Draw the symbol and explain the operation of each
of the following timed contacts of a mechanical
timing relay:
a. On-delay timer—NOTC contact
b. On-delay timer—NCTO contact
c. Off-delay timer—NOTO contact
d. Off-delay timer—NCTC contact
3. Name five pieces of information usually associated
with a PLC timer instruction.
4. When is the output of a programmed timer
energized?
5. a. What are the two methods commonly used to
represent a timer instruction within a PLC’s
ladder logic program?
b. Which method is preferred? Why?
6. a. Explain the difference between the operation of
a nonretentive timer and that of a retentive timer.
b. Explain how the accumulated count of pro-
grammed retentive and nonretentive timers is
reset to zero.
7. State three advantages of using programmed PLC
timers over mechanical timing relays.
8. For a TON timer:
a. When is the enable bit of a timer instruction
true?
b. When is the timer-timing bit of a timer instruc-
tion true?
c. When does the done bit of a timer change state?
9. For a TOF timer:
a. When is the enable bit of a timer instruction true?
b. When is the timer-timing bit of a timer instruc-
tion true?
c. When does the done bit of a timer change state?
10. Explain what each of the following quantities asso-
ciated with a PLC timer instruction represents:
a. Preset time
b. Accumulated time
c. Time base
11. State the method used to reset the accumulated
time of each of the following:
a. TON timer
b. TOF timer
c. RTO timer
1. a. With reference to the relay schematic diagram in
Figure 7-37, state the status of each light (on or
off) after each of the following sequential events:
i. Power is first applied and switch S1 is open.
ii. Switch S1 has just closed.
iii. Switch S1 has been closed for 5 s.
iv. Switch S1 has just opened.
v. Switch S1 has been opened for 5 s.
b. Design a PLC program and prepare a typical I/O
will execute this hardwired control circuit correctly.
will correctly execute the hardwired relay control
circuit shown in Figure 7-38.
3. Study the ladder logic program in Figure 7-39 and
a. What type of timer has been programmed?
b. What is the length of the time-delay period?
CHAPTER 7 PROBLEMS
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Figure 7-38 Hardwired relay control circuit for Problem 2.
L2
(60 s)
L1
PB1
PB2
Stop
Start
M-1
PS1
OL
Hand
Auto
TD-2
TD-1
TD
M
L1 L2
LS1
LS1
Input Outputs
T4:0
1
10
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
DN
DN
EN
EN
T4:0
T4:0
T4:0
T4:0
SOL A
SOL B
R
Y
1
2
3
4
5
SOL A
SOL B
R
Y
R
Y
c. What is the value of the accumulated time when
power is first applied?
d. When does the timer start timing?
e. When does the timer stop timing and reset itself?
f. When input LS1 is first closed, which rungs are
true and which are false?
g. When input LS1 is first closed, state the status
(on or off) of each output.
h. When the timer’s accumulated value equals the
preset value, which rungs are true and which are
false?
i. When the timer’s accumulated value equals the
preset value, state the status (on or off) of each
output.
j. Suppose that rung 1 is true for 5 s and then
power is lost. What will the accumulated value
of the counter be when power is restored?
c. What is the value of the accumulated time when
power is first applied?
d. When does the timer start timing?
e. When does the timer stop timing and reset itself?
f. When input LS1 is first closed, which rungs are
true and which are false?
g. When input LS1 is first closed, state the status
(on or off) of each output.
h. When the timer’s accumulated value equals the pre-
set value, which rungs are true and which are false?
i. When the timer’s accumulated value equals the
preset value, state the status (on or off) of each
output.
j. Suppose that rung 1 is true for 5 s and then
power is lost. What will the accumulated value
of the counter be when power is restored?
c. When does the timer start timing?
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PL2
T4:5/EN
L1
Inputs
PB2
PB1
PB2
PB1
T4:5
1.0
50
0
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
PL1
T4:5/EN
PL4
L2
Outputs
PL4
PL3
PL2
PL1
T4:5 DN
PL3
T4:5 DN
1
2
3
4
5
6
RES
T4:5
EN
DN
5
4
3
2
1
DN
Input
L1
LS1
LS1
T4:0
1
25
0
TOF
TIMER OFF DELAY
Timer
Time base
Preset
Accumulated
EN
SOL A
SOL B
R
Y
T4:0
DN
T4:0
DN
T4:0
EN
T4:0
EN
R
SOL B
Outputs
L2
Y
SOL A
Y
R
d. When is the timer reset?
e. When will rung 3 be true?
f. When will rung 5 be true?
g. When will output PL4 be energized?
h. Assume that your accumulated time value is up
to 020 and power to your system is lost. What
will your accumulated time value be when
power is restored?
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b. The input is true, and EN is 1, TT is 1, and
DN is 1.
c. The input is false, and EN is 0, TT is 0, and
DN is 0.
d. The input is true, and EN is 1, TT is 0, and
DN is 1.
10. Study the off-delay timer ladder logic program in
Figure 7-44, and from each of the conditions stated,
determine whether the timer is reset, timing, or
timed out or if the conditions stated are not possible.
a. The input is true, and EN is 0, TT is 0, and
DN is 1.
i. What happens if inputs PB1 and PB2 are both
true at the same time?
a. What is the purpose of interconnecting the two
timers?
b. How much time must elapse before output PL is
energized?
c. What two conditions must be satisfied for timer
T4:2 to start timing?
d. Assume that output PL is on and power to the
system is lost. When power is restored, what will
the status of this output be?
e. When input PB2 is on, what will happen?
f. When input PB1 is on, how much accumulated
time must elapse before rung 3 will be true?
7. You have a machine that cycles on and off during
its operation. You need to keep a record of its total
run time for maintenance purposes. Which timer
would accomplish this?
8. Write a ladder logic program that will turn on a
light, PL, 15 s after switch S1 has been turned on.
9. Study the on-delay timer ladder logic program
in Figure 7-43, and from each of the conditions
stated, determine whether the timer is reset, tim-
ing, or timed out or if the conditions stated are not
possible.
a. The input is true, and EN is 1, TT is 1, and
DN is 0.
T4:2
T4:1
L1
PB1
PB2
PB2
PB1
PB1
Inputs
T4:1
1.0
2900
0
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
EN
DN
T4:2
1.0
1780
0
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
EN
DN
DN
DN
PL
L2
Output
PL
RES
RES
T4:2
T4:1
1
2
3
4
Figure 7-43 On-delay timer ladder logic program for
Problem 9.
Input
T4:0
1.0
10
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:0
T4:0
T4:0
DN
EN
TT
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• Solenoid A is de-energized.
• The agitate motor starts automatically and runs
for 3 min to mix the liquid.
• When the agitate motor stops, solenoid B is ener-
gized to empty the tank.
• When the tank is completely empty, the empty
sensor switch opens to de-energize solenoid B.
• The start button is pressed to repeat the sequence.
14. When the lights are turned off in a building, an exit
door light is to remain on for an additional 2 min,
and the parking lot lights are to remain on for an
additional 3 min after the door light goes out. Write
a program to implement this process.
15. Write a program to simulate the operation of a se-
quential taillight system. The light system consists
of three separate lights on each side of the car. Each
set of lights will be activated separately, by either
the left or right turn signal switch. There is to be a
1 s delay between the activation of each light, and
a 1-s period when all the lights are off. Ensure that
when both switches are on, the system will not op-
erate. Use the least number of timers possible. The
sequence of operation should be as follows:
• The switch is operated.
• Light 1 is illuminated.
• Light 2 is illuminated 1 s later.
• Light 3 is illuminated 1 s later.
• Light 3 is illuminated for 1 s.
• All lights are off for 1 s.
• The system repeats while the switch is on.
b. The input is true, and EN is 1, TT is 1, and
DN is 1.
c. The input is true, and EN is 1, TT is 0, and
DN is 1.
d. The input is false, and EN is 0, TT is 1, and
DN is 1.
e. The input is false, and EN is 0, TT is 0, and
DN is 0.
11. Write a program for an “anti–tie down circuit” that
will disallow a punch press solenoid from operat-
ing unless both hands are on the two palm start
buttons. Both buttons must be pressed at the same
time within 0.5 s. The circuit also will not allow the
operator to tie down one of the buttons and operate
the press with just one button. (Hint: Once either of
the buttons is pressed, begin timing 0.5 s. Then, if
both buttons are not pressed, prevent the press sole-
noid from operating.)
12. Modify the program for the control of traffic lights
in two directions so that there is a 3-s period when
both directions will have their red lights illuminated.
13. Write a program to implement the process illus-
trated in Figure 7-45. The sequence of operation is
to be as follows:
• Normally open start and normally closed stop
pushbuttons are used to start and stop the
process.
• When the start button is pressed, solenoid A ener-
gizes to start filling the tank.
• As the tank fills, the empty level sensor switch
closes.
• When the tank is full, the full level sensor switch
closes.
Figure 7-44 Off-delay timer ladder logic program for
Problem 10.
Input
T4:0
1.0
10
0
TOF
TIMER OFF DELAY
Timer
Time base
Preset
Accumulated
EN
DN
T4:0
T4:0
T4:0
DN
EN
TT
Figure 7-45 Process for Problem 13.
SOL B
Motor
Full
sensor
switch
Empty
sensor
switch
Start/stop
control station
SOL A
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149
All PLCs include both up-counters and down-
counters. Counter instructions and their function
in ladder logic are explained in this chapter. Typi-
cal examples of PLC counters include the follow-
ing: straight counting in a process, two counters
used to give the sum of two counts, and two
counters used to give the difference between
two counts.
Chapter Objectives
8.1 List and describe the functions of PLC counter
instructions
8.2 Describe the operating principle of a transitional, or
one-shot, contact
8.3 Analyze and interpret typical PLC counter ladder
logic programs
8.4 Apply the PLC counter function and associated
circuitry to control systems
8.5 Apply combinations of counters and timers to control
systems
8
Programming Counters
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150 Chapter 8 Programming Counters
or program sources for counting. The two methods used
to represent a counter within a PLC’s ladder logic pro-
gram are the coil format and the block format. Figure 8-4
shows a typical coil-formatted up-counter instruction.
The up-counter increments its accumulated value by 1
each time the counter rung makes a false-to-true transi-
tion. When the accumulated count equals the preset count
the counter output is energized or set to 1. Shown as part
of the instruction are the:
Counter type
Counter address
Counter preset value
Accumulated count
The counter reset instruction must be used in conjunc-
tion with the counter instruction. Up-counters are always
reset to zero. Down-counters may be reset to zero or
to some preset value. Some manufacturers include the
reset function as a part of the general counter instruc-
tion, whereas others dedicate a separate instruction for
resetting the counter. Figure 8-5 shows a coil-formatted
counter instruction with a separate instruction for reset-
ting the counter. When programmed, the counter reset
coil (CTR) is given the same reference address as the
8.
1 Counter Instructions
Programmed counters can serve the same function as me-
chanical counters. Figure 8-1 shows the construction of a
simple mechanical counter. Every time the actuating lever
is moved over, the counter adds one number; the actuating
lever then returns automatically to its original position.
Resetting to zero is done with a pushbutton located on the
side of the unit.
Electronic counters, such as those shown in Figure 8-2,
can count up, count down, or be combined to count up
and down. Although the majority of counters used in in-
dustry are up-counters, numerous applications require the
implementation of down-counters or of combination up/
down-counters.
All PLC manufacturers offer some form of counter
instruction as part of their instruction set. One common
counter application is keeping track of the number of items
moving past a given point as illustrated in Figure 8-3.
Counters are similar to timers except that they do not
operate on an internal clock but are dependent on external
Figure 8-1 Mechanical counter.
Reset
button
Actuating
lever
0000312
Figure 8-2 Electronic counters.
Figure 8-3 Counter application.
PLC
Figure 8-4 Coil-formatted up-counter instruction.
XXX
PR: YYY
AC: 000
Counter address
Accumulated
counter value
Preset counter
value
Increments
counter by 1
for each
false-to-true
transition.
Type of
counter
CTU
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Programming Counters Chapter 8 151
of the input signal. The counter will either increment
or decrement whenever the count input transfers from
an off state to an on state. The counter will not operate
on the trailing edge, or on-to-off transition, of the input
condition.
Some manufacturers require the reset rung or line to be
true to reset the counter, whereas others require it to be
false to reset the counter. For this reason, it is wise to con-
sult the PLC’s operations manual before attempting any
programming of counter circuits.
PLC counters are normally retentive; that is, whatever
count was contained in the counter at the time of a proces-
sor shutdown will be restored to the counter on power-up.
The counter may be reset, however, if the reset condition
is activated at the time of power restoration.
PLC counters can be designed to count up to a preset
value or to count down to a preset value. The up-counter
is incremented by 1 each time the rung containing the
counter is energized. The down-counter decrements by 1
each time the rung containing the counter is energized.
These rung transitions can result from events occurring
in the program, such as parts traveling past a sensor or
actuating a limit switch. The preset value of a program-
mable controller counter can be set by the operator or can
be loaded into a memory location as a result of a program
decision.
Figure 8-7 illustrates the counting sequence of an up-
counter and a down-counter. The value indicated by the
counter is termed the accumulated value. The counter will
increment or decrement, depending on the type of coun-
ter, until the accumulated value of the counter is equal to
or greater than the preset value, at which time an output
will be produced. A counter reset is always provided to
cause the counter accumulated value to be reset to a pre-
determined value.
counter (CTU) that it is to reset. In this example the reset
instruction is activated whenever the CTR rung condi-
tion is true.
Figure 8-6 shows a block-formatted counter. The
instruction block indicates the type of counter (up or
down), along with the counter’s preset value and accu-
mulated or current value. The counter has two input con-
ditions associated with it, namely, the count and reset.
All PLC counters operate, or count, on the leading edge
Figure 8-5 Coil-formatted counter and reset instructions.
XXX
XXX
PR: YYY
AC: 000
Count
rung
Resets
counter
Reset
rung
Same
address
CTU
CTR
Figure 8-6 Block-formatted counter instruction.
Count
line
Input
module
Type of
counter
Preset value
Accumulated
value
Output
line
Reset
line
Figure 8-7 Counter counting sequence.
Limit
switch
Accumulated ⫽ Preset ⫽
Up-counter Down-counter
Counter—up
ⴙ4
Counter
value
Off
On
Output
Parts
sensor
Accumulated ⫽ Preset ⫽
Counter—down
ⴚ5
Counter
value
Off
On
Output
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8.2 Up-Counter
The up-counter is an output instruction whose function is
to increment its accumulated value on false-to-true transi-
tions of its instruction. It thus can be used to count false-
to-true transitions of an input instruction and then trigger
an event after a required number of counts or transitions.
The up-counter output instruction will increment by 1
each time the counted event occurs.
Figure 8-8 shows the program and timing diagram for
an SLC 500 Count-Up Counter. This control application
is designed to turn the red pilot light on and the green pilot
light off after an accumulated count of 7. The operation of
the program can be summarized as follows:
• Operating pushbutton PB1 provides the off-to-on
transition pulses that are counted by the counter.
• The preset value of the counter is set for 7.
L1 L2
Inputs Outputs
PB1 (Count)
PB1 (Count)
PB2 (Reset)
Counter done bit Red PL
Counter done bit Green PL
Green PL
Red PL
PB2 (Reset)
Rung 1
Rung 2
Rung 3
Rung 4
I:1/0
I:1/0
C5:1/DN O:2/0
O:2/1
O:2/0
O:2/1
C5:1
C5:1/DN
I:1/1
I:1/1
(a)
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
7
0
RES
CU
DN
G
R
Figure 8-8 Simple up-counter program. (a) Program. (b) Timing diagram.
Rung 1
Rung 2
Rung 3
Rung 4
(count)
Preset
value (7)
Accumulated
value
False
True 1 2 3 4 5 6 7
1
2
3
4
5
6 7
(b)
(reset)
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• Each false-to-true transition of rung 1 increases the
counter’s accumulated value by 1.
• After 7 pulses, or counts, when the preset counter
value equals the accumulated counter value, output
DN is energized.
• As a result, rung 2 becomes true and energizes
output O:2/0 to switch the red pilot light on.
• At the same time, rung 3 becomes false and de-
energizes output O:2/1 to switch the green pilot
light off.
• The counter is reset by closing pushbutton PB2,
which makes rung 4 true and resets the accumulated
count to zero.
• Counting can resume when rung 4 goes false again.
The Allen-Bradley SLC 500 counter file is file 5 (Fig-
ure 8-9). Each counter is composed of three 16-bit words,
collectively called a counter element. These three data
words are the control word, preset word, and accumulated
word. Each of the three data words shares the same base
address, which is the address of the counter itself. There
can be up to 256 counter elements. Addresses for counter
file 5, counter element 3 (C5:3), are listed below.
C5 5 counter file 5
:3 5 counter element 3 (0–255 counter elements per
file)
C5:3/DN is the address for the done bit of the counter.
C5:3/CU is the address for the count-up enable bit of
the counter.
C5:3/CD is the address for the count-down enable bit
of the counter.
C5:3/OV is the address for the overflow bit of the
counter.
C5:3/UN is the address for the underflow bit of the
counter.
C5:3/UA is the address for the update accumulator bit
of the counter.
Figure 8-10 shows the counter table for the Allen-
Bradley SLC 500 controller. The control word uses status
control bits consisting of the following:
Count-Up (CU) Enable Bit—The count-up en-
able bit is used with the count-up counter and is true
whenever the count-up counter instruction is true. If
the count-up counter instruction is false, the CU bit
is false.
Count-Down (CD) Enable Bit—The count-down
enable bit is used with the count-down counter and is
true whenever the count-down counter instruction is
true. If the count-down counter instruction is false, the
CD bit is false.
Done (DN) Bit—The done bit is true whenever the
accumulated value is equal to or greater than the pre-
set value of the counter, for either the count-up or the
count-down counter.
Overflow (OV) Bit—The overflow bit is true when-
ever the counter counts past its maximum value,
which is 32,767. On the next count, the counter will
wrap around to 32,768 and will continue counting
Figure 8-9 SLC 500 counter ﬁle.
Counter address
C5:3 Bit 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
Word
0
Word
1
Word
2
C5:3.0 CU CD DN OV UN UA Internal use (not addressable)
C5:3.1 Preset value
C5:3.2 Accumulated value
File number
Counters
5
File type Counter number
C5:3
Figure 8-10 SLC 500 counter table.
Counter Table
C5:0
C5:1
C5:2
C5:3
C5:4
C5:5
/CU
0
0
0
0
0
0
/CD
0
0
0
0
0
0
/DN
0
0
0
0
0
0
/OV
0
0
0
0
0
0
/UN
0
0
0
0
0
0
/UA
0
0
0
0
0
0
.PRE
0
0
0
50
0
0
.ACC
0
0
0
0
0
0
C5: Counter
Table:
Address C5:3
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from there toward 0 on successive false-to-true transi-
tions of the count-up counter.
Underflow (UN) Bit—The underflow bit will go true
when the counter counts below 32,768. The counter
will wrap around to 132,767 and continue counting
down toward 0 on successive false-to-true rung transi-
tions of the count-down counter.
Update Accumulator (UA) Bit—The update accu-
mulator bit is used only in conjunction with an exter-
nal HSC (high-speed counter).
The preset value (PRE) word specifies the value that
the counter must count to before it changes the state of the
done bit. The preset value is the set point of the counter
and ranges from 232,768 through 132,767. The number
is stored in binary form, with any negative numbers being
stored in 2’s complement binary.
The accumulated value (ACC) word is the current count
based on the number of times the rung goes from false
to true. The accumulated value either increments with a
false-to-true transition of the count-up counter instruc-
tion or decrements with a false-to-true transition of the
count-down counter instruction. It has the same range as
the preset: 232,768 through 132,767. The accumulated
value will continue to count past the preset value instead
of stopping at the preset like a timer does.
Figure 8-11 shows an example of the count-up coun-
ter and its status bits used in the SLC 500 controller
instruction set. The address for counters begins at C5:0
and continues through C5:255. The information to be en-
tered includes:
Counter Number—This number must come from the
counter file. In the example shown, the counter num-
ber is C5:0, which represents counter file 5, counter 0
in that file. The address for this counter should not be
used for any other count-up counter.
Preset Value—The preset value can range from
232,768 to 132,767. In the example shown, the pre-
set value is 10.
Accumulated Value—The accumulated value can
also range from 232,768 through 132,767. Typi-
cally, as in this example, the value entered in the
accumulated word is 0. Regardless of what value is
entered, the reset instruction will reset the accumu-
lated value to 0.
Figure 8-12 shows the timer/counter menu tab from
the RSLogix toolbar. Several timer and counter instruc-
tions appear when this tab is selected. The first three are
timer instructions that are covered in Chapter 7. The next
two instructions from the left are the up-counter (CTU)
and down-counter (CTD) instructions. To the right of the
CTU and CTD instructions is the reset (RES) instruction,
which is used by both counters and timers. The counter
CTU (Count-Up)—Increments the accumulated
value at each false-to-true transition and retains
the accumulated value when an off/on power cycle
occurs.
CTD (Count-Down)—Decrements the accumu-
lated value at each false-to-true transition and retains
the accumulated value when an on/off power cycle
occurs.
HSC (High-Speed Counter)—Counts high-speed
pulses from a high-speed input.
Figure 8-13 shows a PLC counter program used to stop
a motor from running after 10 operations. The operation
of the program can be summarized as follows:
• Up-counter C5:0 counts the number of off/on opera-
tions of the motor.
• The preset value of the counter is set to 10.
Figure 8-11 Count-up counter instruction.
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
10
0
C5:0/CU
Counter enable bit
C5:0/DN
Counter done bit
C5:0/OV
Overflow status bit
C5:0 The reset instruction resets
the counter's accumulated
value back to zero.
CU
DN
RES
Figure 8-12 Counter selection toolbar.
TON TOF RTO CTU CTD RES
User Bit Timer/Counter Input/Output Compare
HSC
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• A counter done bit examine-off instruction is
programmed in series with the motor output
instruction.
• A motor output examine-on instruction is used to
increment the accumulated value of the counter for
each off/on operation.
• After the count of 10 is reached the counter done
bit examine-off instruction goes false preventing the
motor from being started.
• Closure of the reset pushbutton resets the accumu-
lated count to zero.
Figure 8-14 shows a PLC can-counting program that
uses three up-counters. The operation of the program can
• Counter C5:2 counts the total number of cans com-
ing off an assembly line for final packaging.
• Each package must contain 10 parts.
• When 10 cans are detected, counter C5:1 sets bit
B3/1 to initiate the box closing sequence.
• Counter C5:3 counts the total number of packages
filled in a day. (The maximum number of packages
per day is 300.)
• A pushbutton is used to restart the total part and
package count from zero daily.
One-Shot Instruction
Figure 8-15 shows the program for a one-shot, or tran-
sitional, contact circuit that is often used to automati-
cally clear or reset a counter. The program is designed
to generate an output pulse that, when triggered, goes on
for the duration of one program scan and then goes off.
The one-shot can be triggered from a momentary signal
or from a signal that comes on and stays on for some time.
Whichever signal is used, the one-shot is triggered by the
leading-edge (off-to-on) transition of the input signal. It
stays on for one scan and goes off. It stays off until the
trigger goes off, and then comes on again. The one-shot is
perfect for resetting both counters and timers since it stays
on for one scan only.
Some PLCs provide transitional contacts or one-shot
instructions in addition to the standard NO and NC contact
instructions. The off-to-on transitional contact instruc-
tion, shown in Figure 8-16a, is programmed to provide a
one-shot pulse when the referenced trigger signal makes
a positive (off-to-on) transition. This contact will close
for exactly one program scan whenever the trigger signal
goes from off to on. The contact will allow logic continu-
ity for one scan and then open, even though the trigger-
ing signal may stay on. The on-to-off transitional contact,
shown in Figure 8-16b, provides the same operation as
the off-to-on transitional contact instruction, except that
it allows logic continuity for a single scan whenever the
trigger signal goes from an on to an off state.
The conveyor motor PLC program of Figure 8-17 il-
lustrates the application of an up-counter along with a
programmed one-shot (OSR) transitional contact instruc-
tion. The counter counts the number of cases coming off
the conveyor. When the total number of cases reaches 50,
the conveyor motor stops automatically. The trucks being
loaded will take a total of only 50 cases of this particular
Figure 8-13 PLC counter program used to stop a motor from running after 10 operations.
CU
CTU
COUNT-UP COUNTER
Counter C5:0
Preset 10
Accumulated 0
DN
RES
C5:0
Reset
Motor
Motor
Motor
Output
Stop Start
Motor
C5:0/DN
Stop
Start
Inputs
L1
Start
Reset
Reset
Stop
Start
Reset
OL
L2
M
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product; however, the count can be changed for different
product lines. The operation of the program can be sum-
marized as follows:
• The momentary start button is pressed to start the
conveyor motor M1.
• The passage of cases is sensed by the proximity
switch.
• Cases move past the proximity switch and incre-
ment the counter’s accumulated value with each
false-to-true transition of the switch.
• After a count of 50, the done bit of the counter changes
state to stop the conveyor motor automatically and
reset the counter’s accumulated value to zero.
Figure 8-15 One-shot, or transitional, contact program.
A
A
A
Trigger
input
Internal
relay contact
One-shot
output
Internal
relay coil
L1 Input
Figure 8-16 Transitional contact instructions.
On
Off
On
Off
One
scan
(a) Off-to-on transitional contact
Symbol
On
Off
On
Off
One
scan
(b) On-to-off-transitional contact
Symbol
Figure 8-14 Can-counting program.
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
10
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:2
32767
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:3
300
0
Parts
Packages
PROX-SW
Inputs
L1
Reset PB
Close box
B3/1
C5:1
C5:1/DN
B3/1
C5:3
C5:2
PROX-SW
Reset PB
CU
CU
DN
CU
DN
DN
RES
RES
RES
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• The conveyor motor can be stopped and started
manually at any time without loss of the accumu-
lated count.
• The accumulated count of the counter can be reset
manually at any time by means of the count reset
button.
The Allen-Bradley SLC 500 one-shot rising (OSR)
instruction is an input instruction that triggers an event
to occur one time. The OSR instruction is placed in the
ladder logic before the output instruction. When the rung
conditions preceding the OSR instructions go from false-
to-true, the OSR instruction goes true also but for only
one scan. Figure 8-18 illustrates the operation of an OSR
rung which can be summarized as follows:
• The OSR, one-shot rising instruction is used to
make the counter reset instruction (RES) true for
one scan when limit switch input LS1 goes from
false to true.
• The OSR is assigned a Boolean bit (B3:0/0) that is
not used anywhere else in the program.
• The OSR instruction must immediately precede the
output instruction.
• When the limit switch closes the LS1 and OSR, input
instructions go from false to true. The OSR instruc-
tion conditions the rung so that the counter C5:1 reset
output instruction goes true for one program scan.
• The output reset instruction goes false and remains
false for successive scans until the input makes an-
other false-to-true transition.
• The OSR bit is set to 1 as long as the limit switch
remains closed.
• The OSR bit is reset to 0 when the limit switch is
opened.
Applications for the OSR instruction include freez-
ing rapidly displayed LED values. Figure 8-19 shows a
one-shot instruction used to send data to an output LED
display. The one-shot allows the rapidly changing ac-
cumulated time from the timer to be frozen to ensure a
readable, stable display. The operation of the program is
• The accumulated value of timer T4:1 is converted to
Binary Coded Decimal (BCD) and moved to output
word O:6 where an LED display is connected.
Figure 8-17 Case-counting program.
O:2/0
O:2/0
Inputs Output
L1 L2
Stop
Start
Proximity
switch
OL
I:1/1 M1
Reset
I:1/1 I:1/2
O:2/0
C5:0/DN
I:1/3
I:1/4
RES
C5:0
C5:0/DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
50
0
CU
DN O:2/0
I:1/2
I:1/3
I:1/4
B3:0/0
OSR
Figure 8-18 One-shot rising (OSR) instruction.
C5:1
B3:0/0
One PLC scan
RES
OSR rung
Input
B3:0/0
OSR
L1
LS1
LS1
LS1
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• When the timer is running, SW (I:1/1) closed, the
accumulated value changes rapidly.
• Closing the momentary pushbutton PB (I:1/0) will
freeze and display the value at that point in time.
The alarm monitor PLC program of Figure 8-20 illus-
trates the application of an up-counter used in conjunction
with the programmed timed oscillator circuit studied in
Chapter 7. The operation of the program can be summa-
rized as follows:
• The alarm is triggered by the closing of float switch FS.
• The light will flash whenever the alarm condition
is triggered and has not been acknowledged,
Figure 8-20 Alarm monitor program.
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:5
1.0
1
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
1
0
L2
T4:6
C5:1
DN
T4:5
DN
C5:1
DN
FS
FS
SS
C5:1
RES
Light
Light
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:6
1.0
1
0
OFF ON
L1
Inputs
FS
SS
DN
T4:5
DN
EN
EN
CU
DN
DN
DN
Figure 8-19 OSR instruction used to freeze rapidly displayed LED values.
B3:0/0
OSR
TOD
To BCD
Source
Destination
T4:1.ACC
O:6
O:6
I:1/1
PB
SW
I:1/0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
1000
0
EN
DN
I:1/0
I:1/1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0000
Output
Inputs
L1
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even if the alarm condition clears in the
meantime.
• The alarm is acknowledged by closing selector
switch SS.
• The light will operate in the steady on mode when
the alarm trigger condition still exists but has been
acknowledged.
8.3 Down-Counter
The down-counter instruction will count down or decre-
ment by 1 each time the counted event occurs. Each time
the down-count event occurs, the accumulated value is
decremented. Normally the down-counter is used in con-
junction with the up-counter to form an up/down-counter.
Figure 8-21 shows the program and timing diagram for
a generic, block-formatted up/down-counter. The opera-
tion of the program can be summarized as follows:
• Separate count-up and count-down inputs are
provided.
• Assuming the preset value of the counter is 3 and
the accumulated count is 0, pulsing the count-up
input (PB1) three times will switch the output light
from off to on.
• This particular PLC counter keeps track of the num-
ber of counts received above the preset value. As a
result, three additional pulses of the count-up input
(PB1) produce an accumulated value of 6 but no
change in the output.
• If the count-down input (PB2) is now pulsed
four times, the accumulated count is reduced to
2 (6 2 4). As a result, the accumulated count
drops below the preset count and the output light
switches from on to off.
• Pulsing the reset input (PB3) at any time will reset the
accumulated count to 0 and turn the output light off.
Figure 8-21 Generic up/down-counter program. (a) Program. (b) Counting
diagram.
Count
up
Count
down
UDC
PR: 003
AC: 000
Inputs
L1
Light
Light
Output
L2
Reset
PB3
PB3
PB2
PB2
PB1
PB1
(a)
(b)
Count up
Count down
Counter
accumulated
value
Output
Reset
On
Off
On
Off
On
Off
On
Off
1 2 3 4 5 6
1 2 3 4
0
1 2
3
4 5
6 5
4 3
2
0
Preset value
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Not all counter instructions count in the same man-
ner. Some up-counters count only to their preset values,
and additional counts are ignored. Other up-counters
keep track of the number of counts received above the
counter’s preset value. Conversely, some down-counters
will simply count down to zero and no further. Other
down-counters may count below zero and begin count-
ing down from the largest preset value that can be set
for the PLC’s counter instruction. For example, a PLC
up/down-counter that has a maximum counter preset
limit of 999 may count up as follows: 997, 998, 999,
000, 001, 002, and so on. The same counter would count
down in the following manner: 002, 001, 000, 999, 998,
997, and so on.
One application for an up/down-counter is to keep
count of the cars that enter and leave a parking garage.
Figure 8-22 shows a typical PLC program that could be
used to implement this. The operation of the program can
• As a car enters, the enter switch triggers the up-
counter output instruction and increments the accu-
mulated count by 1.
• As a car leaves, the exit switch triggers the down-
counter output instruction and decrements the accu-
mulated count by 1.
• Because both the up- and down-counters have the
same address, C5:1, the accumulated value will be
the same in both instructions as well as the preset.
• Whenever the accumulated value of 150 equals the
preset value of 150, the counter output is energized
by the done bit to light up the Lot Full sign.
• A reset button has been provided to reset the accu-
mulated count.
Figure 8-23 shows an example of the count-down
counter instruction used as part of the Allen-Bradley SLC
500 controller instruction set. The information to be en-
tered into the instruction is the same as for the count-up
counter instruction.
The CTD instruction decrements its accumulated value
by 1 every time it is transitioned. It sets its done bit when
the accumulated value is equal to or greater than the preset
value. The CTD instruction requires the RES instruction
to reset its accumulated value and status bits. Because it
resets its accumulated value to 0, the CTD instruction then
Figure 8-22 Parking garage counter.
Lot full
light
Lot full
light
C5:1/DN
Enter
switch
Enter
switch
Exit
switch
Exit
switch
Reset
Reset
Inputs
L1
C5:1
Output L2
CTU
COUNT-UP COUNTER
Counter C5:1
Preset 150
Accumulated 0
CTD
COUNT-DOWN COUNTER
Counter C5:1
Preset 150
Accumulated 0
CU
CD
DN
DN
RES
Figure 8-23 Count-down counter instruction.
CTD
COUNT-DOWN COUNTER
Counter
Preset
Accumulated
C5:0
10
0
C5:0/CD
Counter enable bit
C5:0/DN
Counter done bit
C5:0/UN
Underflow status bit
C5:0 The reset instruction resets
the counter's accumulated
value back to zero.
CD
DN
RES
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counts negative when it transitions. If the CTD instruction
were used by itself with a positive preset value, its done
bit would be reset when the accumulated value reached 0.
Then, counting in a negative direction, the accumulated
value would never reach its preset value and set the done
bit. However, the preset can be entered with a negative
value; then the done bit is cleared when the accumulated
value becomes less than the preset value.
Figure 8-24 shows an up/down-counter program that
will increase the counter’s accumulated value when push-
button PB1 is pressed and will decrease the counter’s ac-
cumulated value when pushbutton PB2 is pressed. Note
that the same address is given to the up-counter instruc-
tion, the down-counter instruction, and the reset instruc-
tion. All three instructions will be looking at the same
address in the counter file. When input A goes from false
to true, one count is added to the accumulated value.
When input B goes from false to true, one count is sub-
tracted from the accumulated value. The operation of the
program can be summarized as follows:
• When the CTU instruction is true, C5:2/CU will be
true, causing output A to be true.
• When the CTD instruction is true, C5:2/CD will be
true, causing output B to be true.
• When the accumulated value is greater than or equal
to the preset value, C5:2/DN will be true, causing
output C to be true.
• Input C going true will cause both counter instruc-
tions to reset. When reset by the RES instruction,
the accumulated value will be reset to 0 and the
done bit will be reset.
Figure 8-25 illustrates the operation of the up/down-
counter program used to provide continuous monitor-
ing of items in process. An in-feed photoelectric sensor
counts raw parts going into the system, and an out-feed
photoelectric sensor counts finished parts leaving the
machine. The number of parts between the in-feed
and out-feed is indicated by the accumulated count of
the counter. Counts applied to the up-input are added,
and counts applied to the down-input are subtracted.
follows:
• Before start-up, the system is completely empty of
parts, and the counter is reset manually to 0.
• When the operation begins, raw parts move through
the in-feed sensor, with each part generating an up
count.
Figure 8-24 Up/down-counter program.
Input A
Input B
Input C
L1 L2
Inputs
PB1
PB2
Reset
Outputs
Input A
Input B
Input C
CU
DN
CD
C5:2
C5:2
C5:2
CTU
COUNT-UP COUNTER
Counter C5:2
Preset 10
Accumulated 0
CTD
COUNT-DOWN COUNTER
Counter C5:2
Preset 10
Accumulated 0
C5:2
Output B
Output C
Output A
RES
Output A
Output B
Output C
CU
CU
C
B
A
DN
DN
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one scan time. If the input changes faster than one scan
period, the count value will become unreliable because
counts will be missed. When this situation occurs, you
need to use a high-speed counter input or a separate coun-
ter I/O module designed for high-speed applications.
8.4 Cascading Counters
Depending on the application, it may be necessary to
count events that exceed the maximum number allowable
per counter instruction. One way of accomplishing this
count is by interconnecting, or cascading, two counters.
The program of Figure 8-26 illustrates the application of
• After processing, finished parts appearing at the out-
feed sensor generate down counts, so the accumu-
lated count of the counter continuously indicates the
number of in-process parts.
• The counter preset value is irrelevant in this ap-
plication. It does not matter whether the counter
outputs are on or off. The output on-off logic is not
used. We have arbitrarily set the counter’s preset
values to 50.
The maximum speed of transitions that you can count
is determined by your program’s scan time. For a reli-
able count, your counter input signal must be fixed for
Figure 8-25 In-process monitoring program. (a) Process. (b) Program.
(a)
In-feed
(raw parts)
Out-feed
(finished parts)
Photoelectric
sensor
Photoelectric
sensor
Material
processing system
(b)
RESET
Reset
IN-Feed
count
IN-Feed
count
OUT-Feed
count
OUT-Feed
count
Inputs
L1
Reset to
zero
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
Accumulated = No. in-process parts
C5:1
50
0
CTD
COUNT-DOWN COUNTER
Counter
Preset
Accumulated
C5:1
50
0
C5:1
CU
CD
DN
DN
RES
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time of day or to log data pertaining to the operation of the
process. The logic used to implement a clock as part of a
PLC’s program is straightforward and simple to accom-
plish. A single timer instruction and counter instructions
are all you need.
Figure 8-28 illustrates a timer-counter program that
produces a time-of-day clock measuring time in hours
and minutes. The operation of the program can be sum-
marized as follows:
• An RTO timer instruction (T4:0) is programmed
first with a preset value of 60 seconds.
• The T4:0 timer times for a 60-s period, after which
its done bit is set.
• This, in turn, causes the up-counter (C5:0) of rung
001 to increment 1 count.
• On the next processor scan, the timer is reset and
begins timing again.
• The C5:0 counter is preset to 60 counts, and each
time the timer completes its time-delay period, its
count is incremented.
• When the C5:0 counter reaches its preset value of
60, its done bit is set.
• This, in turn, causes the up-counter (C5:1) of rung
002, which is preset for 24 counts, to increment
1 count.
• Whenever the C5:1 counter reaches its preset value
of 24, its done bit is set to reset itself.
the technique. The operation of the program can be sum-
marized as follows:
• The output of the first counter is programmed into
the input of the second counter.
• The status bits of both counters are programmed in
series to produce an output.
• These two counters allow twice as many counts to
be measured.
Another method of cascading counters is sometimes
used when an extremely large number of counts must be
stored. For example, if you require a counter to count up
to 250,000, it is possible to achieve this by using only
two counters. Figure 8-27 shows how the two counters
would be programmed for this purpose. The operation of
• Counter C5:1 has a preset value of 500 and counter
C5:2 has a preset value of 500.
• Whenever counter C5:1 reaches 500, its done bit re-
sets counter C5:1 and increments counter C5:2 by 1.
• When the done bit of counter C5:1 has turned
on and off 500 times, the output light becomes
energized. Therefore, the output light turns on after
500 3 500, or 250,000, transitions of the count
input.
Some PLCs include a real-time clock as part of their
instruction set. A real-time clock allows you to display the
Figure 8-26 Counting beyond the maximum count.
Count
button
Inputs
L1
Light
Light
PB1 B3:0/0
C5:0/DN C5:1/DN
C5:0/DN
PB2
Output L2
Reset
button
C5:1
32000
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
32000
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
C5:1
PB1
PB2
CU
DN
CU
DN
RES
RES
OSR
B3:0/0
OSR
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• The time of day is generated by examining the cur-
rent, or accumulated, count or time for each counter
and the timer.
• Counter C5:1 indicates the hour of the day in
24-h military format, while the current minutes
are represented by the accumulated count value of
counter C5:0.
• The timer displays the seconds of a minute as its
current, or accumulated, time value.
The 24-hour clock can be used to record the time of
an event. Figure 8-29 illustrates the principle of this tech-
nique. In this application the time of the opening of a
pressure switch is to be recorded. The operation of the
• The circuit is set into operation by pressing the reset
button and setting the clock for the time of day.
• This starts the 24-hour clock and switches the set
indicating light on.
• Should the pressure switch open at any time, the
clock will automatically stop and the trip indicating
light will switch on.
• The clock can then be read to determine the time of
opening of the pressure switch.
Figure 8-28 24-hour clock program.
001
002
003
004
005
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
T4:0
1.0
60
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
60
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
24
0
000
T4:0
C5:0
C5:1
Seconds
T4:0/DN
T4:0/DN
C5:0/DN
C5:0/DN
C5:1/DN
Minutes
Hours
EN
CU
CU
DN
DN
DN
RES
RES
RES
Figure 8-27 Cascading counters for extremely large counts.
Reset
Reset
Reset
C5:1
PB1
PB2
Inputs
L1
Count
Count
Output
L2
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
500
0
C5:1
DN
C5:1
DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:2
500
0
C5:2
DN
C5:2
CU
DN
CU
DN
RES
RES
Light
Light
B3:0/0
OSR
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Figure 8-31 illustrates an example of cutting objects to
a specified length. The object is advanced for a specified
distance and measured by encoder pulses to determine the
correct length for cutting.
Figure 8-32 shows a counter program used for length
measurement. This system accumulates the total length
of random pieces of bar stock moved on a conveyor.
follows:
• Count input pulses are generated by the magnetic
sensor, which detects passing teeth on a conveyor
drive sprocket.
8.5 Incremental Encoder-Counter
Applications
The incremental optical encoder shown in Figure 8-30
creates a series of square waves as its shaft is rotated. The
encoder disk interrupts the light as the encoder shaft is
rotated to produce the square wave output waveform.
The number of square waves obtained from the output of
the encoder can be made to correspond to the mechanical
movement required. For example, to divide a shaft revolu-
tion into 100 parts, an encoder could be selected to supply
100 square wave-cycles per revolution. By using a counter to
countthosecycles,wecouldtellhowfartheshafthadrotated.
Figure 8-29 Monitoring the time of an event.
Internal
B3:0/0
Internal
B3:0/0
Set
Outputs
L2
Set
Trip
Pressure
switch
Pressure
switch
Inputs
L1
Reset
Reset
Internal
B3:0/0
Trip
Internal
B3:0/0
Internal
B3:0/0
RTO
RETENTIVE TIMER ON
Timer
Time base
Preset
Accumulated
T4:0
1.0
60
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
24
0
Seconds
C5:0/DN
C5:0/DN
C5:1/DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
60
0
T4:0/DN
T4:0/DN
Minutes
Hours
C5:1
C5:0
T4:0
EN
CU
CU
DN
DN
DN
RES
RES
RES
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counter to accumulate counts only when bar stock
is moving.
• The counter is reset by closing the reset button.
8.6 Combining Counter
and Timer Functions
Many PLC applications use both the counter function and
the timer function. Figure 8-33 illustrates an automatic
stacking program that requires both a timer and counter.
• If 10 teeth per foot of conveyor motion pass the
sensor, the accumulated count of the counter would
indicate feet in tenths.
• The photoelectric sensor monitors a reference
point on the conveyor. When activated, it pre-
vents the unit from counting, thus permitting the
Figure 8-31 Cutting objects to a speciﬁed length.
Pulses
Rotary
encoder
Programmable
controller
Wood
Cutter
control
Figure 8-30 Optical incremental encoder.
Source: Photo courtesy Avtron, www.avtron.com.
Optical
encoder
Generated pulses
Optical
sensor
Light
source
Optical
disk
Lines
Figure 8-32 Counter used for length measurement. (a) Process. (b) Program.
(a)
Magnetic
sensor
Sprocket
Conveyor Reflector
Photoelectric
sensor
fl
(b)
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
10
0
C5:1
Magnetic
sensor
Inputs
Photo
sensor
L1 10 counts per foot
Magnetic
sensor
Reset
Reset
Photo
sensor
CU
DN
RES
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Figure 8-33 Automatic stacking program. (a) Process. (b) Program.
(b)
Outputs
L2
Stop
M2
C5:1
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
5
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
15
0
T4:1
DN
T4:1
DN
Stop
T4:1
C5:1
DN
M1 M2
M1
M1
M2
M2
T4:1
DN
M2
Photo
sensor
Photo
sensor
Stop
Stop
Start
Start
Inputs
L1
M2 run time
Number of plates
EN
CU
DN
DN
RES
RES
(a)
Complete stack
Metal plates
M1
Conveyor
M2
Conveyor
Light
source
Sensor
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• After 15 plates have been stacked, conveyor M1
stops and conveyor M2 begins running.
• After conveyor M2 has been operated for 5 s, it
stops and the sequence is repeated automatically.
• The done bit of the timer resets the timer and the
counter and provides a momentary pulse to auto-
matically restart conveyor M1.
Figure 8-34 shows a motor lock-out program. This
program is designed to prevent a machine operator from
In this process, conveyor M1 is used to stack metal plates
onto conveyor M2. The photoelectric sensor provides an
input pulse to the PLC counter each time a metal plate
drops from conveyor M1 to M2. When 15 plates have
been stacked, conveyor M2 is activated for 5 s by the PLC
as follows:
• When the start button is pressed, conveyor M1
begins running.
Figure 8-34 Motor lock-out program.
Source: This material and associated copyrights are proprietary to, and used with the permission of Schneider Electric.
Outputs
L2
Reset-PB
Reset-PB
C5:0
RES
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:0
1
300
0
EN
5 min
DN
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1
3600
0
EN
1 hr
DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
6
0
CU
DN
T4:0
EN
T4:0
EN
T4:1
DN
T4:1
DN
C5:0
DN
L
L
U
Motor
Motor
Motor
OL
Lock-out
light
Lock-out
light
Lock-out
light
OL
OL
Reset
PB
OL relay
Stop
Stop
Start
Start
Inputs
L1
OL
T4:0
DN
Lock-out
light
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• Sensor pulses continue but do not affect the PLC
counter.
• The number of parts for the past minute is repre-
sented by the accumulated value of the counter.
• The sequence is reset by momentarily opening and
closing the start switch.
A timer is sometimes used to drive a counter when
an extremely long time-delay period is required. For
example, if you require a timer to time to 1,000,000 s,
you can achieve this by using a single timer and counter.
Figure 8-36 shows how the timer and counter would be
programmed for such a purpose. The operation of the pro-
gram can be summarized as follows:
• Timer T4:0 has a preset value of 10,000, and coun-
ter C5:0 has a preset value of 100.
• Each time the timer T4:0 input contact closes for
10,000 s, its done bit resets timer T4:0 and incre-
ments counter C5:0 by 1.
• When the done bit of timer T4:0 has turned on
and off 100 times, the output light becomes
energized.
• Therefore, the output light turns on after 10,000 3
100, or 1,000,000, seconds after the timer input
contact closes.
starting a motor that has tripped off more than 5 times in
an hour. The operation of the program can be summarized
as follows:
• The normally open overload (OL) relay contact
momentarily closes each time an overload current is
sensed.
• Every time the motor stops due to an overload con-
dition, the motor start circuit is locked out for 5 min.
• If the motor trips off more than 5 times in an hour,
the motor start circuit is permanently locked out and
cannot be started until the reset button is actuated.
• The lock-out pilot light is switched on whenever a
permanent lock-out condition exists.
Figure 8-35 shows a product part flow rate program.
This program is designed to indicate how many parts pass
a given process point per minute. The operation of the
• When the start switch is closed, both the timer and
counter are enabled.
• The counter is pulsed for each part that passes the
parts sensor.
• The counting begins and the timer starts timing
through its 1-minute time interval.
• At the end of 1 minute, the timer done bit causes the
counter rung to go false.
Figure 8-35 Product ﬂow rate program.
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
60
0
EN
DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
0
0
CU
DN
Inputs
L1
Start SW
Start SW
Start SW
Sensor
Off On
Total
parts
1 min timer
T4:1
DN
C5:1
RES
Start SW
Sensor
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Figure 8-36 Timer driving a counter to produce an extremely long time-delay period.
Timer
input
Timer
input
Timer
input
Input
L1
S1
Light
Output
L2
Light
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:0
1.0
10000
0
EN
DN
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:0
100
0
CU
DN
C5:0
DN
T4:0
DN
T4:0
DN
C5:0
RES
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1. Name the three forms of PLC counter instructions,
and explain the basic operation of each.
2. State four pieces of information usually associated
with a PLC counter instruction.
3. In a PLC counter instruction, what rule applies to
the addressing of the counter and reset instructions?
4. When is the output of a PLC counter energized?
5. When does the PLC counter instruction increment
or decrement its current count?
6. The counter instructions of PLCs are normally
retentive. Explain what this means.
7. a. Compare the operation of a standard Examine-
on contact instruction with that of an off-to-on
transitional contact.
b. What is the normal function of a transitional
contact used in conjunction with a counter?
8. Explain how an OSR (one-shot rising) instruction
can be used to freeze rapidly changing data.
9. Identify the type of counter you would choose for
each of the following situations:
a. Count the total number of parts made during
each shift.
b. Keep track of the current number of parts in a
stage of a process as they enter and exit.
c. There are 10 parts in a full hopper. As parts
leave, keep track of the number of parts remain-
ing in the hopper
10. Describe the basic programming process involved
in the cascading of two counters.
11. a. When is the overflow bit of an up-counter
set?
b. When is the underflow bit of a down-counter
set?
12. Describe two common applications for counters.
13. What determines the maximum speed of transitions
that a PLC counter can count? Why?
I:1/1
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
50
0
CU
DN
C5:1/DN O:2/0
C5:1/DN O:2/1
I:1/2 C5:1
Res
Rung 1
Rung 2
Rung 3
Rung 4
a. What type of counter has been programmed?
b. When would output O:2/0 be energized?
c. When would output O:2/1 be energized?
d. Suppose your accumulated value is 24 and you
lose ac line power to the controller. When power
is restored to your controller, what will your ac-
cumulated value be?
e. Rung 4 goes true and while it is true, rung 1
goes through five false-to-true transitions of rung
conditions. What is the accumulated value of the
counter after this sequence of events?
f. When will the count be incremented?
g. When will the count be reset?
a. Suppose the input pushbutton is actuated from
off to on and remains held on. How will the
status of output B3:0/9 be affected?
b. Suppose the input pushbutton is now released to
the normally off position and remains off. How
will the status of output B3:0/9 be affected?
a. What type of counter has been programmed?
b. What input address will cause the counter to
increment?
CHAPTER 8 PROBLEMS
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• Turns on a light anytime the accumulated value
of the counter is less than 20.
• Turns on a second light when the accumulated
value of the counter is equal to or greater than 20.
• Resets the counter to 0 when a selector switch is
closed.
will execute the following control circuit correctly:
• Turns on a nonretentive timer when a switch is
closed (preset value of timer is 10 s).
• Resets timer automatically through a pro-
grammed transitional contact when it times out.
• Counts the number of times the timer goes
to 10 s.
• Resets counter automatically through a second
programmed transitional contact at a count of 5.
• Latches on a light at the count of 5.
• Resets light to off and counter to 0 when a selec-
tor switch is closed.
will correctly execute the industrial control process
in Figure 8-40. The sequence of operation is as
follows:
• Product in position (limit switch LS1 contacts
close).
• The start button is pressed and the conveyor
motor starts to move the product forward toward
position A (limit switch LS1 contacts open when
the actuating arm returns to its normal position).
• The conveyor moves the product forward to
position A and stops (position detected by 8 off-
to-on output pulses from the encoder, which are
counted by an up-counter).
• A time delay of 10 s occurs, after which the con-
veyor starts to move the product to limit switch
LS2 and stops (LS2 contacts close when the actu-
ating arm is hit by the product).
c. What input address will cause the counter to
decrement?
d. What input address will reset the counter to a
count of zero?
e. When would output O:6/2 be energized?
f. Suppose the counter is first reset, and then
input I:2/6 is actuated 15 times and input I:3/8
is actuated 5 times. What is the accumulated
count value?
the following counter specifications:
• Counts the number of times a pushbutton is
closed.
• Decrements the accumulated value of the counter
each time a second pushbutton is closed.
L1
Input
B3:0/1
B3:0/1
B3:0/9
Input
Input
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:2
25
0
CU
DN
I:2/6
CTD
COUNT-DOWN COUNTER
Counter
Preset
Accumulated
C5:2
25
0
CU
DN
C5:2
DN
O:6/2
I:3/8
C5:2
RES
I:4/1
Figure 8-40 Control process for Problem 6.
LS1 LS2
Forward
Position
A
Encoder
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As a result:
a. What is the accumulated count of counter CTU?
b. What is the accumulated count of counter CTD?
c. What is the state of output A?
d. What is the state of output B?
e. What is the state of output C?
8. Write a program to implement the process il-
lustrated in Figure 8-42. An up-counter must be
programmed as part of a batch-counting operation
to sort parts automatically for quality control. The
counter is installed to divert 1 part out of every
• An emergency stop button is used to stop the pro-
cess at any time.
• If the sequence is interrupted by an emergency
stop, counter and timer are reset automatically.
7. Answer the following questions with reference
to the up/down-counter program shown in Fig-
ure 8-41. Assume that the following sequence of
events occurs:
• Input C is momentarily closed.
• 20 on/off transitions of input A occur.
• 5 on/off transitions of input B occur.
Input A
PB1
PB2
Reset
Input B
Input C
L1 L2
Inputs Outputs
Input A
Input B
Input C
CU
DN
CD
C5:2
C5:2
C5:2
CTU
COUNT-UP COUNTER
Counter C5:2
Preset 10
Accumulated 0
CTD
COUNT-DOWN COUNTER
Counter C5:2
Preset 10
Accumulated 0
C5:2
CU
DN
CU
DN
Output B
Output C
Output A
RES
Output A
Output B
Output C
A
B
C
Quality control line
Gate
solenoid drive
Parts conveyer
line
Proximity
switch
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kit. The controller must stop the take-up spool
at a predetermined amount of resistors (100). A
worker on the floor will then cut the resistor strip
and place it in the kit. The circuit operates as
follows:
• A start/stop pushbutton station is used to turn the
spool motor drive on and off manually.
• A through-beam sensor counts the resistors as
they pass by.
• A counter preset for 100 (the amount of
resistors in each kit) will automatically stop
the take-up spool when the accumulated count
reaches 100.
• A second counter is provided to count the grand
total used.
• Manual reset buttons are provided for each
counter.
11. Write a program that will latch on a light 20 s after
an input switch has been turned on. The timer will
continue to cycle up to 20 s and reset itself until
the input switch has been turned off. After the third
time the timer has timed to 20 s, the light will be
unlatched.
12. Write a program that will turn a light on when a
count reaches 20. The light is then to go off when a
count of 30 is reached.
13. Write a program to implement the box-stacking
process illustrated in Figure 8-44. This applica-
tion requires the control of a conveyor belt that
feeds a mechanical stacker. The stacker can stack
various numbers of cartons of ceiling tile onto
each pallet (depending on the pallet size and the
preset value of the counter). When the required
number of cartons has been stacked, the conveyor
is stopped until the loaded pallet is removed and
an empty pallet is placed onto the loading area. A
photoelectric sensor will be used to provide count
pulses to the counter after each carton passes by.
In addition to a conveyor motor start/stop station,
a remote reset button is provided to allow the
operator to reset the system from the forklift after
an empty pallet is placed onto the loading area.
1000 for quality control or inspection purposes.
The circuit operates as follows:
• A start/stop pushbutton station is used to turn the
conveyor motor on and off.
• A proximity sensor counts the parts as they pass
by on the conveyor.
• When a count of 1000 is reached, the counter’s
output activates the gate solenoid, diverting the
part to the inspection line.
• The gate solenoid is energized for 2 s, which
allows enough time for the part to continue to the
quality control line.
• The gate returns to its normal position when the
2-s time period ends.
• The counter resets to 0 and continues to
accumulate counts.
• A reset pushbutton is provided to reset the
counter manually.
9. Write a program that will increment a counter’s
accumulated value 1 count every 60 s. A second
counter’s accumulated value will increment 1
count every time the first counter’s accumulated
value reaches 60. The first counter will reset when
its accumulated value reaches 60, and the second
counter will reset when its accumulated value
reaches 12.
10. Write a program to implement the process illus-
trated in Figure 8-43. A company that makes elec-
tronic assembly kits needs a counter to count and
control the number of resistors placed into each
Spool
motor drive
Through-beam
sensor
Reflector
Cartons of
ceiling tile
Pallet
Sensor
Stacker
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14. Write a program to operate a light according to the
following sequence:
• A momentary pushbutton is pressed to start the
sequence.
• The light is switched on and remains on for 2 s.
• The light is then switched off and remains off
for 2 s.
• A counter is incremented by 1 after this
sequence.
• The sequence then repeats for a total of 4 counts.
• After the fourth count, the sequence will stop and
the counter will be reset to zero.
The operation of this system can be summarized
as follows:
• The conveyor is started by pressing the start
button.
• As each box passes the photoelectric sensor, a
count is registered.
• When the preset value is reached (in this
case 12), the conveyor belt turns off.
• The forklift operator removes the loaded pallet.
• After the empty pallet is in position, the forklift
operator presses the remote reset button, which
then starts the whole cycle over again.
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176
The program control instructions covered in this
chapter are used to alter the program scan from
its normal sequence. The use of program con-
trol instructions can shorten the time required to
complete a program scan. Portions of the pro-
gram not being utilized at any particular time can
be jumped over, and outputs in speciﬁc zones in
the program can be left in their desired states.
Typical industrial program control applications
are explained.
Chapter Objectives
9.1 State the purpose of program control instructions
9.2 Describe the operation of the master control reset
instruction and develop an elementary program
illustrating its use
9.3 Describe the operation of the jump instruction and the
label instruction
9.4 Explain the function of subroutines
9.5 Describe the immediate input and output instructions
function
9.6 Describe the forcing capability of the PLC
9.7 Describe safety considerations built into PLCs and
programmed into a PLC installation
9.8 Explain the differences between standard and safety
PLCs
9.9 Describe the function of the selectable timed interrupt
and fault routine files
9.10 Explain how the temporary end instruction can be
used to troubleshoot a program
9
Program Control Instructions
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Program Control Instructions Chapter 9 177
TND (Temporary End)—Makes a temporary end
that halts program execution.
MCR (Master Control Reset)—Clears all set non-
retentive output rungs between the paired MCR
instructions.
SUS (Suspend)—Identifies conditions for debugging
and system troubleshooting.
Hardwired master control relays are used in relay con-
trol circuitry to provide input/output power shutdown of
an entire circuit. Figure 9-2 shows a typical hardwired
master control relay circuit. In this circuit, unless the mas-
ter control relay coil is energized, there is no power flow
to the load side of the MCR contacts.
PLC manufacturers offer a form of a master control relay
as part of their instruction set. These instructions function
in a similar manner to the hardwired master control relay;
that is, when the instruction is true, the circuit functions
normally, and when the instruction is false, nonretentive
outputs are switched off. Because these instructions are not
hardwired but programmed, for safety reasons they should
not be used as a substitute for a hardwired master control
relay, which provides emergency I/O power shutdown.
A Master Control Reset (MCR) instruction is an out-
put coil instruction that functions like a master control
9.
1 Master Control Reset Instruction
Several output-type instructions, which are often referred
to as override instructions, provide a means of execut-
ing sections of the control logic if certain conditions are
met. These program control instructions allow for greater
program flexibility and greater efficiency in the program
scan. Portions of the program not being utilized at any
particular time can be jumped over, and outputs in specific
zones in the program can be left in their desired states.
Program control instructions are used to enable or dis-
able a block of logic program or to move execution of a
program from one place to another place. Figure 9-1 shows
the Program Control menu tab for the Allen-Bradley
SLC 500 PLC and its associated RSLogix software. The
program control commands can be summarized as follows:
JMP (Jump to Label)—Jump forward/backward to a
corresponding label instruction.
LBL (Label)—Specifies label location.
JSR (Jump to Subroutine)—Jump to a designated
subroutine instruction.
RET (Return from Subroutine)—Exits current sub-
routine and returns to previous condition.
SBR (Subroutine)—Identifies the subroutine program.
Figure 9-1 Program Control menu tab.
JMP LBL JSR RET SBR TND
Program Control
MCR SUS
Ascii Control Ascii String Micro
Figure 9-2 Hardwired master control relay.
Source: This material and associated copyrights are proprietary to, and used with the permission of
Schneider Electric.
CR2
MCR
MCR MCR
Master stop
Master start
CR4
CR
M1
OL
CR1
M2
OL
M1
L1 L2
MCR
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178 Chapter 9 Program Control Instructions
it has no address. Figure 9-4 shows the programming of
an MCR fenced zone with the zone true. The operation
• The MCR zone is enclosed by a start fence, which
is a rung with a conditional MCR, and an end fence,
which is a rung with an unconditional MCR.
• Input A of the start rung is true so all outputs act
according to their rung logic as if the zone did not
exist.
Figure 9-5 shows the programmed MCR fenced zone
with the zone false. The operation of the program can be
• When the MCR in the start fence is false, all rungs
within the zone are treated as false. The scan ig-
nores the inputs and de-energizes all nonretentive
outputs (that is, the output energize instruction, the
on-delay timer, and the off-delay timer).
• All retentive devices, such as latches, retentive tim-
ers, and counters, remain in their last state.
• Input A of the start rung is false so output A and
T4:1 will be false and output B will remain in its last
state.
• The input conditions in each rung will have no
effect on the output conditions.
relay. MCR coil instructions are used in pairs and can
be programmed to control an entire circuit or to control
only selected rungs of a circuit. In the program of Fig-
ure 9-3, the MCR is programmed to control an entire
circuit. The operation of the program can be summarized
as follows:
• When the MCR instruction is false, or de-energized,
all nonretentive (nonlatched) rungs below the MCR
will be de-energized even if the programmed logic
for each rung is true.
• All retentive rungs will remain in their last state.
• The MCR instruction establishes a zone in the user
program in which all nonretentive outputs can be
turned off simultaneously.
• Retentive instructions should not normally be placed
within an MCR zone because the MCR zone main-
tains retentive instructions in the last active state
when the instruction goes false.
• An off-delay timer will start timing when in a de-
energized MCR zone.
Allen-Bradley SLC 500 controllers use the master
control reset instruction to set up single or multiple zones
within a program. The MCR instruction is used in pairs
to disable or enable a zone within a ladder program, and
Figure 9-3 Master Control Reset (MCR) instruction.
ON/OFF
L1 L2
Inputs
Level switch PL1
LS1 SOL
LS2 SOL
L
U
Stop Start
M
M
M
PL1
When MCR
is de-energized,
all nonretentive
outputs
de-energize.
When MCR
is de-energized,
all retentive
outputs remain
in last state.
Outputs
SOL
ON/OFF
Stop
Start
Level
switch
LS1
LS2
MCR
MCR
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Figure 9-5 MCR fenced zone with the zone false.
End fence
L1
Inputs
Input C
Input D
Input E
Input A
Input B
L2
Outputs
Output A
Output B
Start fence
OFF
ON
Input C
Input A
T4:1
1.0
10
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
Input D
Input E
Input B Output A
Latch output B
Unlatch output B
MCR
MCR
L
EN
DN
U
Figure 9-4 MCR fenced zone with the zone true.
L1 L2
Outputs
Output A
Output B
Inputs
Input C
Input C
Input A
T4:1
1.0
10
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
Input D
Input D
Input E
Input E
Input B Output A
Latch output B
Unlatch output B
Start fence
End fence
Active
ON
ON
Input A
Input B
MCR
MCR
DN
EN
L
U
A common application of an MCR zone control in-
volves examining one or more fault bits as part of the start
fence and enclosing the portion of the program you want
de-energized in case of a fault in the MCR zone. In case of
a detected fault condition, the outputs in that zone would
be de-energized automatically.
If you start instructions such as timers or counters in an
MCR zone, instruction operation ceases when the zone is
disabled. The TOF timer will activate when placed inside
a false MCR zone. When troubleshooting a program that
contains an MCR zone, you need to be aware of which
rungs are within zones in order to correctly edit the circuit.
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MCR-controlled areas must contain only two MCR
instructions—one to define the start and one to define the
end. Never overlap or nest MCR zones. Any additional
MCR instructions, or a jump instruction programmed
to jump to an MCR zone, could produce unexpected
and damaging results to your program and to machine
operation.
9.2 Jump Instruction
In PLC programming it is sometimes desirable to be able
to jump over certain program instructions when certain
conditions exist. The jump (JMP) instruction is an output
instruction used for this purpose. When the jump instruc-
tion is used, the PLC will not execute the instructions of a
rung that is jumped. The jump instruction is often used to
jump over instructions not pertinent to the machine’s op-
eration at that instant. In addition, sections of a program
may be programmed to be jumped should a production
fault occur.
Some manufacturers provide a skip instruction, which
is essentially the same as the jump instruction.
The program of Figure 9-6 illustrates the use of a
jump instruction in conjunction with Allen-Bradley
SLC 500 programmable controllers. Addresses Q2:0
through Q2:255 are the addresses used for the jump
(JMP) instructions. The label (LBL) instruction is a
target for the jump instruction. In addition, the jump
instruction with its associated label must have the same
address. The area of the program that the processor
jumps over is defined by the locations of the jump and
label instructions in the program. If the jump coil is en-
ergized, all logic between the jump and label instruc-
tions is bypassed and the processor continues scanning
after the LBL instruction.
follows:
• When the switch is open the jump instruction is not
activated.
• With the switch open, closing PB turns on all three
pilot lights.
• When the switch is closed the jump (JMP) instruc-
tion will activate.
• With the switch closed, pressing PB turns on pilot
lights PL1 and PL3 only.
• Rung 3 is skipped over during the PLC program
scan so PL2 is not turned on.
Figure 9-7 illustrates the effect on input and output in-
structions of jumped rungs in a program. The label in-
struction is used to identify the ladder rung that is the
target destination but does not contribute to logic continu-
ity. For practical purposes the label instruction is always
considered to be logically true. The operation of the pro-
• When rung 4 has logic continuity, the processor is
instructed to jump to rung 8 and continue to execute
the main program from that point.
• Jumped rungs 5, 6, and 7 are not scanned by the
processor.
• Input conditions for the jumped rungs are not exam-
ined and outputs controlled by these rungs remain in
their last state.
• Any timers or counters programmed within the
jump area cease to function and will not update
themselves during this period. For this reason they
should be programmed outside the jumped section
in the main program zone.
You can jump to the same label from multiple jump
locations, as illustrated in the program of Figure 9-8. In
this example, there are two jump instructions addressed
Q2:20. There is a single label instruction addressed
Q2:20. The scan can then jump from either jump instruc-
tion to label Q2:20, depending on whether input A or
input D is true.
It is possible to jump backward in the program, but this
should not be done an excessive number of times. Care
must be taken that the scan does not remain in a loop too
long. The processor has a watchdog timer that sets the
maximum allowable time for a total program scan. If this
time is exceeded, the processor will indicate a fault and
shut down.
The forward jump is similar to an MCR instruction in
that both permit an input logic condition to skip over a
block of PLC ladder logic. The main difference between
Figure 9-6 Jump (JMP) operation.
LBL
Q2:0
JMP
Q2:0
PL3
PL3
PL2
PL1
PB
Switch
Switch
PL2
PB
PB
PL1 L2
L1
Outputs
Inputs
1
2
3
4
PB
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the two is in how the outputs are handled when the in-
structions are executed. The MCR instruction sets all
nonretentive outputs to the false state and keeps the reten-
tive outputs in their last state. The JMP instruction leaves
all outputs in their last state. You should never jump into
a Master Control Reset zone. If you do, instructions that
are programmed within the MCR zone starting at the
LBL instruction and ending at the end MCR instruc-
tion will always be evaluated as though the MCR zone is
true, without consideration to the state of the start MCR
instruction.
9.3 Subroutine Functions
In addition to the main ladder logic program, PLC pro-
grams may also contain additional program files known as
subroutines. A subroutine is a short program that is used
by the main program to perform a specific function. Large
programs are often broken into subroutine program files,
which are called and executed from the main program. In
the SLC 500 series PLCs, the main ladder logic program
is in program file two (shown as LAD 2). Ladder logic
programs for subroutines can be placed in file number
three (LAD 3) through file number 255 (LAD 255).
Figure 9-7 Effect on input and output instructions of jumped rungs.
PL1
L1 L2
Outputs
M
M
PL2
Inputs
TS1
LS4
SOL3 SOL4
SOL3
PL2
SOL2
SOL1
SOL1
SOL3
LS3
TS1
LS2
LS2
LS1
LS1
LS1
PS1 PL1
PS1
LLS1
LLS1
PB3
PB3
PB2
PB1
PB1 PB2
LS4
SOL2
SOL3
SOL4
1
2
3
4
5
6
7
8
9
10
T4:6
1.0
5
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
M
M
M
T4:6
DN
Heater
JMP
Q2:1
Q2:1
LBL
Heater
Heater
Timers should be
programmed outside
the jumped section.
Jumped program rungs
are not scanned by the
processor.
Input conditions are not
examined, and outputs
remain in their last state.
DN
T4:6
LS3
PL2
PL1
EN
DN
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Use of subroutines is a valuable tool in PLC program-
ming. At times it is better to construct programs that con-
sist of several subroutines than a lengthy single program.
When programs are written with subroutines, each sub-
routine can be tested individually for functionality. These
subroutines can then be called from the main program as
illustrated in Figure 9-9.
When a subroutine is called from the main program,
the program is able to escape from the main program
and go to a program subroutine to perform certain func-
tions and then return to the main program. In situations
in which a machine has a portion of its cycle that must be
repeated several times during one machine cycle, the sub-
routine can save a great deal of duplicate programming.
The sequence of rungs could be programmed one time
into a subroutine and just called when needed.
The subroutine concept is the same for all program-
mable controllers, but the method used to call and return
from a subroutine uses different commands, depending on
the PLC manufacturer. The subroutine-related instructions
used in the Allen-Bradley PLCs shown in Figure 9-10 are
the jump to subroutine (JSR) output instruction, the sub-
routine (SBR) input instruction, and the return (RET) out-
put instruction.
The subroutine instructions can be summarized as
follows:
Jump to Subroutine (JSR)—The JSR instruction is
an output instruction that causes the scan to jump to
the program file designated in the instruction. It is the
only parameter entered in the instruction. When rung
conditions are true for this output instruction, it causes
the processor to jump to the targeted subroutine file.
Each subroutine must have a unique file number
(decimal 3–255).
Subroutine (SBR)—The SBR instruction is the first
input instruction on the first rung in the subroutine
file. It serves as an identifier that the program file is
a subroutine. This file number is used in the JSR in-
struction to identify the target to which the program
should jump. It is always true, and although its use is
optional, it is still recommended.
Return (RET)—The RET instruction is an output
instruction that marks the end of the subroutine file.
It causes the scan to return to the main program at the
instruction following the JSR instruction where it ex-
ited the program. The scan returns from the end of the
file if there is no RET instruction. The rung containing
the RET instruction may be conditional if this rung
precedes the end of the subroutine. In this way, the
processor omits the balance of a subroutine only if its
rung condition is true.
The jump to subroutine (JSR), subroutine (SBR), and
return (RET) instructions are used to direct the controller
to execute a subroutine file. Figure 9-11 shows a materials
Figure 9-9 Main program with a call from a subroutine.
Main program rungs
Jumps
Unconditional return
Subroutine area
Returns to next
instruction after JSR
JSR
SBR
RET
Figure 9-10 Allen-Bradley subroutine-related instructions.
JSR
JUMP-TO-SUBROUTINE
SBR file number U:3
SBR
SUBROUTINE
RET
RETURN
Figure 9-8 Jump-to-label from two locations.
T4:1
1.0
10
0
Q2:5
Input A
Output A
Input B
Input C
Q2:5
Input D
Output C
Input E
Output D
Q2:5
LBL
Input F
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
EN
DN
JMP
JMP
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(a)
Solenoid
Weight
sensor
Pilot light
Figure 9-11 Flashing pilot light subroutine. (a) Process. (b) Program.
OFF/ON
Sensor
Sensor
Sensor
SBR
SUBROUTINE
Main program
file 2
Subroutine
file 3
(b)
L1 L2
Inputs
Stop
Stop
M1
Outputs
PL1
PL1
OFF/ON SOL
SOL
Motor
Motor
Motor
T4:1/EN
T4:0/DN
T4:1/DN
JSR
JUMP-TO-SUBROUTINE
SBR file number
T4:0
1.0
1
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
DN
EN
T4:1
1.0
1
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
DN
EN
RET
RETURN
U:3
Sensor
PL1
Start
Start
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conveyor system with a flashing pilot light as a subrou-
tine. The operation of the program can be summarized as
follows:
• If the weight on the conveyor exceeds a preset
value, the solenoid is de-energized and pilot light
PL1will begin flashing.
• When the weight sensor switch closes, the JSR is
activated and directs the processor scan to jump to
the subroutine U:3.
• The subroutine program is scanned and pilot light
PL1 begins flashing.
• When the weight sensor switch opens, the proces-
sor will no longer scan the subroutine area and pilot
light PL1 will return to its normal on state.
The Allen-Bradley SLC 500 controller main program
is located in program file 2 whereas subroutines are as-
signed to program file numbers 3 to 255. Each subroutine
must be programmed in its own program file by assigning
it a unique file number. Figure 9-12 illustrates the proce-
dure for setting up a subroutine and can be summarized
as follows:
• Note each ladder location where a subroutine should
be called.
• Create a subroutine file for each location. Each sub-
routine file should begin with an SBR instruction.
• At each ladder location where a subroutine is called,
program a JSR instruction specifying the subroutine
file number.
• The RET instruction is optional.
– The end of a subroutine program will cause a re-
turn to the main program.
– If you want to end a subroutine program before it
executes to the end of program file, a conditional
return (RET) instruction may be used.
An optional SBR instruction is the header instruction
that stores incoming parameters. This feature lets you
pass selected values to a subroutine before execution
so the subroutine can perform mathematical or logi-
cal operations on the data and return the results to the
main program. For example, the program shown in Fig-
ure 9-13 will cause the scan to jump from the main pro-
gram file to program file 4 when input A is true. When
the scan jumps to program file 4, data will also be passed
from N7:30 to N7:40. When the scan returns to the main
program from program file 4, data will be passed from
N7:50 to N7:60.
Nesting subroutines allows you to direct program flow
from the main program to a subroutine and then to another
subroutine, as illustrated in Figure 9-14. Nested subrou-
tines make complex programming easier and program op-
eration faster because the programmer does not have to
continually return from one subroutine to enter another.
Programming nested subroutines may cause scan time
problems because while the subroutine is being scanned,
the main program is not. Excessive delays in scanning the
main program may cause the outputs to operate later than
required. This situation may be avoided by updating criti-
cal I/O using immediate input and/or immediate output
instructions.
9.4 Immediate Input and Immediate
Output Instructions
The immediate input and immediate output instructions
interrupt the normal program scan to update the input
image table file with current input data or to update an
output module group with the current output image table
file data. These instructions are intended to be used only
for time-critical I/O data.
The immediate input (IIN) Allen-Bradley PLC-5 in-
struction is used to read an input condition before the I/O
update is performed. This operation interrupts the pro-
gram scan when it is executed. After the immediate input
instruction is executed, normal program scan resumes.
This instruction is used with critical input devices that re-
quire updating in advance of the I/O scan.
Figure 9-12 Setting up a subroutine ﬁle.
JSR
JUMP-TO-SUBROUTINE
SBR file number 3
Main program
file 2
SBR
SUBROUTINE
RET
RETURN
Subroutine
file 3
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Figure 9-13 Passing subroutine parameters.
RETURN ( )
Return parameter N7:50
RET
JSR
Input A
Execution resumes
JUMP-TO-SUBROUTINE
Program file 4
Input parameter N7:30
Return parameter N7:60
SUBROUTINE
Input parameter N7:40
Subroutine
file 4
Main ladder program
Return parameter returned to the
address that you specified in the
JSR instruction when execution
returns to the main ladder program.
SBR
Input parameter passed to the
SBR instruction when execution
jumps to the subroutine file.
Figure 9-14 Nested subroutines.
JSR
JUMP-TO-SUBROUTINE
SBR file 3
JSR
JUMP-TO-SUBROUTINE
SBR file 4
Main program
file 2
SBR
SUBROUTINE
RET
RETURN
Level 1
file 3
JSR
JUMP-TO-SUBROUTINE
SBR file 5
SBR
SUBROUTINE
RET
RETURN
Level 2
file 4
SBR
SUBROUTINE
RET
RETURN
Level 3
file 5
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The operation of the immediate input instruction is il-
lustrated in Figure 9-15. When the program scan reaches
the immediate input instruction, the scan is interrupted
and the bits of the addressed word are updated. The
immediate input is most useful if the instruction asso-
ciated with the critical input device is at the middle or
toward the end of the program. The immediate input is
not needed near the beginning of the program since the
I/O scan has just occurred at that time. Although the im-
mediate input instruction speeds the updating of bits, its
scan-time interruption increases the total scan time of the
program. The operation of the program can be summa-
rized as follows:
• When the scan reaches a true IIN instruction, the
scan is interrupted.
• The processor updates 16 bits in the input
image table at the location indicated on the IIN
instruction.
• The two-digit address on the IIN instruction is com-
posed of the rack number (first digit) and the I/O
group number (second digit) containing the input or
inputs and needs immediate updating.
The immediate output (IOT) Allen-Bradley PLC-5 in-
struction is a special version of the output energize in-
struction used to update the status of an output device
before the I/O update is performed. The immediate output
is used with critical output devices that require updating in
advance of the I/O scan. When the program scan reaches
the immediate output instruction, the scan is interrupted
and the bits of the addressed word are updated. The opera-
tion of the immediate output instruction is illustrated in
Figure 9-16 and can be summarized as follows:
• When the program scan reaches a true IOT instruc-
tion, the scan is interrupted and the data in the
output image table at the word address on the in-
struction are transferred to the real-world outputs.
• In this example, the IOT instruction follows the out-
put energize instruction.
• Thus, the output image table word is updated first,
and then the data are transferred to the real-world
outputs.
Figure 9-15 Immediate input instruction.
Ι/O scan
Program scan
Immediate input instruction
interrupts program scan
and examines bits in word
I:012 here in program
Returns to
program
scan
Word I:012
2
Module
group
(input)
Rack 1
12
IIN
Immediate output instruction
interrupts program scan
Word O:013
Returns to
program
scan
Module
group
(output)
Rack 1
12
IIN
13
IOT
O:013
I:012
07 11
I/O scan
Program scan
3
Figure 9-16 Immediate output instruction.
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• This allows the programmer to update only sections
of the inputs to be used throughout the rest of the
program.
The immediate output with mask (IOM) instruction is
shown in Figure 9-18. The IOM operates on the physi-
cal outputs assigned to a particular word of a slot. When
the IOM rung is true, the program scan is interrupted to
update output data to the module located in the slot speci-
fied in the instruction. These data are then available to the
commands in the ladder following the IOM instruction.
The parameters entered are basically the same as those
entered for the IIM instruction.
Processor communication with the local chassis is
many times faster than communication with the remote
chassis. This is due to the fact that local I/O scan is
synchronous with the program scan and communication
is in parallel with the processor, whereas the remote
I/O scan is asynchronous with the program scan and
communication with remote I/O is serial. For this rea-
son, fast-acting devices should be wired into the local
chassis.
9.5 Forcing External I/O Addresses
The force function is essentially a manual override con-
trol function. Forcing allows the PLC user to turn an ex-
ternal input or output on or off from the keyboard of the
programming device. This is accomplished regardless of
the actual state of the field device. The forcing capability
allows a machine or process to continue operation until a
faulty field device can be repaired. It is also valuable dur-
ing start-up and troubleshooting of a machine or process
to simulate the action of portions of the program that have
not yet been implemented.
Forcing inputs manipulates the input image table file
bits and thus affects all areas of the program that use those
bits. The forcing of inputs is done just after the input scan.
When we force an input address, we are forcing the sta-
tus bit of the instruction at the I/O address to an on or
The Allen-Bradley SLC 500 PLC’s immediate I/O
instructions contain a few improvements over those of
the PLC-5. The SLC 500’s instructions, which are called
immediate input with mask (IIM) and immediate out-
put with mask (IOM), allow the programmer to specify
which of the 16 bits are to be copied from an input mod-
ule to the input image data table (or from the output
image table to an output module). The other bits in the
input image table or output module are not affected by
these instructions. In addition, the SLC 500 instructions
allow you to input or output a series of data words from
a single input module or output a series of data words to
an output module.
The immediate input with mask (IIM) instruction is
shown in Figure 9-17. The IIM instruction operates on
the inputs assigned to a particular word of a slot. When
the IIM rung is true, the program scan is interrupted, and
data from a specific input slot are transferred through
the mask to the input data file. These data are then avail-
able to the commands in the ladder following the IIM
instruction. The following parameters are entered in the
instruction:
Slot Specifies the slot and word that contain the data
to be updated. For example, I:3.0 means the input of
slot 3, word 0.
Mask Specifies either a hex constant or a register
address. For the mask, a 1 in the bit position passes
data from the source to the destination. A 0 inhibits
or blocks bits from passing from the source to the
destination.
Length Used to transfer more than one word per
slot.
The program operation of the instruction is summarized
as follows:
• The IIM instruction retrieves data from I:1.0 and
passes it through the mask.
• The mask permits only the four least significant bits
to be moved to the input register I:1.0.
Figure 9-17 Immediate input with mask (IIM) instruction.
IIM IOM SYC MSG IIE IID
Input/Output Compare Compute/Math
RPI REF
Move/Logic
IIM
Immediate Input w/ Mask
Slot
Mask
Length
I:1.0
000Fh
1
Figure 9-18 Immediate output with mask (IOM) instruction.
IIM IOM SYC MSG IIE IID
Input/Output Compare Compute/Math
RPI REF
Move/Logic
IOM
Immediate Output w/ Mask
Slot
Mask
Length
O:4.0
0FFFh
2
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bit of the output instruction at the address is usually not
affected. Figure 9-20 illustrates how an output is forced
on. The operation of the program can be summarized as
follows:
• The processor ignores the actual state of solenoid
output O:2/5.
• The programming device sets the force state in the
output force data file and the PLC implements the
force to turn solenoid output O:2/5 on even though
the output image table file indicates that the user
logic is setting the point to off.
• M output O:2/6 remains off because the status
bit of output O:2/5 is not affected by the force
instruction.
• Not all brands of PLCs operate this way. For ex-
ample, forcing an output with a GE Fanuc controller
will cause the contacts that have the same address as
the output to also change to the appropriate state.
off state. Figure 9-19 illustrates how an input is forced
on. The operation of the program can be summarized as
follows:
• The processor ignores the actual state of input limit
switch I:1/3.
• Although limit switch I:1/3 is off (0 or false) the
processor considers it as being in the on (1 or true)
state.
• The program scan records this, and the program is
executed with this forced status.
• In other words, the program is executed as if the
limit switch were actually closed.
Forcing outputs affects only the addressed output ter-
minal. Therefore, since the output image table file bits are
unaffected, your program will be unaffected. The forcing
of outputs is done just before the output image table file is
updated. When we force an output address, we are forcing
only the output terminal to an on or off state. The status
OFF
Input image table
1
Input module
I:1/3
Field input
device
Programming terminal
forces the state of
input I:1/3 ON (1)
O:2/5
O:2/5 O:2/6
I:1/3
M
L2
Outputs
Input
OFF
ON
ON
L1
Force ON
O:2/5
O:2/6
I:1/3
Ladder
logic
program
Forces Exist
Forces Enabled
ONLINE
Actual state of input
device (0) ignored
Figure 9-19 Forcing an input on.
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file online. With RSLogix 500 software, the steps are as
follows:
1. Open the program file in which you want to force
the logic on or off.
Overriding of physical inputs on conventional relay
control systems can be accomplished by installing
hardwire jumpers. With PLC control hardwire jumpers
are not necessary because the input data table values
can be forced to an on or off state. The force func-
tion allows you to override the actual status of external
input circuits by forcing external data bits on or off.
Similarly, you can override the processor logic and
status of output data file bits by forcing output bits
on or off. By forcing outputs off, you can prevent the
controller from energizing those outputs even though
the ladder logic, which normally controls them, may
be true. In other instances, outputs may be forced on
even though logic for the rungs controlling those out-
puts may be false.
Figure 9-21 shows the forces version of the data table
with bit I:1/3 forced on. You can enter and enable or
disable forces while you are monitoring your file off-
line, or in any processor mode while monitoring your
Figure 9-20 Forcing an output on.
O:2/5
O:2/5 O:2/6
I:1/3
M
L2
Outputs
Input
OFF
ON
OFF
L1
Force ON
Force ON
O:2/5
O:2/6
I:1/3
Ladder
logic
program
Forces Exist
Forces Enabled
ONLINE
Output image table
Status of bit
O:2/5 remains
at 0
Field output
devices
O:2/5
O:2/6
OFF
ON
Output module
M
0 0
Figure 9-21 Forces version of the data table with bit I:1/3
forced on.
Offset 15
.
14
.
13
.
12
.
11
.
10
.
9
.
8
.
7
.
6
.
5
.
4
.
2
.
.
1
.
.
0
.
.
3
1
I:1.0
I:2.0
.
Data File I1 (bin) . . INPUT Forces
Radix:
Columns:
Symbol:
Desc:
Enable Remove All Data File Help
I:1.0/3
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example, if maintenance personnel are performing rou-
tine maintenance on a de-energized motor, the machine
may suddenly become energized by someone forcing the
motor to turn on. This is why a hardwired master control
circuit is required for the I/O rack. The hardwired circuit
will provide a method of physically removing power to
the I/O system, thereby ensuring that it is impossible to
energize any inputs or outputs when the master control
is off.
9.6 Safety Circuitry
Sufficient emergency circuits must be provided to stop ei-
ther partially or totally the operation of the controller or
the controlled machine or process. These circuits should
be hardwired outside the controller so that in the event of
total controller failure, independent and rapid shutdown
is available.
Figure 9-23 shows typical safety wiring requirements
for a PLC installation. The safety requirements of this in-
stallation can be summarized as follows:
• A main disconnect switch is installed on the
incoming power lines as a means of removing
power from the entire programmable controller
system.
• The main power disconnect switch should be lo-
cated where operators and maintenance personnel
have quick and easy access to it. Ideally, the discon-
nect switch is mounted on the outside of the PLC
enclosure so that it can be accessed without opening
the enclosure.
• In addition to disconnecting electrical power, you
should de-energize, lock out, and tag all other
sources of power (pneumatic and hydraulic) before
you work on a machine or process controlled by the
controller.
• An isolation transformer is used to isolate the con-
troller from the main power distribution system and
step the voltage down to 120 VAC.
• A hardwired master control relay is included to pro-
vide a convenient means for emergency controller
shutdown. Because the master control relay allows
the placement of several emergency-stop switches
in different locations, its installation is important
from a safety standpoint.
• Overtravel limit switches or mushroom head emer-
gency stop pushbuttons are wired in series so that
when one of them opens, the master control is
de-energized.
• This removes power to input and output device
circuits. Power continues to be supplied to the
2. With the right mouse button, click the I/O bit you
want to force.
3. From the menu that appears, select Go to Data Table
or select Force On or Force Off.
4. From the associated data table that appears, click on
the Forces button.
5. The Forces version of the data table appears with
the selected bit highlighted. Click on this bit with
the right mouse button.
6. From the menu that appears, you can force the se-
lected bit on or off.
Exercise care when you use forcing functions. If
used incorrectly, force functions can cause injuries
to persons working around a system, and/or equip-
ment damage. For this reason, forcing functions should
be used only by personnel who completely understand
the circuit and the process machinery or driven equip-
ment (Figure 9-22). You must understand the potential
effect that forcing given inputs or outputs will have on
machine operation in order to avoid possible personal
injury and equipment damage. Before using a force
function, check whether the force acts on the I/O point
only or whether it acts on the user logic as well as on the
I/O point. Most programming terminals and PLC CPUs
provide some visible means of alerting the user that a
force is in effect.
In situations in which rotating equipment is involved,
the force instruction can be extremely dangerous. For
Figure 9-22 Exercise care when you use forcing functions.
Source: Courtesy Givens Engineering Inc.
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electromechanical component must not be dependent on
electronic components (hardware or software). Any part
can fail, including the switches in a master control relay
circuit. The failure of one of these switches would most
likely cause an open circuit, which would be a safe power-
off failure. However, if one of these switches shorts out, it
no longer provides any safety protection. These switches
should be tested periodically to ensure that they will stop
machine motion when needed. Never alter these circuits
to defeat their function. Serious injury or machine dam-
age could result.
controller power supply so that any diagnostic
indicators on the processor module can still be
observed.
• Note that the master control relay is not a substitute
for a disconnect switch. When you are replacing
any module, replacing output fuses, or working on
equipment, the main disconnect switch should be
pulled and locked out.
The master control relay must be able to inhibit all
machine motion by removing power to the machine I/O
devices when the relay is de-energized. This hardwired
L
Power
mains
Main disconnect switch
Step-down
isolation transformer
Fuse
120
VAC
Emergency stop switches
Emergency
stop
Overtravel
limit
switch
Stop Start
MCR
Machine
start/stop
buttons
Master
control
relay
PLC Control Panel
PLC power
supply
GND
L3
L2
L1
MCR
L1 2
MCR
L1 L2
PLC output module
PLC input module
L2
8
7
6
5
4
3
2
1
L1
L2
8
7
6
5
4
3
2
1
L1
Figure 9-23 Safety wiring requirements for a PLC installation.
Source: Courtesy Minarik Automation Control.
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• Safety PLCs use power supplies designed specifi-
cally for use in safety control systems and redun-
dant backplane circuitry between the controller and
I/O modules.
Safety considerations should be developed as part
of the PLC program. A PLC program for any applica-
tion will be only as safe as the time and thought spent
on both personnel and hardware considerations make
it. One such consideration involves the use of a motor
starter auxiliary seal-in contact, shown in Figure 9-25,
in place of the programmed contact referenced to the
output coil instruction. The use of the field-generated
starter auxiliary contact status in the program is more
costly in terms of field wiring and hardware, but it is
safer because it provides positive feedback to the pro-
cessor about the exact status of the motor. Assume,
for example, that the OL contact of the starter opens
under an overload condition. The motor, of course,
would stop operating because power would be lost to
the starter coil. If the program was written using an
examine-on contact instruction referenced to the out-
put coil instruction as the seal-in for the circuit, the
processor would never know that power had been lost
to the motor. When the OL was reset, the motor would
restart instantly, creating a potentially unsafe operating
condition.
Another safety consideration concerns the wiring
of stop buttons. A stop button is generally considered
a safety function as well as an operating function. As
such, all stop buttons should be wired using a nor-
mally closed contact programmed to examine for an on
Safety PLCs, such as the one shown in Figure 9-24,
are now available for applications that require more ad-
vanced safety functionality. A safety PLC is typically
certified by third parties to meet rigid safety and reliabil-
ity requirements of international standards. Both stan-
dard and safety PLCs have the ability to perform control
functions but a standard PLC was not initially designed
to be fault tolerant and fail-safe. That is the fundamental
difference.
Some of the differences between standard and safety
PLCs include the following:
• A standard PLC has one microprocessor that
executes the program, Flash memory area that
stores the program, RAM for making calcula-
tions, ports for communications, and I/O for
detection and control of the machine. In contrast,
a safety PLC has redundant microprocessors,
Flash and RAM that are continuously moni-
tored by a watchdog circuit, and a synchronous
detection circuit. Redundancy is duplication. The
probability of hazards arising from one malfunc-
tion in an electrical circuit can be minimized
by creating partial or complete redundancy
(duplication).
• Standard PLC inputs provide no internal means for
testing the functionality of the input circuitry. By
contrast, safety PLCs have an internal output circuit
associated with each input for the purpose of testing
the input circuitry. Inputs are driven both high and
low for very short cycles during runtime to verify
their functionality.
7
12
4
6
11
5
1 2
10
3 7
8
9
Number Feature
1 Module status indicators
2 Alphanumeric display
3 Node address switches
4 Baud rate switches
5 USB port
6 DeviceNet communication connector
7 Terminal connectors
8 Input status indicators
9 Output status indicators
10 IP address desplay switch
11 Ethernet connector
12 Service switch
Figure 9-24 Safety PLC.
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9.7 Selectable Timed Interrupt
The selectable timed interrupt (STI) instruction is used
to interrupt the scan of the main program file automati-
cally, on a time basis, to scan a specified subroutine file.
For Allen-Bradley SLC 500 controllers, the time base at
which the program file is executed and the program file
assigned as the selectable timed interrupt file are deter-
mined by the values stored in words S:30 and S:31 of the
status section of the data files. The value in S:30 stores
the time base, which may be from 1 through 32,767, at
10 millisecond increments. Word S:31 stores the program
file assigned as the selectable interrupt file, which may be
any program file from 3 through 999. Entering a 0 in the
time-base word disables the selectable timed interrupt.
condition (Figure 9-26). Using a normally open contact
programmed to examine for an off condition will pro-
duce the same logic but is not considered to be as safe.
Assume that the latter configuration is used. If, by some
chain of events, the circuit between the button and the
input point were to be broken, the stop button could be
depressed forever, but the PLC logic could never react
to the stop command because the input would never
be true. The same holds true if power were lost to the
stop button control circuit. If the normally closed wir-
ing configuration is used, the input point receives power
continuously unless the stop function is desired. Any
faults occurring with the stop circuit wiring, or a loss
of circuit power, would effectively be equivalent to an
intentional stop.
L1 L2
Stop
Stop
Start
Start
OL
Output
Inputs Ladder logic program
Main power
contact
Auxiliary
contact
M
M
M
Starter
auxiliary
contact
Starter
auxiliary
contact
Figure 9-25 Motor starter programmed using the starter auxiliary seal-in contact.
L1
Stop M
M
L2
OL
Output
M
M
Stop
Start
Start
Figure 9-26 Wiring of stop buttons.
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does not exist, the processor shuts down. When there is a
fault routine, and the fault is recoverable, the fault routine
is executed. If the fault is nonrecoverable, the fault rou-
tine is scanned once and shuts down. Either way, the fault
routine allows for an orderly shutdown.
9.9 Temporary End Instruction
The temporary end (TND) instruction is an output in-
struction used to progressively debug a program or
conditionally omit the balance of your current program
file or subroutines. When rung conditions are true, this
instruction stops the program scan, updates the I/O, and
resumes scanning at rung 0 of the main program file.
Figure 9-28 illustrates the use of the TND instruction
in troubleshooting a program. The TND instruction lets
your program run only up to this instruction. You can
move it progressively through your program as you debug
each new section. You can program the TND instruction
Programming the selectable timed interrupt is done
when a section of program needs to be executed on a
time basis rather than on an event basis. For example, a
program may require certain calculations to be executed
at a repeatable time interval for accuracy. These calcula-
tions can be accomplished by placing this programming
in the selectable timed-interrupt file. This instruction can
also be used for process applications that require periodic
lubrication.
The immediate input and immediate output instruc-
tions are often located in a selectable timed interrupt
file, so that a particular section of program is updated
on a timed basis. This process could be done on a high-
speed line, when items on the line are being examined
and the rate at which they pass the sensor is faster than
the scan time of the program. In this way, the item can
be scanned multiple times during the program scan, and
the appropriate action may be taken before the end of
the scan.
The selectable timed disable (STD) instruction
is generally paired with the selectable timed enable
(STE) instruction to create zones in which STI inter-
rupts cannot occur. Figure 9-27 illustrates the use of
the STD and STE instructions and can be summarized
as follows:
• In this program, the STI instruction is assumed to be
in effect.
• The STD and STE instructions in rungs 6 and 12
are included in the ladder program to avoid having
STI subroutine execution at any point in rungs 7
through 11.
• The STD instruction (rung 6) resets the STI enable
bit, and the STE instruction (rung 12) sets the
enable bit again.
• The first pass bit S:1/15 and the STE instruction in
rung 0 are included to ensure that the STI function
is initialized after a power cycle.
9.8 Fault Routine
Allen-Bradley SLC 500 controllers allow you to des-
ignate a subroutine file as a fault routine. If used, it
determines how the processor responds to a program-
ming error. The program file assigned as the fault rou-
tine is determined by the value stored in word S:29 of
the status file. Entering a 0 in word S:29 disables the
fault routine.
There are two kinds of major faults that result in a
processor fault: recoverable and nonrecoverable faults.
When the processor detects a major fault, it looks for a
fault routine. If a fault routine exists, it is executed; if one
S:1
15
2
1
0
Program file 3
3
4
5
8
9
10
7
14
15
16
17
13
11
STE
SELECTABLE TIMED ENABLE
6
STD
SELECTABLE TIMED DISABLE
12
STE
SELECTABLE TIMED ENABLE
End
STΙ execution
will not occur
between STD
and STE.
Figure 9-27 Selectable timed disable (STD) and selectable
timed enable (STE) instructions.
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unconditionally, or you can condition its rung according
to your debugging needs.
9.
10 Suspend Instruction
The suspend (SUS) instruction is used to trap and identify
specific conditions during system troubleshooting and
program debugging. Figure 9-29 shows a suspend instruc-
tion in a ladder logic rung. The execution of the instruc-
tion can be summarized as follows:
• When you program the SUS instruction, you must
enter a suspend ID number (number 100 is used in
this example).
• When the rung is true, the SUS output
instruction places the controller in the suspend
mode and the PLC immediately terminates
scan cycling.
• All ladder logic outputs are de-energized, but other
status files have the data present when the suspend
instruction is executed.
• The SUS instruction writes the suspend ID number
(100) to S:7 as it executes.
• You can include several SUS instructions in a pro-
gram, each with a different suspend ID and read S:7
to determine which SUS instruction caused the PLC
to halt.
• Status file S:8 will contain the number of the pro-
gram file that was executing when the SUS instruc-
tion executed.
T4:2
1.0
5
0
DN
EN
TON
TIMER ON DELAY
Timer
Time base (sec)
Preset
Accumulated
SW 12
T4:3
1.0
50
0
DN
EN
RTO
RETENTIVE TIMER ON
Timer
Time base (sec)
Preset
Accumulated
SW 3
LT4
T4:2
DN
TND
SW 13
L
SW 1
U
LT1
LT1
SW 2
RES
T4:3
PB 1
Main program
Remainder of main program
Temporary end
L1
Inputs
SW 13
Figure 9-28 Temporary end (TND) instruction.
100
Suspend
Suspend ID
SUS
Figure 9-29 Suspend (SUS) instruction.
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1. a. Two MCR output instructions are to be pro-
grammed to control a section of a program.
Explain the programming procedure to be
followed.
b. State how the status of the output devices within
the fenced zone will be affected when the MCR
instruction makes a false-to-true transition.
c. State how the status of the output devices within
the fenced zone will be affected when the MCR
instruction makes a true-to-false transition.
2. What is the main advantage of the jump
instruction?
3. What types of instructions are not normally in-
cluded inside the jumped section of a program?
Why?
4. a. What is the purpose of the label instruction in
the jump-to-label instruction pair?
b. When the jump-to-label instruction is executed,
in what way are the jumped rungs affected?
5. a. Explain what the jump-to-subroutine instruction
allows the program to do.
b. In what type of machine operation can this
instruction save a great deal of duplicate
programming?
6. What advantage is there to the nesting of
subroutines?
7. a. When are the immediate input and immediate
output instructions used?
b. Why is it of little benefit to program an immedi-
ate input or immediate output instruction near
the beginning of a program?
8. a. What does the forcing capability of a PLC allow
the user to do?
b. Outline two practical uses for forcing functions.
c. Why should extreme care be exercised when
using forcing functions?
9. Why should emergency stop circuits be hardwired
instead of programmed?
10. State the function of each of the following in the
basic safety wiring for a PLC installation:
a. Main disconnect switch
b. Isolation transformer
c. Emergency stops
d. Master control relay
11. Compare standard and safety PLCs with regard to:
a. Processors
b. Input circuitry
c. Output circuitry
d. Power supplies
12. When programming a motor starter circuit, why is
it safer to use the starter seal-in auxiliary contact
in place of a programmed contact referenced to the
output coil instruction?
13. When programming stop buttons, why is it safer
to use an NC pushbutton programmed to examine
for an on condition than an NO pushbutton pro-
grammed to examine for an off condition?
14. Explain the selectable timed interrupt function.
15. Explain the function of the fault routine file.
16. How is the temporary end instruction used to trou-
bleshoot a program?
1. Answer the questions, in sequence, for the MCR
program in Figure 9-30, assuming the program has
just been entered and the PLC is placed in the RUN
mode with all switches turned off.
a. Switches S2 and S3 are turned on. Will outputs
PL1 and PL2 come on? Why?
b. With switches S2 and S3 still on, switch S1 is
turned on. Will output PL1 or PL2 or both come
on? Why?
c. With switches S2 and S3 still on, switch S1 is
turned off. Will both outputs PL1 and PL2 de-
energize? Why?
d. With all other switches off, switch S6 is turned
on. Will the timer time? Why?
e. With switch S6 still on, switch S5 is turned on.
Will the timer time? Why?
f. With switch S6 still on, switch S5 is turned off.
What happens to the timer? If the timer was an
CHAPTER 9 PROBLEMS
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RTO type instead of a TON, what would happen
to the accumulated value?
2. Answer the questions, in sequence, for the jump-to-
label program in Figure 9-31. Assume all switches
are turned off after each operation.
a. Switch S3 is turned on. Will output PL1 be ener-
gized? Why?
b. Switch S2 is turned on first, then switch S5
is turned on. Will output PL4 be energized?
Why?
c. Switch S3 is turned on and output PL1 is ener-
gized. Next, switch S2 is turned on. Will output
PL1 be energized or de-energized after turning
on switch S2? Why?
d. All switches are turned on in order according
to the following sequence: S1, S2, S3, S5, S4.
Which pilot lights will turn on?
L1 L2
Outputs
MCR
MCR
S5
EN
DN
S6
MCR
S1
MCR
S2 PL1
S3
L
PL2
S4
U
PL2
1
2
3
4
5
6
7
8
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1:0
10
0
S2
S3
S4
S5
S6
PL1
PL2
S1
S4
10
LBL
L1 L2
Outputs
Inputs
S1
10
JMP
1
S2
S3
S5
2
4
3
5
PL1
PL4
PL3
PL2
PL2
PL3
PL4
PL1
S2
S3
S4
S5
S1
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3. Answer the questions, in sequence, for the jump-
to-subroutine and return program in Figure 9-32.
Assume all switches are turned off after each
operation.
a. Switches S1, S3, S4, and S5 are all turned
on. Which pilot light will not be turned on?
Why?
b. Switch S2 is turned on and then switch S4
is turned on. Will output PL3 be energized?
Why?
c. To what rung does the RET instruction return the
program scan?
Main program file 2
S1
S2
S3
S5 PL4
S4 PL3
PL2
PL1
JSR
JUMP-TO-SUBROUTINE
SBR file number
SBR
SUBROUTINE
Subroutine file 3
RET
RETURN
U:3
L1
Inputs
S2
S3
S4
S5
S1
L2
Outputs
PL2
PL3
PL4
PL1
4. Answer the questions, in sequence, for Figure 9-33.
Assume all switches are turned off after each
operation.
a. Switches S2, S12, and S5 are turned on in order.
Will output PL5 be energized? Why?
b. All switches except S7 are turned off. Will RTO
start timing? Why?
c. Switches S3 and S8 are turned on in order. Will
pilot light PL2 come on? Why?
d. When will timer TON function?
e. Assume all switches are turned on. In what order
will the rungs be scanned?
f. Assume all switches are turned off. In what
order will the rungs be scanned?
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S1
Main program file 2
SBR
SUBROUTINE
20
JMP
1
S2
S3
S4
S5
S7
S8
S9
S13
S12
S11
S10
2
3
4
20
JMP
5
6
20
JMP
7
8
LBL
LBL
9
T4:3/DN
10
11
12
JMP
12
13
RET
RETURN
15
PL2
PL3
PL4
PL5
PL6
PL1
S6
14
JSR
JUMP-TO-SUBROUTINE
SBR file number U:3
EN
DN
RES
EN
DN
RTO
RETENTIVE TIMER ON
Timer
Time base (sec)
Preset
Accumulated
T4:3
1.0
50
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:6
1.0
5
0
T4:3
Subroutine file 3
12
20
L1
Inputs
S2
S3
S4
S1
S6
S7
S8
S5
S10
S11
S9
S12
S13
L2
Outputs
PL2
PL3
PL1
PL4
PL5
PL6
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200
Data manipulation involves transferring data and
operating on data with math functions, data con-
versions, data comparison, and logical opera-
tions. This chapter covers both data manipulation
instructions that operate on word data and those
that operate on ﬁle data, which involve multiple
words. Data manipulations are performed inter-
nally in a manner similar to that used in micro-
computers. Examples of processes that need
these operations on a fast and continuous basis
are studied.
Chapter Objectives
10.1 Execute data transfer of word and file level
instructions from one memory location to another
10.2 Interpret data transfer and data compare instructions
as they apply to a PLC program
10.3 Compare the operation of discrete I/Os with that of
multibit and analog types
10.4 Understand the basic operation of PLC closed-loop
control systems
10
Data Manipulation
Instructions
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Data Manipulation Instructions Chapter 10 201
can be placed in two broad categories: data transfer and
data comparison.
The manipulation of entire words is an important fea-
ture of a programmable controller. This feature enables
PLCs to handle inputs and outputs containing multiple bit
configurations such as analog inputs and outputs. Arith-
metic functions also require data within the programma-
ble controller to be handled in word or register format.
To simplify the explanation of the various data manipula-
tion instructions available, the instruction protocol for the
Allen-Bradley SLC 500 families of PLCs will be used.
Again, even though the format and instructions vary with
each manufacturer, the concepts of data manipulation re-
main the same.
Figure 10-2 shows the Move/Logical menu tab for the
SLC 500 PLC and its associated RSLogix software. The
MOV (Move)—Moves the source value to the
destination.
MVM (Masked Move)—Moves data from a source
location to a selected portion of the destination.
AND (And)—Performs a bitwise AND operation.
OR (Or)—Performs a bitwise OR operation.
XOR (Exclusive Or)—Performs a bitwise XOR
operation.
NOT (Not)—Performs a bitwise NOT operation.
CLR (Clear)—Sets all bits of a word to zero.
10.2 Data Transfer Operations
Data transfer instructions simply involve the transfer of
the contents from one word or register to another. Fig-
ure 10-3a and b illustrate the concept of moving numeri-
cal binary data from one memory location to another.
Figure 10-3a shows the original data are in register N7:30
and N7:20. Figure 10-3b shows that after the data trans-
fer has occurred register N7:20 now holds a duplicate of
the information that is in register N7:30. The previously
existing data stored in register N7:20 have been replaced
with those of N7:30. This process is referred to as writing
over the existing data.
10.
1 Data Manipulation
Data manipulation instructions allow numerical data
stored in the controller’s memory to be operated on within
the control program. This category of word operation in-
structions allows the user to truly exploit the computer
capabilities of the PLC.
The use of data manipulation extends a controller’s ca-
pability from that of simple on/off control based on bi-
nary logic, to quantitative decision making involving data
comparisons, arithmetic, and conversions—which in turn
can be applied to analog and positioning control.
There are two basic classes of instructions to accom-
plish data manipulation: instructions that operate on word
data and those that operate on file, or block, data, which
involve multiple words.
Each data manipulation instruction requires two or
more words of data memory for operation. The words of
data memory in singular form may be referred to either
as registers or as words, depending on the manufacturer.
The terms table or file are generally used when a consecu-
tive group of related data memory words is referenced.
Figure 10-1 illustrates the difference between a word and
a file. The data contained in files and words will be in
the form of binary bits represented as series of 1s and 0s.
A group of consecutive elements or words in an Allen-
Bradley SLC 500 are referred to as a file.
The data manipulation instructions allow the move-
ment, manipulation, or storage of data in either single- or
multiple-word groups from one data memory area of the
PLC to another. Use of these PLC instructions in applica-
tions that require the generation and manipulation of large
quantities of data greatly reduces the complexity and
quantity of the programming required. Data manipulation
Figure 10-1 Data ﬁles, words, and bits.
1
1
0
1
1
0
0
1
0
1
1
1
0
0
0
0
1
1
0
0
1
1
0
1
1
1
1
1
1
1
1
1
0
1
1
0
0
1
1
1
1
1
0
1
0
1
0
1
1
1
0
1
0
1
0
1
1
1
0
0
0
0
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
0
1
1
1
0
0
0
0
1
0
1
1
1
1
0
1
1
1
1
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
1
0
1
0
1
0
1
0
1
0
0
1
1
1
1
0
1
1
1 0 1 1 0 1 1 1 0 1 0 1 0 1 1
Binary bit
Word or
register
File or
table
Section of
memory map
Figure 10-2 Move/Logical menu tab.
MOV MVM AND OR XOR NOT
Move/Logical
Compare Compute/Math
CLR
File/Misc
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202 Chapter 10 Data Manipulation Instructions
• The instruction may be programmed with input
conditions preceding it, or it may be programmed
unconditionally.
The move with mask (MVM) instruction differs slightly
from the MOV instruction because a mask word is involved
in the move. The data being moved must pass through the
mask to get to their destination address. Masking refers
to the action of hiding a portion of a binary word before
transferring it to the destination address. The operation of
a mask word can be summarized as follows:
• The pattern of characters in the mask determines
which source bits will be passed through to the des-
tination address.
• The bits in the mask that are set to zero (0) do not
pass data.
• Only the bits in the mask that are set to one (1) will
pass the source data through to the destination.
• Bits in the destination are not affected when the cor-
responding bits in the mask are zero.
• The MVM instruction is used to copy the desired
part of a 16-bit word by masking the rest of the
value.
Figure 10-5 shows an example of a mask move (MVM)
instruction. This instruction transfers data through the
mask from the source address, B3:0, to the destination
address, B3:4. The operation of the program can be sum-
marized as follows:
• The mask may be entered as an address or in hexa-
decimal format, and its value will be displayed in
hexadecimal.
Data transfer instructions can address almost any loca-
tion in the memory. Prestored values can be automatically
retrieved and placed in any new location. That location
may be the preset register for a timer or counter or even
an output register that controls a seven-segment display.
SLC 500 controllers use a block-formatted move (MOV)
instruction to accomplish data moves. The MOV instruc-
tion is used to copy the value in one register or word to an-
other. This instruction copies data from a source register
to a destination register. Figure 10-4 shows an example of
the MOV instruction. The operation of the program can
• When the rung is true, input switch A closed, the
value stored at the source address, N7:30, is copied
into the destination address, N7:20.
• When the rung goes false, input switch A opened,
the destination address will retain the value unless it
is changed elsewhere in the program.
• The source value remains unchanged and no data
conversion occurs.
Figure 10-3 Data transfer concept.
1 1 0
1 1 1 1 1 1 1 1
0 0 0 0 0
0 0 1
1 1 1 0 0 1 1 1
0 0 1 0 0
N7:20
N7:28
N7:29
N7:30
N7:31
N7:20
N7:28
N7:29
N7:30
N7:31
Original data stored in registers N7:30 and N7:20
1 1 0
1 1 1 1 1 1 1 1
0 0 0 0 0
1 1 0
1 1 1 1 1 1 1 1
0 0 0 0 0
Data transferred from register N7:30 to N7:20
(b)
(a)
N7:30
N7:20
MOV
MOVE
Source
Destination
N7:30
N7:20
N7: Integer table
A
L1
Input
A
Figure 10-4 SLC 500 block-formatted move instruction. Figure 10-5 Masked move (MVM) instruction.
B3:0
B3:1
FF0F
B3:4
MVM
MASKED MOVE
Source
1010101010101010
Mask
Destination
1010101011001010
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
1 1 1 1 1 1 1 10 0 0 0 1 1 1 1
1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0
1 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0
Source B3:0
Unchanged
Mask FF0F
Destination B3:4 before instruction
went true
Destination B3:4 after instruction
went true
A
L1
Input
A
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• Where there is a 1 in the mask, data will pass from
the source to the destination.
• Where there is a 0 in the mask, data in the destina-
tion will remain in their last state.
• Status in bits 4–7 are unchanged due to zeroes in the
mask (remained in their last state).
• Status in bits 0–3 and 8–15 were copied from the
source to destination when the MVM instruction went
true.
• The mask must be the same word size as the source
and destination.
The bit distribute (BTD) instruction is used to move
bits within a word or between words, as illustrated in
Figure 10-6. On each scan, when the rung that contains
the BTD instruction is true, the processor moves the bit
field from the source word to the destination word. Bits
are lost if they extend beyond the destination word; the
bits are not wrapped to the next higher word. To move data
within a word, enter the same address for the source and
destination. The source data will remain unchanged but
the instruction writes over the destination with the speci-
fied bits.
The program of Figure 10-7 illustrates how the move
(MOV) instruction can be used to create variable preset
timer values. A two-position selector switch is operated
to select one of two preset timer values. Operation of the
• When the selector switch is in the open 10 s posi-
tion, rung 2 has logic continuity and rung 3 does
not.
• As a result, the value 10 stored at the source ad-
dress, N7:1, is copied into the destination address,
T4:1.PRE.
• Therefore, the preset value of timer T4:1 will
change from 0 to 10.
• When pushbutton PB1 is closed, there will be
a 10 s delay period before the pilot light is
energized.
• When the selector switch is in the closed 5 s posi-
tion, rung 3 has logic continuity and rung 2 does
not.
• As a result, the value 5 stored at the source address,
N7:2, is copied into the destination address, T4:1.
PRE.
• Closing pushbutton PB1 will now result in a
5 s time-delay period before the pilot light is
energized.
Figure 10-6 Bit distribute (BTD) instruction.
(a) Moving bits within words.
N70:22
3
N70:22
10
6
BTD
BIT FIELD DISTRIBUTION
Source
Source bit
Destination
Destination bit
Length
1 0 1 1 0 1 1 0 1 1 0 1 N70:22
00
08
Destination bit
N70:22/10
Source bit
N70:22/3
07
15
(b) Moving bits between words.
0 1 1
1 0 1 1 1 0 1 N7:020
00
08 07
15
1
1
0 1 1
0 1 1 0 1 N7:022
00
08 07
15
N7:020
3
N7:022
5
10
BTD
BIT FIELD DISTRIBUTION
Source
Source bit
Destination
Destination bit
Length
Source bit N7:020/3
Destination bit N7:022/5
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The program of Figure 10-8 illustrates how the move
(MOV) instruction can be used to create variable preset
counter values. The operation of the program can be sum-
marized as follows:
• Limit switch LSI is programmed to the input of
up-counter C5:1 and counts the number of parts
coming off a conveyor line onto a storage rack.
• Three different types of products are run on this
line.
• The storage rack has room for only 300 boxes of
product A or 175 boxes of product B or 50 boxes of
product C.
• Three momentary switches are used to select the de-
sired preset counter value depending on the product
line (A, B, or C) being manufactured.
• A reset button is provided to reset the accumulated
count to 0.
• A pilot lamp is switched on to indicate when the
storage rack is full.
• The program has been constructed so that normally
only one of the three switches will be closed at any
one time. If more than one of the preset counter
switches is closed, the last value is selected.
A file is a group of related consecutive words in the
data table that have a defined start and end and are used to
store information. For example, a batch process program
may contain several separate recipes in different files that
can be selected by an operator.
In some instances it may be necessary to shift complete
files from one location to another within the programma-
ble controller memory. Such data shifts are termed file-
to-file shifts. File-to-file shifts are used when the data in
one file represent a set of conditions that must interact
with the programmable controller program several times
and, therefore, must remain intact after each operation.
Because the data within this file must also be changed by
the program action, a second file is used to handle the data
changes, and the information within that file is allowed
to be altered by the program. The data in the first file,
however, remain constant and therefore can be used many
times. Other types of data manipulation used with file in-
structions include word-to-file and file-to-word moves, as
illustrated in Figure 10-9.
Files allow large amounts of data to be scanned quickly
and are useful in programs requiring the transfer, com-
parison, or conversion of data. Most PLC manufacturers
display file instructions in block format on the program-
ming terminal screen. Figure 10-10 compares the SLC
Figure 10-7 Move instruction used to change the preset time of a timer.
T4:1
1.0
0
0
DN
EN
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
L1
PB1
PL1
L2
Output
PB1
SS1
SS1
SS1
5 s
10 s
Inputs
N7:1
10
T4:1.PRE
0
MOV
MOVE
Source
Destination
N7:2
5
T4:1.PRE
0
MOV
MOVE
Source
Destination
T4:/DN
1
2
3
4
PL1
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500 controller word and file addressing. The addressing
formats can be summarized as follows:
• The address that defines the beginning of a file or
group of words starts with the pound sign #.
• The # prefix is omitted in a single word or element
address.
• Address N7:30 is a word address that represents a
single word: word number 30 in integer file 7.
• Address #N7:30 represents the starting address of
a group of consecutive words in integer file 7. The
length is eight words, which is determined by the
instruction where the file address is used.
Figure 10-8 Move instruction used to change the preset count of a counter.
L1
LS1
A
C
B
Inputs
Reset
C5:1
0
0
DN
CU
CTU
COUNT UP
Counter
Preset
Accumulated
N7:1
300
C5:1.PRE
0
MOV
MOVE
Source
Destination
Full
N7:2
175
C5:1.PRE
0
MOV
MOVE
Source
Destination
N7:3
50
C5:1.PRE
0
MOV
MOVE
Source
Destination
C5:1/DN
C5:1
RES
1
2
3
4
5
6
LS1
A
C
B
Reset
L2
Output
Full
Figure 10-9 Moving data using ﬁle instructions.
Word-to-file move
File
Word Word
File File File
File-to-word move
File-to-file move
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The file arithmetic and logic (FAL) instruction is used
to copy data from one file to another and to do file math
and file logic. This instruction is available only on Allen-
Bradley PLC-5 and ControlLogix platforms. An example
of the FAL instruction is shown in Figure 10-11.
The basic operation of the FAL instruction is similar
in all functions and requires the following parameters and
PLC-5 addresses to be entered in the instruction:
Control
• Is the first entry and the address of the control struc-
ture in the control area (R) of processor memory.
• The processor uses this information to run the
instruction.
• The default file for the control file is data file 6.
• The control element for the FAL instruction must be
unique for that instruction and may not be used to
control any other instruction.
• The control element is made up of three words.
• The control word uses four control bits: bit 15
(enable bit), bit 13 (done bit), bit 11 (error bit), and
bit 10 (unload bit).
Length
• Is the second entry and represents the file length.
• This entry will be in words, except for the
floating-point file, for which the length is in
elements. (A floating-point element consists of
two words.)
• The maximum length possible is 1000 elements.
Enter any decimal number from 1 to 1000.
Position
• Is the third entry and represents the current location
in the data block that the processor is accessing.
• It points to the word being operated on.
• The position starts with 0 and indexes to 1 less than
the file length.
• You generally enter a 0 to start at the beginning of
a file. You may also enter another position at which
you want the FAL to start its operation.
• When the instruction resets, however, it will reset
the position to 0.
• You can manipulate the position from the program.
Mode
• Is the fourth entry and represents the number of file
elements operated on per program scan. There are
three choices: all mode, numeric mode, and incre-
mental mode.
All Mode
• For this mode you enter the letter A.
• In the all mode, the instruction will transfer the
complete file of data in one scan.
• The enable (EN) bit will go true when the instruc-
tion goes true and will follow the rung condition.
• When all of the data have been transferred, the done
(DN) bit will go true. This change will occur on the
same scan during which the instruction goes true.
• If the instruction does not go to completion due to an
error in the transfer of data (such as trying to store
too large or too small a number for the data-table
Figure 10-10 SLC 500 word and ﬁle address.
Integer Table
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
N7:30/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:31/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:32/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:33/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:34/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:35/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:36/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:37/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Radix: Table: N7: Integer
File
#N7:30
Length = 8
N7:37
Word
N7:30
Binary
Figure 10-11 File arithmetic/logic (FAL) instruction.
EN
DN
ER
FAL
File arith/logical
Control
Length
Position
Mode
Destination
Expression
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type), the instruction will stop at that point and set
the error (ER) bit. The scan will continue, but the in-
struction will not continue until the error bit is reset.
• If the instruction goes to completion, the enable bit
and the done bit will remain set until the instruction
goes false, at which point the position, the enable
bit, and the done bit will all be reset to 0.
Numeric Mode
• For this mode you enter a decimal number (1–1000).
• In the numeric mode, the file operation is distributed
over a number of program scans.
• The value you enter sets the number of elements to
be transferred per scan.
• The numeric mode can decrease the time it takes
to complete a program scan. Instead of waiting for
the total file length to be transferred in one scan, the
numeric mode breaks up the transfer of the file data
into multiple scans, thereby cutting down on the in-
struction execution time per scan.
Incremental mode
• For this mode you enter the letter I.
• In the incremental mode, one element of data is
operated on for every false-to-true transition of the
instruction.
• The first time the instruction sees a false-to-true
transition and the position is at 0, the data in the first
element of the file are operated on. The position will
remain at 0 and the UL bit will be set. The EN bit
will follow the instruction’s condition.
• On the second false-to-true transition, the position
will index to 1, and data in the second word of the
file will be operated on.
• The UL bit controls whether the instruction will
operate just on data in the current position, or
whether it will index the position and then trans-
fer data. If the UL bit is reset, the instruction—on
a false-to-true transition of the instruction—will
operate on the data in the current position and set
the UL bit. If the UL bit is set, the instruction—on
a false-to-true transition of the instruction—will
index the position by 1 and operate on the data in
their new position.
Destination
• Is the fifth entry and is the address at which the pro-
cessor stores the result of the operation.
• The instruction converts to the data type specified
by the destination address.
• It may be either a file address or an element address.
Expression
• Is the last entry and contains addresses, program
constants, and operators that specify the source of
data and the operations to be performed.
• The expression entered determines the function of
the FAL instruction.
• The expression may consist of file addresses, ele-
ment addresses, or a constant and may contain only
one function because the FAL instruction may per-
form only one function.
Figure 10-12 shows an example of a file-to-file copy
function using the FAL instruction. The operation of the
• When input A goes true, data from the expression
file #N7:20 will be copied into the destination file
#N7:50.
• The length of the two files is set by the value en-
tered in the control element word R6:1.LEN.
• In this instruction, we have also used the ALL
mode, which means all of the data will be trans-
ferred in the first scan in which the FAL instruction
sees a false-to-true transition.
• The DN bit will also come on in that scan unless an
error occurs in the transfer of data, in which case the
ER bit will be set, the instruction will stop operation
at that position, and then the scan will continue at
the next instruction.
Figure 10-12 File-to-ﬁle copy function using the FAL
instruction.
EN
DN
ER
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
R6:1
6
0
All
#N7:50
528
#N7:20
L1
A
Input Ladder logic program
528
621
778
986
342
135
N7:20
File
N7:25
528
621
778
986
342
135
N7:50
Destination
#N7:50
Expression
#N7:20
N7:55
A
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Figure 10-13 shows an example of a file-to-word copy
function using the FAL instruction. The operation of the
• With each false-to-true rung transition of input A,
the processor reads one word of integer file N29.
• The processor starts reading at word 0, and writes
the image into word 5 of integer file N29.
• The instruction writes over any data in the
destination.
Figure 10-14 shows an example of a word-to-file copy
function using the FAL instruction. It is similar to the file-
to-word copy function except that the instruction copies
data from a word address into a file. The operation of the
• The expression is a word address (N7:100) and the
destination is a file address (#N7:101).
• If we start with position 0, the data from N7:100
will be copied into N7:101 on the first false-to-true
transition of input A.
• The second false-to-true transition of input A will
copy the data from N7:100 into N7:102.
• On successive false-to-true transitions of the instruc-
tion, the data will be copied into the next position in
the file until the end of the file, N7:106, is reached.
The exceptions to the rule that file addresses must take
consecutive words in the data table are in the timer, counter,
and control data files for the FAL instruction. In these three
data files, if you designate a file address, the FAL instruc-
tion will take every third word in that file and make a file of
preset, accumulated, length, or position data within the cor-
responding file type. This might be done, for example, so
that recipes storing values for timer presets can be moved
into the timer presets, as illustrated in Figure 10-15.
The file copy (COP) instruction and the fill file (FLL)
instruction are high-speed instructions that operate more
quickly than the same operation with the FAL instruc-
tion. Unlike the FAL instruction, there is no control ele-
ment to monitor or manipulate. Data conversion does not
Figure 10-13 File-to-word copy function using the FAL
instruction.
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
R6:6
5
0
INC
N29:5
#N29:0
Word 0
1
2
3
4
Word
Word N29:5
File # N29:0
First move
Second move
Fifth move
Fourth move
Third move
EN
DN
ER
L1
A
Input
A FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
R6:2
6
0
Incremental
#N7:101
0
N7:100
N7:101
N7:106
N7:100
First move
EN
DN
ER
L1
A
Input
A
Figure 10-14 Word-to-ﬁle copy function using the FAL
instruction.
Figure 10-15 Copying recipes and storing values for timer
presets.
3452
6789
8321
983
#N7:10
Length = 4
Recipe A
T4:0 preset value
T4:1 preset value
T4:2 preset value
T4:3 preset value
#T4:0.PRE
Length = 4
File of timer preset values
778
986
342
135
#N7:20
Length = 4
Recipe B
File-to-file copy
File-to-file copy
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take place, so the source and destination should be the
same file types. An example of the file COP instruction is
shown in Figure 10-16. The operation of the program can
• Both the source and destination are file addresses.
• When input A goes true, the values in file N40 are
copied to file N20.
• The instruction copies the entire file length for each
scan during which the instruction is true.
An example of the fill file (FLL) instruction is shown in
Figure 10-17. It operates in a manner similar to the FAL
instruction that performs the word-to-file copy in the ALL
mode. The operation of the program can be summarized
as follows:
• When input A goes true, the value in N15:5 is cop-
ied into N20:1 through N20:6.
• Because the instruction transfers to the end of the
file, the file will be filled with the same data value in
each word.
The FLL instruction is frequently used to zero all of
the data in a file, as illustrated in the program of Fig-
ure 10-18. The operation of the program can be summa-
rized as follows:
• Momentarily pressing pushbutton PB1 copies the
contents of file #N10:0 into file #N12:0.
• Momentarily pressing pushbutton PB2 then clears
file #N12:0.
• Note that 0 is entered for the source value.
10.3 Data Compare Instructions
Data transfer operations are all output instructions,
whereas data compare instructions are input instructions.
Data compare instructions are used to compare numerical
values. These instructions compare the data stored in two
or more words (or registers) and make decisions based on
the program instructions. Numeric values in two words
of memory can be compared for each of the basic data
compare instructions shown in Figure 10-19, depending
on the PLC.
Data comparison concepts have already been used
with the timer and counter instructions. In both these in-
structions, an output was turned on or off when the ac-
cumulated value of the timer or counter equaled its preset
Figure 10-16 File copy (COP) instruction.
N20:1
N20:2
N20:3
N20:4
N20:5
N20:6
N40:1
N40:2
N40:3
N40:4
N40:5
N40:6
COP
COPY FILE
Source
Destination
Length
#N40:1
#N20:1
6
L1
A
Input
A
COP FLL DDV SCL INT STE
File / Misc
Move/Logical
STS STD PID
File Shift/Sequencer Prog
Location in RSLogix software
Figure 10-17 Fill ﬁle (FLL) instruction.
N20:1
N20:2
N20:3
N20:4
N20:5
N20:6
N15:5
COP FLL DDV SCL INT STE
File / Misc
Move/Logical
STS STD PID
File Shift/Sequencer Prog
Location in RSLogix software
FLL
FILL FILE
Source
Destination
Length
N15:5
#N20:1
6
L1
A
Input
A
Figure 10-18 Using the FLL instruction to change all the
data in a ﬁle to zero.
COP
COPY FILE
Source
Destination
Length
#N10:0
#N12:0
4
FLL
FILL FILE
Source
Destination
Length
0
#N12:0
4
L1
PB1
PB2
Inputs
PB1
PB2
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value. What actually occurred was that the accumulated
numeric data in one memory word was compared to the
preset value of another memory word on each scan of the
processor. When the processor saw that the accumulated
value was equal to the preset value, it switched the output
on or off.
Comparison instructions are used to test pairs of val-
ues to determine if a rung is true. Figure 10-20 shows the
Compare menu tab for the Allen-Bradley SLC 500 PLC
and its associated RSLogix software. The compare in-
structions can be summarized as follows:
LIM (Limit test)—Tests whether one value is within
the limit range of two other values.
MEQ (Masked Comparison for Equal)—Tests
portions of two values to see whether they are equal.
Compares 16-bit data of a source address to 16-bit
data at a reference address through a mask.
EQU (Equal)—Tests whether two values are equal.
NEQ (Not Equal)—Tests whether one value is not
equal to a second value.
LES (Less Than)—Tests whether one value is less
than a second value.
GRT (Greater Than)—Tests whether one value is
greater than a second value.
LEQ (Less Than or Equal)—Tests whether one
value is less than or equal to a second value.
GEQ (Greater Than or Equal)—Tests whether one
value is greater than or equal to a second value.
The equal (EQU) instruction is an input instruction that
compares source A to source B: when source A is equal
to source B, the instruction is logically true; otherwise it
is logically false. Figure 10-21 shows an example of an
EQU logic rung. The operation of the rung can be sum-
marized as follows:
• When the accumulated value of counter T4:0 stored
in source A’s address equals the value in source B’s
address, N7:40, the instruction is true and the output
is energized.
• Source A may be a word address or a floating-point
address.
• Source B may be a word address, a floating-point
address, or a constant value.
• With the equal instruction, the floating-point data
is not recommended because of the exactness re-
quired. One of the other comparison instructions,
such as the limit test, is preferred.
The not equal (NEQ) instruction is an input instruction
that compares source A to source B: when source A is not
equal to source B, the instruction is logically true; other-
wise it is logically false. Figure 10-22 shows an example
of an NEQ logic rung. The operation of the rung can be
• When the value stored at source A’s address, N7:5,
is not equal to 25, the output will be true; otherwise,
the output will be false.
• The value stored at Source A is 30.
• The value stored at Source B is 25.
• Since the two values are not the same the output
will be true or on.
• In all input-comparison instructions, Source A must
be an address and Source B can be an address or a
constant.
Figure 10-19 Basic PLC data compare instructions.
Name
Equal to
Not equal to
Less than
Greater than
Less than or equal to
Greater than or equal to
Symbol
(⫽)
(苷)
(⬍)
(⬎)
(ⱕ)
(ⱖ) Figure 10-21 EQU logic rung.
L2
PL1
PL1
EQU
EQUAL
Source A
T4:0.ACC
Source B
N7:40
Output
Figure 10-22 NEQ logic rung.
L2
Output
PL1
PL1
N7:5
30
25
NEQ
NOT EQUAL
Source A
Source B
Figure 10-20 Compare menu tab.
LIM MEQ EQU NEQ LES GRT
Compare
Bit Input/Output
Timer/Counter
LEQ GEQ
Com
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The greater than (GRT) instruction is an input instruc-
tion that compares source A to source B: when source A
is greater than source B, the instruction is logically true;
otherwise it is logically false. Figure 10-23 shows an ex-
ample of a GRT logic rung. The operation of the rung can
• The instruction is either true or false, depending on
the values being compared.
• When the accumulated value of timer T4:10, stored
at the address of source A, is greater than the con-
stant 200 of source B, the output will be on; other-
wise the output will be off.
The less than (LES) instruction is an input instruction
that compares source A to source B: when source A is less
than source B, the instruction is logically true; otherwise
it is logically false. Figure 10-24 shows an example of an
LES logic rung. The operation of the rung can be sum-
marized as follows:
• The instruction is either true or false, depending on
the values being compared.
• When the accumulated value of counter C5:10,
stored at the address of source A, is less than the
constant 350 of source B, the output will be on; oth-
erwise, it will be off.
The greater than or equal (GEQ) instruction is an input
instruction that compares source A to source B: when
source A is greater than or equal to source B, the instruc-
tion is logically true; otherwise it is logically false. Fig-
ure 10-25 shows an example of a GEQ logic rung. The
operation of the rung can be summarized as follows:
• When the value stored at the address of source A,
N7:55, is greater than or equal to the value stored at
the address of source B, N7:12, the output will be
true; otherwise, it will be false.
• The value stored at source A is 100.
• The value stored at source B is 23.
• Therefore the output will be true or on.
The less than or equal (LEQ) instruction is an input
instruction that compares source A to source B: when
source A is less than or equal to source B, the instruction
is logically true; otherwise it is logically false. Figure 10-26
shows an example of an LEQ logic rung. The operation of
the rung can be summarized as follows:
• When the accumulated count of counter C5:1 is less
than or equal to 457, the pilot light will turn on.
• The accumulated value of the counter is less
than 457.
• Therefore the output will be false or off.
The limit test (LIM) instruction is used to test whether
values are within or outside the specified range. Applica-
tions in which the limit test instruction is used include
allowing a process to operate as long as the temperature is
within or outside a specified range.
Programming the LIM instruction consists of entering
three parameters: low limit, test, and high limit. The limit
test instruction functions in the following two ways:
• The instruction is true if—The lower limit is equal
to or less than the higher limit, and the test param-
eter value is equal to or inside the limits. Otherwise
the instruction is false.
• The instruction is true if—The lower limit has a
value greater than the higher limit, and the instruc-
tion is equal to or outside the limits. Otherwise the
instruction is false.
Figure 10-23 GRT logic rung.
GRT
GREATER THAN
Source A
T4:10.ACC
Source B
200
L2
Output
PL1
PL1
Figure 10-24 LES logic rung.
LES
LESS THAN
Source A
C5:10.ACC
Source B
350
L2
Output
PL1
PL1
Figure 10-25 GEQ logic rung.
GEQ
GREATER THAN OR EQUAL
Source A
Source B
N7:55
100
N7:12
23
L2
Output
PL1
PL1
LEQ
LESS THAN OR EQUAL
Source A
C5:1.ACC
Source B
457
L2
Output
PL1
PL1
Figure 10-26 LEQ logic rung.
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The limit test instruction is said to be circular because
it can function in either of two ways. Figure 10-27 shows
an example of an LIM instruction where the low limit
value is less than the high limit value. The operation of the
logic rung can be summarized as follows:
• The high limit has a value of 50, and the low limit
has a value of 25.
• Instruction is true for values of the test from 25
through 50.
• Instruction is false for test values less than 25 or
greater than 50.
• Instruction is true because the test value is 48.
Figure 10-28 shows an example of an LIM instruction
where the low limit value is greater than the high limit
value. The operation of the logic rung can be summarized
as follows:
• The high limit has a value of 50, and the low limit
has a value of 100.
• Instruction is true for test values of 50 and less than
50 and for test values of 100 and greater than 100.
• Instruction is false for test values greater than 50
and less than 100.
• Instruction is true because the test value is 125.
The masked comparison for equal (MEQ) instruction
compares a value from a source address with data at a com-
pare address and allows portions of the data to be masked.
One application for the MEQ instruction is to compare
the correct position of up to 16 limit switches when the
source contains the limit switch address and the compare
stores their desired states. The mask can block out the
switches you don’t want to compare (Figure 10-29).
Figure 10-30 shows an example of an MEQ instruc-
tion. The operation of the logic rung can be summarized
as follows:
• When the data at the source address match the data
at the compare address bit-by-bit (less masked bits),
the instruction is true.
• The instruction goes false as soon as it detects a
mismatch.
• A mask passes data when the mask bits are set (1); a
mask blocks data when the mask bits are reset (0).
True ( 50) ( 100) True
LIM
LIMIT TEST (CIRC)
Low limit
Test
High limit
N7:28
100
N7:29
125
N7:27
50
50
False
100
L2
Output
PL1
PL1
Figure 10-28 LIM instruction where the low limit value is
greater than the high limit value.
Figure 10-29 MEQ instruction can be used to monitor the
state of limit switches.
Source: Courtesy Jayashree Electrodevices.
LIM
LIMIT TEST (CIRC)
Low limit
Test
High limit
N7:22
25
N7:23
48
N7:24
50
25
False ( 25) False ( 50)
True
50
L2
Output
PL1
PL1
Figure 10-27 LIM instruction where the low limit value is
less than the high limit value.
MEQ
MASKED EQUAL
Source
Mask
Compare
N7:5
N7:6
N7:10
Source N7:5 01010101 0 1 0 1 1 1 1 1
Mask N7:6 11111111
Compare N7:10 01010101 0 1 0 1 x x x x
1 1 1 10 0 0 0
L2
Output
PL1
PL1
Figure 10-30 Masked comparison for equal (MEQ) logic
rung.
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timers. However, the same circuit can be programmed
using only one internal timer along with data compare in-
structions. Figure 10-32 shows the program required to
implement the circuit using only one internal timer. The
operation of the program can be summarized as follows:
• The momentary stop button is closed.
• When the momentary start button is pressed,
SOL A output energizes immediately to switch on
solenoid A.
• SOL A examine-on contact becomes true to seal
in output SOL A and to start on-delay timer T4:1
timing.
• The timer preset time is set to 15 seconds.
• Output SOL D will energize (through the timer
done bit T4:1/DN) after a total time delay of 15 sec-
onds to energize solenoid D.
• Output SOL B will energize after a total time delay
of 5 seconds, when the accumulated time becomes
equal to and then greater than 5 seconds. This, in
turn, will energize solenoid B.
• Output SOL C will energize after a total time delay
of 10 seconds, when the accumulated time becomes
equal to and then greater than 10 seconds. This, in
turn, will energize solenoid C.
Figure 10-33 shows an application of an on-delay
timer program implemented using the EQU instruction.
follows:
• When the switch (S1) is closed, timer T4:1 will
begin timing.
• Both EQU instructions’ source As are addressed to
get the accumulated value from the timer while it is
running.
• The EQU instruction of rung 2 has the value of 5
stored in source B.
• When the accumulated value of the timer reaches 5,
the EQU instruction of rung 2 will become logic
true for 1 second.
• As a result, the latch output will energize to switch
the pilot light PL1 on.
• When the accumulated value of the timer reaches
15, the EQU instruction of rung 3 will be true for
1 second.
• As a result, the unlatch output will energize to
switch the pilot light PL1 off.
• Therefore, when the switch is closed, the pilot light
will come on after 5 seconds, stay on for 10 sec-
onds, and then turn off.
• The mask must be the same element size (16 bits) as
the source and compare addresses.
• You must set mask bits to 1 to compare data. Bits
in the compare address that correspond to 0s in the
mask are not compared.
• If you want the ladder program to change mask
value, store the mask at a data address. Otherwise,
enter a hexadecimal value for a constant mask
value.
• The instruction is true because reference bits XXXX
are not compared.
10.4 Data Manipulation Programs
Data manipulation instructions give new dimension and
flexibility to the programming of control circuits. For ex-
ample, consider the hardwired relay-operated, time-delay
circuit in Figure 10-31. This circuit uses three electrome-
chanical time-delay relays to control four solenoid valves.
The operation of the hardwired circuit can be summarized
as follows:
• When the momentary start pushbutton is pressed
solenoid A is energized immediately.
• Solenoid B is energized 5 s later than solenoid A.
• Solenoid C is energized 10 s later than solenoid A.
• Solenoid D is energized 15 s later than solenoid A.
The hardwired time-delay circuit could be implemented
using a conventional PLC program and three internal
Stop
Start
1TD
2TD
3TD
L2
L1
CR
(5 s)
(5 s)
(5 s)
SOL D
SOL A
SOL B
SOL C
3TD
2TD
1TD
CR
Figure 10-31 Three electromechanical time-delay relays
used to control four solenoid valves.
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Figure 10-33 Timer program implemented using the EQU instruction.
T4:1
1.0
20
0
DN
EN
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
L1
S1
S1
T4:1.ACC
15
EQU
EQUAL
Source A
Source B
L
T4:1.ACC
5
EQU
EQUAL
Source A
Source B
1
2
3
PL1
L2
Output
PL1
U
PL1
SOL A
T4:1
1.0
15
0
DN
EN
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
SOL A
SOL A
SOL B
T4:1.ACC
0
5
GEQ
Source A
Source B
SOL C
T4:1.ACC
0
10
GEQ
Source A
Source B
L1
Inputs
SOL D
T4:1
DN
SOL A
L2
Outputs
SOL B
SOL C
SOL D
Stop
Stop
Start
Start
A
B
C
D
Figure 10-32 Controlling multiple loads using one timer and the GEQ instruction.
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• While the vessel fills, the PLC performs a compari-
son between the vessel’s current weight and a pre-
determined constant programmed in the processor.
Figure 10-34 Counter program implemented using the LES instruction.
Source: Photo courtesy Turck, Inc., www.turck.com.
C5:1
50
0
DN
CU
CTU
COUNT UP
Counter
Preset
Accumulated
L1
Sensor
Sensor
C5:1
DN
C5:1
RES
SOL
C5:1.ACC
0
20
LES
LESS THAN
Source A
Source B
L2
Output
SOL
Figure 10-34 shows an application of an up-counter
program implemented using the LES instruction. The op-
eration of the program can be summarized as follows:
• Up-counter C5:1 will increment by 1 for every
false-to-true transition of the proximity sensor
switch.
• Source A of the LES instruction is addressed to the
accumulated value of the counter and source B has a
constant value of 20.
• The LES instruction will be true as the long as
the value contained in source A is less than that of
source B.
• Therefore, output solenoid SOL will be energized
when the accumulated value of the counter is be-
tween 0 and 19.
• When the counter’s accumulated value reaches 20,
the LES instruction will go false, de-energizing
output solenoid SOL.
• When the counter’s accumulated value reaches its
preset value of 50, the counter reset will be ener-
gized through the counter done bit (C5:1/DN) to
reset the accumulated count to 0.
The use of comparison instructions is generally
straightforward. However, one precaution involves the
use of these instructions in PLC programs used to control
the flow in vessel filling operations (Figure 10-35). This
control scenario can be summarized as follows:
• The receiving vessel has its weight monitored con-
tinuously by the PLC program as it fills.
• When the weight reaches a preset value, the flow is
cut off.
Figure 10-35 Vessel ﬁlling operation.
Source: Courtesy Feige Filling.
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performed by the LED display device. The BCD output
module is used to output data from a specific register or
word location in memory. This type of output module en-
ables a PLC to operate devices that require BCD coded
signals.
Figure 10-38 shows a PLC program that uses a BCD
input interface module connected to a thumbwheel switch
and a BCD output interface module connected to an LED
display board. The program is designed so that the LEDs
display the setting of the thumbwheel switch. Both the
MOV and EQU instructions form part of the program. The
• The LED display board monitors the decimal set-
ting of the thumbwheel switch.
• The MOV instruction is used to move the data from
the thumbwheel switch input to the LED display
output.
• If the programmer uses only the equal instruction,
problems may result.
• As the vessel fills, the comparison for equality will
be false. At the instant the vessel weight reaches the
desired preset value of the equal instruction, the in-
struction becomes true and the flow is stopped.
• However, should the supply system leak additional
material into the vessel, the total weight of the ma-
terial could rise above the preset value, causing the
instruction to go false and the vessel to overfill.
• The simplest solution to this problem is to program
the comparison instruction as a greater than or equal
to instruction. This way, any excess material enter-
ing the vessel will not affect the filling operation.
• It may be necessary, however, to include addi-
tional programming to indicate a serious overfill
condition.
10.5 Numerical Data I/O Interfaces
The expanding data manipulation processing capabilities
of PLCs led to the development of I/O interfaces known
as numerical data I/O interfaces. In general, numerical
data I/O interfaces can be divided into two groups: those
that provide interface to multibit digital devices and those
that provide interface to analog devices.
The multibit digital devices are like the discrete I/O
because processed signals are discrete (on/off). The dif-
ference is that, with the discrete I/O, only a single bit is
required to read an input or control an output. Multibit
interfaces allow a group of bits to be input or output as a
unit. They can be used to accommodate devices that re-
quire BCD inputs or outputs.
The thumbwheel switches (TWS), shown in Fig-
ure 10-36, are typical BCD input devices. Each one of the
four switches provides four binary digits at its output that
correspond to the decimal number selected on the switch.
The conversion from a single decimal digit to four binary
digits is performed by the TWS device. The BCD input
module allows the processor to accept the 4-bit digital
codes and input their data into specific register or word
locations in memory to be used by the control program.
Data manipulation instructions can be used to access the
data from the input module allowing a person to change
set points, timer, or counter presets externally without
modifying the control program.
The seven-segment LED display board, shown in Fig-
ure 10-37, is a typical Binary Coded Decimal (BCD) out-
put device. It displays a decimal number that corresponds
to the BCD value it receives at its input. Conversion of the
four binary bits to a single decimal digit on the display is
7
8
9
10
11
12
13
14
15
L2
L1
0
0
0
0
1
0
1
0
0
1
1
0
1
1
1
0
0
1
2
3
4
5
6
Bit
address
BCD
input module
0
5
6
7
1s units
Decimal
10s units
100s units
1000s units
Thumbwheel
switch
(TWS)
BCD
Figure 10-36 BCD input interface module connected to a
thumbwheel switch.
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• Setting of the thumbwheel switch is compared to
the reference number 1208 stored in source B by the
EQU instruction.
• Pilot light output PL is energized whenever the
input switch S1 is true (closed) and the value of the
thumbwheel switch is equal to 1208.
Input and output modules can be addressed either at
the bit level or at the word level. Analog modules con-
vert analog signals to 16-bit digital signals (input) or
16-bit digital signals to analog values (output). An analog
I/O will allow monitoring and control of analog voltages
and currents. Figure 10-39 illustrates how an analog input
interface operates. The operation of this input module can
• The analog input module contains the circuitry nec-
essary to accept analog voltage or current signals
from field devices.
• The input signal is converted from an analog to a
digital value by an analog-to-digital (A/D) converter
circuit.
• The conversion value, which is proportional to the
analog signal, is passed through the controller’s
data bus and stored in a specific register or word
location in memory for later use by the control
program.
An analog output interface module receives numerical
data from the processor; these data are then translated into
a proportional voltage or current to control an analog field
device. Figure 10-40 illustrates how an analog output in-
terface operates. The operation of this output module can
Figure 10-37 BCD output interface module connected to
a seven-segment LED display board.
7
8
9
10
11
12
13
14
15
L2
L1
0
1
0
0
0
0
0
1
0
0
1
0
1
1
0
0
0
1
2
3
4
5
6
Bit
address
BCD
output module
1s units
Decimal
10s units
100s units
1000s units
LED
display board
BCD
Figure 10-38 Monitoring the setting of a thumbwheel switch.
L1
S1
S1 PL
Thumbwheel switch
1
TWS
TWS
2
3
4
1208
EQU
EQUAL
Source A
Source B
MOV
MOVE
Source
Destination
(All 16 bits)
TWS
LED
PL
LED display
L2
Outputs
(All 16 bits)
LED
1 2 0 8
1 2 0 8
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10.6 Closed-Loop Control
In open-loop control, no feedback loop is employed
and system variations which cause the output to deviate
from the desired value are not detected or corrected. A
closed-loop system utilizes feedback to measure the ac-
tual system operating parameter being controlled such as
temperature, pressure, flow, level, or speed. This feedback
signal is sent back to the PLC where it is compared with
the desired system set-point. The controller develops an
error signal that initiates corrective action and drives the
final output device to the desired value.
PLC set-point control in its simplest form compares
an input value, such as analog or thumbwheel inputs,
to a set-point value. A discrete output signal is provided
if the input value is less than, equal to, or greater than
the set-point value. The temperature control program of
• The function of the analog output module is to ac-
cept a range of numeric values output from the PLC
program and to produce a varying current or voltage
signal required to control a connected analog output
device.
• Data from a specific register or word location in the
CPU memory are passed through the controller’s
data bus to the digital-to-analog (D/A) converter.
• The analog output from the D/A converter is then
used to control the analog output device.
• The level of the analog signal output is based on the
digital value of the data word supplied by the CPU
and manipulated by the control program.
• These output interfaces normally require an external
power supply that meets certain current and voltage
requirements.
Figure 10-39 Analog input interface module.
Thermocouple
L1
1
0
0
1
0
1
1
0
1
0
1
0
1
1
1
0
12
13
14
15
0
1
2
3
4
5
6
7
8
9
10
11
Data
bus
A/D
c
o
n
v
e
r
t
e
r
Variable voltage
or current input
L2
BCD conversion value
Analog output module
Analog
input
device
Figure 10-40 Analog output interface module.
Isolation
Word
data
from
CPU
Digital
to
analog
converter
Voltage
and
current
driver
Load
Amplifier
Analog
output
Servo
motor
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on and sealed-in through the heater examine-on
instruction.
• Once the temperature increases to 598°F the LEQ
instruction goes false but the heater output remains
on until the temperature rises to 603°F.
• At the 603°F point the GEQ instruction and B3:0/2
will both be true and the heater will be switched off.
Several set-point control schemes can be performed by
different PLC models. These include on/off control, pro-
portional (P) control, proportional-integral (PI) control,
and proportional-integral-derivative (PID) control. Each
involves the use of some form of closed-loop control to
maintain a process characteristic such as a temperature,
pressure, flow, or level at a desired value. When a control
system is designed such that it receives operating infor-
mation from the machine and makes adjustments to the
machine based on this operating information, the system
is said to be a closed-loop system.
The block diagram of a closed-loop control system is
shown in Figure 10-42. A measurement is made of the
variable to be controlled. This measurement is then com-
pared to a reference point, or set-point. If a difference
Figure 10-41 is one example of set-point control. In this
application, a PLC is to provide for simple off/on control
of the electric heating elements of an oven. The operation
• Oven is to maintain an average set-point tempera-
ture of 600°F with a variation of about 1 percent
between the off and on cycles.
• The electric heaters are turned on when the temper-
ature of the oven is 597°F or less and will stay on
until the temperature rises to 603°F or more.
• The electric heaters stay off until the temperature
drops to 597°F, at which time the cycle repeats
itself.
• Whenever the less than or equal (LEQ) instruction
is true, a low-temperature condition exists and the
program switches on the heater.
• Whenever the greater than or equal (GEQ) instruc-
tion is true, a high-temperature condition exists and
the program switches off the heater.
• For the program as shown the temperature is
595°F so the LEQ instruction and B3:0/1 will
both be true and the heater output will be switched
Figure 10-41 Set-point control program.
B3:0/1
(Internal)
B3:0/2
(Internal)
LEQ
LESS THAN OR EQUAL
Source A
Source B
GEQ
Source A
Source B
MOV
MOVE
Source
Destination
Low temp.
B3:0/1
Heater
Heater
High temp.
B3:0/2
L1
Inputs
S1
S1
S1
Thermocouple
Thermocouple
Thermocouple
Thermocouple
LED
Heater
L2
Outputs
LED Display
LED
5 9 5
597
603
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the output to degrade closing the valve by different per-
centages, adjusting the valve to maintain a set-point.
Proportional-integral-derivative (PID) control is the
most sophisticated and widely used type of process control.
PID operations are more complex and are mathematically
based. PID controllers produce outputs that depend on the
magnitude, duration, and rate of change of the system error
signal. Sudden system disturbances are met with an aggres-
sive attempt to correct the condition. A PID controller can
reduce the system error to 0 faster than any other controller.
A typical PID control loop is illustrated in Figure 10-44.
The loop measures the process, compares it to a set-point,
and then manipulates the output in the direction which
should move the process toward the set-point. The termi-
nology used in conjunction with a PID loop can be sum-
marized as follows:
• Operating information that the controller receives
from the machine is called the process variable
(PV) or feedback.
• Input from the operator that tells the controller the
desired operating point is called the set-point (SP).
• When operating, the controller determines whether
the machine needs adjustment by comparing (by
subtraction) the set-point and the process variable
(error) exists between the actual and desired levels, the
PLC control program will take the necessary corrective
action. Adjustments are made continuously by the PLC
until the difference between the desired and actual output
is as small as is practical.
With on/off PLC control (also known as two-position
and bang-bang control), the output or final control ele-
ment is either on or off—one for the occasion when the
value of the measured variable is above the set-point and
the other for the occasion when the value is below the
set-point. The controller will never keep the final control
element in an intermediate position. Most residential ther-
mostats are on/off type controllers.
On/off control is inexpensive but not accurate enough
for most process and machine control applications. On/
off control almost always means overshoot and resultant
system cycling. For this reason a deadband usually ex-
ists around the set-point. The deadband or hysteresis of
the control loop is the difference between the on and off
operating points.
Proportional controls are designed to eliminate the
hunting or cycling associated with on/off control. They
allow the final control element to take intermediate po-
sitions between on and off. This permits analog control
of the final control element to vary the amount of energy
to the process, depending on how much the value of the
measured variable has shifted from the desired value.
The process illustrated in Figure 10-43 is an example
of a proportional control process. The PLC analog output
module controls the amount of fluid placed in the holding
tank by adjusting the percentage of valve opening. The
valve is initially open 100 percent. As the fluid level in the
tank approaches the preset point, the processor modifies Figure 10-44 Typical PID control loop.
Σ
Error
Set-point
(SP)
PID equation
Control
variable
(CV)
Process
variable
(PV)
Level
detector
Flow
rate
Figure 10-42 Closed-loop control system.
PLC
controller
Measurement of
variable to be
controlled
(sensor)
Control
element
(heater-valve)
Set-point
(potentiometer)
Figure 10-43 Proportional control process.
Ultrasonic
level sensor
Analog output
4 to 20 mA
analog input
Adjustable
valve
PLC
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to produce a difference (the difference is called the
error).
• Output from the loop is called the control variable
(CV), which is connected to the controlling part of
the process.
• The PID loop takes appropriate action to modify the
process operating point until the control variable
and the set-point are very nearly equal.
Programmable controllers are either equipped with
PID I/O modules that produce PID control or have suf-
ficient mathematical functions of their own to allow PID
control to be carried out. Figure 10-45 shows an SLC 500
PID instruction with typical addresses for the parameters
entered. The PID instruction normally controls a closed
loop using inputs from an analog input module and pro-
vides an output to an analog output module. Explanation
Figure 10-45 SLC 500 PID instruction.
PID
PID
Control
Process
Control
Control
Block
Variable
Variable
Block length
N10:0
N10:28
N10:29
23
of the PID instruction parameters can be summarized as
follows:
• Control Block is the file that stores the data required
to operate the instruction.
• Process Variable (PV) is an element address that
stores the process input value.
• Control Variable (CV) is an element address that
stores the output of the PID instruction.
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Figure 10-46 Logic rungs for Question 14.
(a)
(b)
(c)
GEQ
Source A
Source B
F
N7:601
N7:600
30
E
F
E
Output
L2
L1
Output
LES
LESS THAN
Source A
Source B
D L2
N7:500
250
L1 C
Input
D
C
EQU
EQUAL
Source A
Source B
B L2
Output
N7:400
N7:401
L1 A
Input
B
A
1. In general, what do data manipulation instructions
allow the PLC to do?
2. Explain the difference between a register or word
and a table or file.
3. Into what two broad categories can data manipula-
tion instructions be placed?
4. What takes place with regard to a data transfer
instruction?
5. The MOV instruction is to be used to copy the
information stored in word N7:20 to N7:35.
What address is entered into the source and the
destination?
6. What is the purpose of the mask word in the MVM
instruction?
7. What is the purpose of the bit distribute
instruction?
8. List three types of data shifts used with file
instructions.
9. List the six parameters and addresses that must
be entered into the file arithmetic and logic (FAL)
instruction.
10. Assume the ALL mode has been entered as part of
a FAL instruction. How will this affect the transfer
of data?
11. What is the advantage of using the file copy (COP)
or fill file (FLL) instruction rather than the FAL in-
struction for the transfer of data?
12. What are data compare instructions used for?
13. Name and draw the symbols for the six different
types of data compare instructions.
14. Explain what each of the logic rungs in
Figure 10-46 is instructing the processor to do.
15. What does the limit test (LIM) instruction test
values for?
16. How are multibit I/O interfaces different from the
discrete type?
17. Assume that a thumbwheel switch is set for the
decimal number 3286.
a. What is the equivalent BCD value for this
setting?
b. What is the equivalent binary value for this
setting?
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21. Outline the control process involved with simple
PLC set-point control.
22. Compare the operation of the final control element
in on/off and proportional control systems.
23. Explain the meaning of the following terms as they
apply to a PID control:
a. Process variable
b. Set-point
c. Error
d. Control variable
18. Assume that a thermocouple is connected to an
analog input module. Explain how the tempera-
ture of the thermocouple is communicated to the
processor.
19. Outline the process by which an analog output in-
terface module operates the field device connected
to it.
20. Compare the operation of open-loop and closed-
loop PLC systems.
L1
Inputs
L2
Output
Thumbwheel switch
N7:112
N7:13
LED display
MOV
MOVE
Source
Destination
S1
N7:112
N7:13
4
8
5
S1
1. Study the data transfer program of Figure 10-47
and answer the following questions:
a. When S1 is open, what decimal number will
be stored in integer word address N7:13 of the
MOV instruction?
b. When S1 is on, what decimal number will be
stored in integer word address N7:112 of the
MOV instruction?
c. When S1 is on, what decimal number will ap-
pear in the LED display?
d. What is required for the decimal number 216 to
appear in the LED display?
2. Study the data transfer counter program of Fig-
ure 10-48 and answer the following questions:
a. What determines the preset value of the
counter?
b. Outline the steps to follow to operate the
program so that the PL1 output is energized
after 25 off-to-on transitions of the count
PB input.
3. Construct a nonretentive timer program that will
turn on a pilot light after a time-delay period. Use
a thumbwheel switch to vary the preset time-delay
value of the timer.
4. Study the data compare program of Figure 10-49
a. Will the pilot light PL1 come on whenever
switch S1 is closed? Why?
b. Must switch S1 be closed to change the num-
ber stored in source A of the EQU instruction?
c. What number or numbers need to be set on
the thumbwheel in order to turn on the pilot
light?
CHAPTER 10 PROBLEMS
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L1
Count PB
Count PB
Inputs
L2
Output
PL1
PL1
N7:10
Reset PB
Reset PB
MOV
MOVE
Source
Destination
N7:10
C5:1.PRE
CTU
COUNT UP
Counter
Preset
Accumulated
C5:1
000
000
DN
CU
RES
C5:1/DN
C5:1
L1
Thumbwheel switch
N7:112
EQU
EQUAL
Source A
Source B
S1
N7:112
004
L2
Output
PL1
PL1
S1
5. Study the data compare program in Figure 10-50
a. List the values for the thumbwheel switch that
would allow the pilot light to turn on.
b. If the value in the word N7:112 is 003 and switch
S1 is open, will the pilot light turn on? Why?
c. Assume that source B is addressed to the accu-
mulated count of an up-counter. With S1 closed,
what setting of the thumbwheel switch would be
required to turn the pilot light off when the ac-
cumulated count reaches 150?
6. Write a program to perform the following:
a. Turn on pilot light 1 (PL1) if the thumbwheel
switch value is less than 4.
b. Turn on pilot light 2 (PL2) if the thumbwheel
switch value is equal to 4.
c. Turn on pilot light 3 (PL3) if the thumbwheel
switch value is greater than 4.
d. Turn on pilot light 4 (PL4) if the thumbwheel
switch value is less than or equal to 4.
e. Turn on pilot light 5 (PL5) if the thumbwheel
switch value is greater than or equal to 4.
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L1
Thumbwheel switch
N7:112
GRT
GREATER THAN
Source A
Source B
S1
N7:112
12
L2
Output
PL1
PL1
S1
11. Write a program that will cause a light to come on
only if a PLC counter has a value of 6 or 10.
12. Write a program that will cause a light to come on if
a PLC counter value is less than 10 or more than 30.
13. Write a program for the following: The tempera-
ture reading from a thermocouple is to be read and
stored in a memory location every 5 minutes for
4 hours. The temperature reading is brought in con-
tinuously and stored in address N7:150. File #7:200
is to contain the data from the last full 4-hour
period.
7. Write a program that will copy the value stored at
address N7:56 into address N7:60.
8. Write a program that uses the mask move instruc-
tion to move only the upper 8 bits of the value
stored at address I:2.0 to address O:2.1 and to
ignore the lower 8 bits.
9. Write a program that uses the FAL instruction to
copy 20 words of data from the integer data file,
starting with N7:40, into the integer data file,
starting with N7:80.
10. Write a program that uses the COP instruction to
copy 128 bits of data from the memory area, start-
ing at B3:0, to the memory area, starting at B3:8.
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226
Most PLCs have arithmetic function capabilities.
Basic PLC math instructions include add, sub-
tract, multiply and divide to calculate the sum,
difference, product, and quotient of the content
of word registers. The PLC is capable of doing
many arithmetic operations per scan period for
fast updating of data. This chapter covers the
basic mathematical instructions performed by
PLCs and their applications.
Chapter Objectives
11.1 Analyze and interpret math instructions as they apply
to a PLC program
11.2 Create PLC programs involving math instructions
11.3 Apply combinations of PLC arithmetic functions to
processes
11
Math Instructions
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Math Instructions Chapter 11 227
DIV (Divide)—Divides source A by source B and
stores the result in the math register.
SQR (Square Root)—Calculates the square root
of the source and places the integer result in the
destination.
NEG (Negate)—Changes the sign of the source and
places it in the destination.
TOD (To BCD)—Converts a 16-bit integer source
value to BCD and stores it in the math register or the
destination.
FRD (From BCD)—Converts a BCD value in the
math register or the source to an integer and stores it
in the destination.
Figure 11-2 shows the CPT (compute) instruction used
with SLC 500 controllers. When CPT instruction is ex-
ecuted, then copy, arithmetic, logical, or conversion op-
eration residing in the expression field of this instruction
is performed and the result is sent to the destination. The
execution time of a CPT instruction is longer than that of
a single arithmetic operation and uses more instruction
words.
11.2 Addition Instruction
Most math instructions take two input values, perform
the specified arithmetic function, and output the result
to an assigned memory location. For example, the ADD
instruction performs the addition of two values stored in
the referenced memory locations. How these values are
accessed depends on the controller. Figure 11-3 shows the
11.
1 Math Instructions
Math instructions, like data manipulation instructions,
enable the programmable controller to take on more of
the qualities of a conventional computer. The PLC’s math
functions capability allows it to perform arithmetic func-
tions on values stored in memory words or registers. For
example, assume you are using a counter to keep track
of the number of parts manufactured, and you would
like to display how many more parts must be produced
in order to reach a certain quota. This display would re-
quire the data in the accumulated value of the counter to
be subtracted from the quota required. Other applications
include combining parts counted, subtracting detected de-
fects, and calculating run rates.
Depending on what type of processor is used, various
math instructions can be programmed. The basic four
mathematical functions performed by PLCs are:
Addition—The capability to add one piece of data to
another.
Subtraction—The capability to subtract one piece of
data from another.
Multiplication—The capability to multiply one piece
of data by another.
Division—The capability to divide one piece of data
by another.
Math instructions use the contents of two words or
registers and perform the desired function. The PLC in-
structions for data manipulation (data transfer and data
compare) are used with the math symbols to perform
math functions. Math instructions are all output instruc-
tions. Figure 11-1 shows the Compute/Math menu tab for
the SLC 500 PLC and its associated RSLogix software.
The commands can be summarized as follows:
CPT (Compute)—Evaluates an expression and stores
the result in the destination.
ADD (add)—Adds source A to source B and stores
the result in the destination.
SUB (Subtract)—Subtracts source B from source A
and stores the result in the destination.
MUL (Multiply)—Multiplies source A by source B
and stores the result in the destination.
Figure 11-1 Compute/Math menu tab.
CPT ADD SUB MUL DIV SQR
Compute/Math
Compare
Input/Output
NEG TOD FRD
Move/Logic
Figure 11-2 SLC 500 CPT (compute) instruction.
CPT
Compute
Destination
Expression
Figure 11-3 SLC 500 ADD instruction.
Input
L1
ADD
ADD
Source A
Source B
Destination
N7:0
25
N7:1
50
N7:2
75
SW
SW
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228 Chapter 11 Math Instructions
come on when the sum of the counts from the two coun-
ters is equal to or greater than 350. The operation of the
• Source A of the ADD instruction is addressed to
store the accumulated value of counter C5:0.
• Source B of the ADD instruction is addressed to
store the accumulated value of counter C5:1.
• The value at source A is added to the value at source
B, and the result (answer) is stored at destination
address N7:1.
• Source A of the GEQ (greater than or equal) instruc-
tion is addressed to store the value of the destination
address N7:1.
• Source B of the GEQ instruction contains the con-
stant value of 350.
ADD instruction used with the SLC 500 controllers. The
operation of the logic rung can be summarized as follows:
• When input switch SW is closed the rung will be
true.
• The value stored at the source A address, N7:0 (25),
is added to the value stored at the source B address,
N7:1 (50).
• The answer (75) is stored at the destination address
N7:2.
• Source A and source B can be either values or
addresses that contain values, but A and B cannot
both be constants.
The program of Figure 11-4 illustrates how the ADD
instruction can be used to add the accumulated counts of
two up-counters. This application requires a pilot light to
Figure 11-4 Counter program that uses the ADD instruction.
L1
Inputs
S2
L2
Output
S1
C5:0
350
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
350
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
PL1
C5:0.ACC
C5:1.ACC
N7:1
ADD
ADD
Source A
Source B
Destination
Reset C5:0
GEQ
Source A
Source B
N7:1
350
CU
DN
CU
DN
RES
C5:1
RES
PL1
Reset
S1
S2
125
100
250
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• The GEQ instruction and PL1 output will be true
whenever the accumulated values in the two counters
are equal to or greater than the constant value 350.
• A reset button is provided to reset the accumulated
count of both counters to zero.
When performing math functions, care must be taken
to ensure that values remain in the range that the data
table or file can store; otherwise, the overflow bit will be
set. The arithmetic status bits for the SLC 500 controller
are found in word 0, bits 0 to 3 of the processor status
file S2 (Figure 11-5). After an instruction is executed, the
arithmetic status bits in the status file are updated. The
description of each bit can be summarized as follows:
Carry (C)—Address S2:0/0, is set to 1 when there is
a carry in the ADD instruction or a borrow in the SUB
instruction.
Overflow (O)—Address S2:0/1, is set to 1 when the
result is too large to fit in the destination register.
Zero (Z)—Address S2:0/2, is set to 1 when the result
of the subtract instruction is zero.
Sign (S)—Address S2:0/3, is set to 1 when the result
is a negative number.
11.3 Subtraction Instruction
The SUB (subtract) instruction is an output instruction that
subtracts one value from another and stores the result in
the destination address. When rung conditions are true, the
subtract instruction subtracts source B from source A and
stores the result in the destination. Figure 11-6 shows the
SUB instruction used with the SLC 500 controllers. The
• When input switch SW is closed the rung will be true.
• The value stored at the source B address, N7:05
(322), is subtracted from the value stored at the
source A address, N7:10 (520).
• The answer (198) is stored at the destination ad-
dress, N7:20.
• Source A and source B can be either values or ad-
dresses that contain values, but A and B cannot both
be constants.
The program of Figure 11-7 shows how the SUB func-
tion can be used to indicate a vessel overfill condition.
This application requires an alarm to sound when a supply
system leaks 5 lb or more of raw material into the ves-
sel after a preset weight of 500 lb has been reached. The
• When the start button is pressed, the fill solenoid
(rung 1) and filling indicating light (rung 2) are turned
on and raw material is allowed to flow into the vessel.
• The vessel has its weight monitored continuously by
the PLC program (rung 3) as it fills.
• When the weight reaches 500 lb, the fill solenoid is
de-energized and the flow is cut off.
• At the same time, the filling pilot light indicator is
turned off and the full pilot light indicator (rung 3)
is turned on.
• Should the fill solenoid leak 5 lb or more of raw
material into the vessel, the alarm (rung 5) will en-
ergize and stay energized until the overflow level is
reduced below the 5-lb overflow limit.
Figure 11-5 Processor status ﬁle S2.
Status Table
Address S2:0 Table: S2:Status
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
S2:0/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
S2:1/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
S2:2/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
S2:3/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
S2:4/ 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 1
S2:5/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Figure 11-6 SLC 500 SUB (subtract) instruction.
Input
L1
SUB
SUBTRACT
Source A
Source B
Destination
N7:10
520
N7:05
322
N7:20
198
SW
SW
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11.4 Multiplication Instruction
The multiply (MUL) instruction is an output instruction
that multiplies two values and stores the result in the des-
tination address. Figure 11-8 shows the MUL instruction
used with the SLC 500 controllers. The operation of the
logic rung can be summarized as follows:
• When input switch SW is closed the rung will be
true.
• The data in source A (constant 20) will be multi-
plied by the data in source B (accumulated value of
counter C5:10).
• The resultant answer is placed in the destination
N7:2.
• Similar to previous math instructions, source A and
B in multiplication instructions can be values (con-
stants) or addresses that contain values, but A and B
cannot both be constants.
The program of Figure 11-9 is an example of how
MUL instruction calculates the product of two sources.
follows:
• When input switch SW is closed the MUL instruc-
tion is executed.
• The value stored in source A, address N7:1 (123),
is then multiplied by the value stored in source B,
address N7:2(61).
Figure 11-7 Vessel overﬁll alarm program.
Full
Full
Fill
solenoid
Fill
solenoid
Fill
solenoid
Alarm
Full
Full
Filling
Outputs
L2
Start
Inputs
L1
Weight
transducer
(All 16 bits)
Stop
SUB
SUBTRACT
Source A
Source B
Destination
Ι:012
500
N7:1
GEQ
Source A
Source B
Ι:012
500
GEQ
Source A
Source B
N7:1
5
1
2
3
4
5
Start
Ι:012
Stop
Full
Fill
solenoid
Alarm
Filling
Start
Stop
Figure 11-8 SLC 500 MUL (multiply) instruction.
Input
L1
SW
SW
MUL
MULTIPLY
Source A
Source B
Destination
20
C5:10.ACC
N7:2
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• The product (7503) is placed into destination word
N7:3.
• As a result, the equal instruction becomes true, turn-
ing output PL1 on.
The program of Figure 11-10 is an example of how the
MUL instruction is used as part of an oven temperature
control program. The operation of the program can be
• The PLC calculates the upper and lower deadband,
or off/on limits, about the set-point.
• Upper and lower temperature limits are set automati-
cally at 61 percent regardless of the set-point value.
• Set-point temperature is adjusted by means of the
thumbwheel switch.
• The analog thermocouple interface module is used
to monitor the current temperature of the oven.
• In this example, the set-point temperature is 400°F.
• Therefore, the electric heaters will be turned on when
the temperature of the oven drops to less than 396°F
and stay on until the temperature rises above 404°F.
• If the set-point is changed to 100°F, the deadband
remains at 61 percent, with the lower limit being
99°F and the upper limit being 101°F.
• The number stored in word N7:1 represents the
upper temperature limit, and the number stored in
word N7:2 represents the lower limit.
11.5 Division Instruction
The divide (DIV) instruction divides the value in source A
by the value in source B and stores the result in the desti-
nation and math register. Figure 11-11 shows an example
of the DIV instruction. The operation of the logic rung
• When input switch SW is closed the rung will be true.
• The data in source A (the accumulated value of
counter C5:10) is then divided by the data in source B
(the constant 2).
• The result is placed in the destination N7:3.
• If the remainder is 0.5 or greater, a roundup occurs
in the integer destination.
• The value stored in the math register consists of the
unrounded quotient (placed in the most significant
word) and the remainder (placed in the least signifi-
cant word).
• Some PLCs support the use of floating-decimal as
well as integer (whole number) values. As an exam-
ple, 10 divided by 3 may be expressed as 3.333333
(floating-decimal notation) or 3 with a remainder
of 1.
DIV instruction calculates the integer value that results
from dividing source A by source B. The operation of the
• When input switch SW is closed the DIV instruc-
tion is executed.
• The value stored in source A, address N7:0 (120),
is then divided by the value stored in source B,
address N7:1 (4).
• The answer, 30, is placed in the destination address
N7:5.
• As a result, the equal instruction becomes true, turn-
ing output PL1 on.
Figure 11-9 MUL instruction used to calculate the product of two sources.
Input
L2
PL1
MUL
MULTIPLY
Source A
Source B
Destination
N7:1
123
N7:2
61
N7:3
7503
EQU
EQUAL
Source A
Source B
N7:3
7503
7503
PL1
L1
SW
SW
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the DIV function is used as part of a program to convert
Celsius temperature to Fahrenheit. The operation of the
• The thumbwheel switch connected to the input
module indicates Celsius temperature.
• The program is designed to convert the recorded
Celsius temperature in the data table to Fahrenheit
values for display.
Figure 11-10 The MUL instruction used as part of a temperature control program.
ON/OFF PL1 PL2 Heater
Heater
PL2
PL1
MUL
MULTIPLY
Source A
Source B
Destination
Ι:012
400
0.0100000
N7:0
4
ADD
ADD
Source A
Source B
Destination
High limit
Low limit
Ι:012
400
N7:0
4
N7:1
404
SUB
SUBTRACT
Source A
Source B
Destination
Ι:012
400
N7:0
4
N7:2
396
LES
LESS THAN
Source A
Source B
Ι:013
0
N7:2
396
GRT
GREATER THAN
Source A
Source B
Ι:013
0
N7:1
404
Inputs
L1
ON/OFF
TWS
Thermocouple
input
1
2
3
Ι:013
Ι:012
PL1
PL2
4 0 0
Heater
L2
Outputs
Figure 11-11 SLC 500 DIV (divide) instruction.
Input
L1
DIV
DIVIDE
Source A
Source B
Destination
C5:10.ACC
2
N7:3
SW
SW
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• The following conversion formula forms the basis
for the program:
F 5 (9
__
5
× C)1 32
• In this example, a current temperature reading of
60°C is assumed.
• The MUL instruction multiplies the temperature
(60°C) by 9 and stores the product (540) in address
N7:0.
• Next, the DIV instruction divides 5 into the 540 and
stores the answer (108) in address N7:1.
• Finally, the ADD instruction adds 32 to the value of
108 and stores the sum (140) in address O:13.
• Thus 60°C 5 140°F.
11.6 Other Word-Level Math
Instructions
The program of Figure 11-14 is an example of the square
root (SQR) instruction. The operation of the logic rung
• When input switch SW is closed the SQR instruc-
tion is executed.
• The number whose square root we want to deter-
mine (144) is placed in the source.
• The function calculates the square root and places it
(12) in the destination.
• If the value of the source is negative, the instruction
will store the square root of the absolute (positive)
value of the source at the destination.
Figure 11-12 DIV instruction used to calculate the value that results
from dividing source A by source B.
Input
L2
PL1
DIV
DIVIDE
Source A
Source B
Destination
N7:0
120
N7:1
4
N7:5
30
EQU
EQUAL
Source A
Source B
N7:5
30
30
PL1
L1
SW
SW
Input
Thumbwheel
switch
L1
1
2
3
Output
MUL
MULTIPLY
Source A
Source B
Destination
Ι:012
60
9
N7:0
540
DIV
DIVIDE
Source A
Source B
Destination
N7:0
540
5
N7:1
108
ADD
ADD
Source A
Source B
Destination
N7:1
108
32
O:013
140
Ι:012 O:013
0 6 0
L2
LED
Display
Fahrenheit
Celsius
1 4 0
Figure 11-13 Program for converting Celsius temperature
to Fahrenheit.
Figure 11-14 SLC 500 SQR (square root) instruction.
Input
L1
SQR
SQUARE ROOT
Source A
Destination
N7:101
144
N7:105
12
SW
SW
( 144 = 12)
√
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Figure 11-17 is an example of the TOD instruction. The
• When input switch SW is closed the TOD instruc-
tion is executed.
• The binary bit pattern at the source address N7:23 is
converted into a BCD bit pattern of the same deci-
mal value at the destination address O:20.
• The source displays the value 10, which is the
correct decimal value; however, the destination
displays the value 16.
• The processor interprets all bit patterns as binary;
therefore the value 16 is the binary interpretation of
the BCD bit pattern.
• The bit pattern for 10 BCD is the same as the bit
pattern for 16 binary.
The convert from BCD (FRD) instruction is used to con-
vert binary-coded decimal (BCD) values to integer values.
This instruction could be used to convert data from a BCD
external source, such as a BCD thumbwheel switch, to the
binary format in which the processor operates.The program
of Figure 11-18 is an example of the FRD instruction. The
• When input switch SW is closed the FDR instruc-
tion is executed.
• The BCD bit pattern stored at the source address
I:30 is converted into a binary bit pattern of the same
decimal value at the destination address, N7:24.
The program of Figure 11-15 is an example of the
negate (NEG) instruction. This math function changes
the sign of the source value from positive to negative.
The operation of the logic rung can be summarized as
follows:
• When input switch SW is closed the NEG instruc-
tion is executed.
• The positive value 101 stored at the source address
N7:52 is negated to 2101 and stored in destination
address N7:53.
• Positive numbers will be stored in straight binary
format, and negative numbers will be stored as 2’s
complement.
The program of Figure 11-16 is an example of the clear
(CLR) instruction. The operation of the logic rung can be
• When input switch SW is closed the CLR instruc-
tion is executed.
• Upon execution it sets all bits of a word to zero.
• In this example it changes the value of all bits stored
in the destination address N7:22 to 0.
The convert to BCD (TOD) instruction is used to
convert 16-bit integers into binary-coded decimal
(BCD) values. This instruction could be used when
transferring data from the processor (which stores data
in binary format) to an external device, such as an LED
display, that functions in BCD format. The program of
Figure 11-15 SLC 500 NEG (negate) instruction.
Input
L1
NEG
NEGATE
Source A
Destination
N7:52
101
N7:53
–101
SW
SW
Figure 11-16 SLC 500 CLR (clear) instruction.
N7:22
0000000000000000
Input
L1
CLR
CLEAR
Destination
SW
SW
Input
L1
TOD
TO BCD
Source
Destination
N7:23
10
O:20
16
SW
SW
Figure 11-17 SLC 500 TOD (convert to BCD) instruction.
Figure 11-18 SLC 500 FRD (convert from BCD)
instruction.
Input
L1
FRD
FROM BCD
Source
Destination
I:30
16
N7:24
10
SW
SW
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data in file address N7:25 to the data stored in file
address N7:50 and store the result in file address
N7:100.
• The rate per scan is set at All, so the instruction
goes to completion in one scan.
The program of Figure 11-21 is an example of the file
subtract function of the FAL instruction. The operation of
the logic rung can be summarized as follows:
• When input switch SW is closed the rung goes true
and the processor subtracts a program constant
(255) from each word of file address N10:0 and
The scale data (SCL) instruction is used to allow very
large or very small numbers to be enlarged or reduced by
the rate value. When rung conditions are true, this instruc-
tion multiplies the source by a specified rate. The rounded
result is then added to an offset value and placed in the
destination. The program of Figure 11-19 is an example
of the SCL instruction. The operation of the logic rung
• When input switch SW is closed the SCL instruc-
tion is executed.
• The number 100 stored at the source address, N7:0,
is multiplied by 25,000, divided by 10,000, and
added to 127.
• The result, 377, is placed in the destination address,
N7:1.
You can use SCL instruction to scale data from your
analog module and bring it into the limits prescribed by the
process variable or another analog module. For instance,
you can use the SCL instruction to convert a 4–20 mA
input signal to a PID process variable, or to scale an ana-
log input to control an analog output.
11.7 File Arithmetic Operations
File arithmetic functions include file add, file subtract,
file multiply, file divide, file square root, file convert from
BCD, and file convert to BCD. The file arithmetic and
logic (FAL) instruction can combine an arithmetic opera-
tion with file transfer. The arithmetic operations that can
be implemented with the FAL are ADD, SUB, MULT,
DIV, and SQR.
The file add function of the FAL instruction can be
used to perform addition operations on multiple words.
The program of Figure 11-20 is an example of the file add
function of the FAL instruction. The operation of the logic
rung can be summarized as follows:
and the expression tells the processor to add the
Figure 11-19 SLC 500 SCL (scale) instruction.
Input
L1
SW
SW
SCL
SCALE
Source
Rate (/10000)
Offset
Destination
N7:0
100
25000
127
N7:1
377
Input
L1
SW
SW
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
#N7:25 + #N7:50
R6:1
4
0
All
#N7:100
25
234
1256
77
N7:25
N7:28
= N7:100
= N7:103
+ N7:50
+
+
+ N7:53
50
22
456
100
75
256
1712
177
#N7:25 #N7:50 #N7:100
EN
DN
ER
Figure 11-20 SLC 500 ﬁle add function of the FAL
instruction.
Figure 11-21 SLC 500 ﬁle subtract function of the FAL
instruction.
680
950
20
100
N10:0
N10:3
= N7:255
= N7:258
– 255 425
695
–235
–155
#N10:0 #N7:255
Input
L1
SW
SW
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
#N10:0 – 255
R6:5
4
0
2
#N7:255
EN
DN
ER
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• The rate per scan is set at All, so the instruction
goes to completion in one scan.
divide function of the FAL instruction. The operation of
and the data in file address F8:20 is divided by the
data in file address F8:100, with the result stored in
element address F8:200.
• The mode is Incremental, so the instruction operates
on one set of elements for each false-to-true transi-
tion of the instruction.
stores the result at the destination file address,
N7:255.
• The rate per scan is set at 2, so it will take 2 scans
from the moment the instruction goes true to com-
plete its operation.
multiply function of the FAL instruction. The operation of
and the data in file address N7:330 is multiplied by
the data in element address N7:23, with the result
stored at the destination file address N7:500.
Figure 11-22 SLC 500 ﬁle multiply function of the FAL
instruction.
20
240
–78
321
N7:330
N7:333
= N7:500
= N7:503
N7:23 100
* 2000
24000
–7800
32100
#N7:330 #N7:500
N7:23
Input
L1
SW
SW
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
#N7:330 * N7:23
R6:8
4
0
All
#N7:500
EN
DN
ER
Figure 11-23 SLC 500 ﬁle divide function of the FAL
instruction.
Input
L1
SW
SW
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
#F8:20 / #F8:100
R6:7
4
1
Incremental
F8:200
100
25
1.33
586
F8:20
F8:23
= F8:200
÷ F8:100
F8:103
1000
2
1.5
3
0.1
#F8:20 #F8:100 F8:200
EN
DN
ER
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1. Explain the function of math instructions as applied
to the PLC.
2. Name the four basic math functions performed by
PLCs.
3. What standard format is used for PLC math
instructions?
4. Would math instructions be classified as input or
output instructions?
5. With reference to the instruction of Figure 11-24,
what is the value of the number stored at source B
if N7:3 contains a value of 60 and N7:20 contains a
value of 80?
what is the value of the number stored at the desti-
nation if N7:3 contains a value of 500?
what is the value of the number stored at the des-
tination if N7:3 contains a value of 40 and N7:4
contains a value of 3?
what is the value of the number stored at the
destination if N7:3 contains a value of 15 and N7:4
contains a value of 4?
what is the value of the number stored at N7:20 if
N7:3 contains a value of 2345?
what will be the value of each of the bits in word
B3:3 when the rung goes true?
what is the value of the number stored at N7:101?
list the values that will be stored in file #N7:10
when the rung goes true.
Figure 11-24 Instruction for Question 5.
ADD
ADD
Source A
Source B
Destination
N7:3
N7:4
N7:20
SUB
SUBTRACT
Source A
Source B
Destination
N7:3
338
N7:20
DIV
DIVIDE
Source A
Source B
Destination
N7:3
N7:4
N7:20
MUL
MULTIPLY
Source A
Source B
Destination
N7:3
N7:4
N7:20
NEG
NEGATE
Source
Destination
N7:3
N7:20
CLR
CLEAR
Destination B3:3
0000111100001111
SQR
SQUARE ROOT
Source A
Destination
N7:101
N7:105
4
FAL
FILE ARITH/LOGICAL
Control
Length
Position
Mode
Destination
Expression
#N11:0 10
R6:0
5
0
All
#N7:10
328
150
10
32
0
File #N11:0
EN
DN
ER
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(3) N7:1
(4) Source B of the GEQ instruction
d. Will output PL1 be energized at this point?
Why?
2. Answer each of the following with reference to the
overfill alarm program shown in Figure 11-33.
a. Assume that the vessel is filling and has reached
the 300-lb point. State the status of each of the
logic rungs (true or false) at this point.
b. Assume that the vessel is filling and has reached
the 480-lb point. State the value of the number
stored in each of the following words at this
point:
(1) I:012
(2) N7:1
c. Assume that the vessel is filled to a weight of
502 lb. State the status of each of the logic rungs
(true or false) for this condition.
1. Answer each of the following with reference to the
counter program shown in Figure 11-32.
a. Assume the accumulated count of counters C5:0
and C5:1 to be 148 and 36, respectively. State
the value of the number stored in each of the fol-
lowing words at this point:
(1) C5:0.ACC
(2) C5:1.ACC
(3) N7:1
(4) Source B of the GEQ instruction
b. Will output PL1 be energized at this point?
Why?
c. Assume the accumulated count of counters C5:0
and C5:1 to be 250 and 175, respectively. State
the value of the number stored in each of the fol-
lowing words at this point:
(1) C5:0.ACC
(2) C5:1.ACC
CHAPTER 11 PROBLEMS
L1
Inputs
S2
L2
Output
S1
C5:0
350
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
350
0
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
PL1
C5:0.ACC
C5:1.ACC
N7:1
ADD
ADD
Source A
Source B
Destination
Reset C5:0
GEQ
Source A
Source B
N7:1
350
CU
DN
CU
DN
RES
C5:1
RES
PL1
Reset
S1
S2
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c. Assume that the set-point temperature is 600°F
and the thermocouple input module indicates a
temperature of 608°F. What is the status (energized
or not energized) of each of the following outputs?
(1) PL1
(2) PL2
(3) Heater
4. With reference to the Celsius to Fahrenheit con-
version program shown in Figure 11-35, state the
value of the number stored in each of the following
words for a thumbwheel setting of 035:
a. I:012
b. N7:0
c. N7:1
d. O:013
5. Design a program that will add the values stored
at N7:23 and N7:24 and store the result in N7:30
whenever input A is true, and then, when input B is
true, will copy the data from N7:30 to N7:31.
6. Design a program that will take the accumulated
value from TON timer T4:1 and display it on a
4-digit, BCD format set of LEDs. Use address
d. Assume that the vessel is filled to a weight of
510 lb. State the value of the number stored in
each of the following words for this condition:
(1) I:012
(2) N7:1
e. With the vessel filled to a weight of 510 lb, state
the status of each of the logic rungs (true or
false).
3. Answer the following with reference to the tem-
perature control program shown in Figure 11-34.
a. Assume that the set-point temperature is 600°F.
At what temperature will the electric heaters be
turned on and off?
b. Assume that the set-point temperature is 600°F
and the thermocouple input module indicates a
temperature of 590°F. What is the value of the
number stored in each of the following words at
this point?
(1) I:012
(2) I:013
(3) N7:0
(4) N7:1
(5) N7:2
Full
Fill
solenoid
Fill
solenoid Full
Full
Filling
Outputs
L2
Inputs
L1
Weight
transducer
(All 16 bits)
SUB
SUBTRACT
Source A
Source B
Destination
GEQ
Source A
Source B
GEQ
Source A
Source B
N7:1
5
1
2
3
4
5
Start
Ι:012
Stop
Ι:012
500
Full
Fill
solenoid
Alarm
Start
Stop
Filling
Ι:012
500
N7:1
Fill
solenoid
Alarm
Full
Filling
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• Multiply the value in N7:3 by 25 and store the
result in N7:4.
• Divide the value in N7:4 by 35 and store the
result in F8:0.
8. a. There are three part conveyor lines (1-2-3) feed-
ing a main conveyor. Each of the three conveyor
lines has its own counter. Construct a PLC pro-
gram to obtain the total count of parts on the
main conveyor.
b. Add a timer to the program that will update the
total count every 30 s.
O:023 for the LEDs. Include the provision to
change the preset value of the timer from a set of
4-digit BCD thumbwheels when input A is true.
Use address I:012 for the thumbwheels.
7. Design a program that will implement the follow-
ing arithmetic operation:
• Use a MOV instruction and place the value 45 in
N7:0 and 286 in N7:1.
• Add the values together and store the result in N7:2.
• Subtract the value in N7:2 from 785 and store the
result in N7:3.
ON/OFF PL1 PL2 Heater
Heater
PL2
PL1
MUL
MULTIPLY
Source A
Source B
Destination
Ι:012
400
0.0100000
N7:0
4
ADD
ADD
Source A
Source B
Destination
Ι:012
400
N7:0
4
N7:1
404
SUB
SUBTRACT
Source A
Source B
Destination
Ι:012
400
N7:0
4
N7:2
396
LES
LESS THAN
Source A
Source B
Ι:013
0
N7:2
396
GRT
GREATER THAN
Source A
Source B
Ι:013
0
N7:1
404
Inputs
L1
ON/OFF
TWS
Thermocouple
input
1
2
3
Ι:013
Ι:012 PL1
PL2
Heater
L2
Outputs
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10. With reference to the math instruction program
shown in Figure 11-37, when the input goes
true, what value will be stored at each of the
following?
a. N7:3
b. N7:4
c. N7:5
d. N7:6
11. Two part conveyor lines, A and B, feed a main
conveyor line M. A third conveyor line, R,
removes rejected parts a short distance away from
the main conveyor. Conveyors A, B, and R have
parts counters connected to them. Construct a PLC
program to obtain the total parts output of main
conveyor M.
12. A main conveyor has two conveyors, A and B,
feeding it. Feeder conveyor A puts six-packs of
canned soda on the main conveyor. Feeder con-
veyor B puts eight-packs of canned soda on the
main conveyor. Both feeder conveyors have coun-
ters that count the number of packs leaving them.
Construct a PLC program to give a total can count
on the main conveyor.
9. With reference to math instruction program shown
in Figure 11-36, when the input goes true, what
value will be stored at each of the following?
a. N7:3
b. N7:5
c. F8:1
Input
Thumbwheel
switch
L1
Celsius
Fahrenheit
1
2
3
Output
MUL
MULTIPLY
Source A
Source B
Destination
Ι:012
9
N7:0
DIV
DIVIDE
Source A
Source B
Destination
N7:0
5
N7:1
ADD
ADD
Source A
Source B
Destination
N7:1
32
O:013
Ι:012 O:013
L2
LED
Display
MUL
MULTIPLY
Source A
Source B
Destination
N7:3
N7:4
4
N7:5
ADD
ADD
Source A
Source B
Destination
N7:1
208
N7:2
114
N7:3
DIV
DIVIDE
Source A
Source B
Destination
N7:5
5.000000
F8:1
Input
Input
MUL
MULTIPLY
Source A
Source B
Destination
N7:3
2
N7:4
SUB
SUBTRACT
Source A
Source B
Destination
N7:1
80
N7:2
20
N7:3
ADD
ADD
Source A
Source B
Destination
N7:4
24
N7:5
SQR
SQUARE ROOT
Source
Destination
N7:5
N7:6
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242
12
Sequencer and Shift
Register Instructions
Chapter Objectives
12.1 Identify and describe the various forms of mechanical
sequencers and explain the basic operation of each
12.2 Interpret and explain information associated
with PLC sequencer output, compare, and load
instructions
12.3 Compare the operation of an event-driven and a time-
driven sequencer
12.4 Describe the operation of bit and word shift registers
12.5 Interpret and develop programs that use shift registers
This chapter explains how the PLC sequencer
and shift register functions operate and how
they can be applied to control problems. The se-
quencer instruction evolved from the mechanical
drum switch, and it can handle complex sequenc-
ing control problems more easily than does the
drum switch. Shift registers are often used to
track parts on automated manufacturing lines by
shifting either status or values through data ﬁles.
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Sequencer and Shift Register Instructions Chapter 12 243
• Each location where there was a peg is represented
by a 1 (on), and the positions where there were no
pegs are each represented by a 0 (off).
Sequencer switches are useful whenever a repeatable
operating pattern is required. One example is the timed
12.
1 Mechanical Sequencers
Sequencer instructions are designed to operate much like
the mechanical rotating cam limit switch shown in Fig-
ure 12-1. These mechanical type sequencers are often
referred to as drum switches, rotary switches, stepper
switches, or cam switches. They are often used to control
machinery that has a repetitive cycle of operation.
Figure 12-2 illustrates the operation of a cam-operated
sequencer switch. An electric motor is used to drive the
cams. A series of leaf-spring mounted contacts interacts
with the cam so that in different degrees of rotation of the
cam, various contacts are closed and opened to energize
and de-energize various electrical devices. As the cams
rotate, load devices connected to the contacts can change
from an on to an off state, from an off to an on state, or
remain at the same state.
Figure 12-3 illustrates a typical mechanical drum-
operated sequencer switch. The switch consists of a series
of normally open contact blocks that are operated by pegs
located on the motor-driven drum. The operation of this
sequencer can be summarized as follows:
• Pegs are placed at specific locations around the
circumference of the drum to operate the contact
blocks.
• When the drum is rotated, contacts that align with
the pegs will close, whereas the contacts where
there are no pegs will remain open.
• The presence of a peg can be interpreted as logic 1,
or on, and the absence of a peg as logic 0, or off.
• The equivalent sequencer data table illustrates the
logic state for the first four steps of the drum cylinder.
Figure 12-1 Rotating cam limit switch.
Symbol
Enclosure
Switch assembly
Figure 12-2 Mechanical cam-operated sequencer.
Motor
Contacts
Cam
Figure 12-3 Mechanical drum-operated sequencer switch.
Equivalent sequencer data table
1
0
1
0
0
1
1
1
2
4
1
3
0
0
1 1
1
0
1
0
1
0
0
1
1
1
1
0
0
0
1
0
0
1
0
1
0 1
0 0
0
1
0
1
1 0
0
0
0
0
0
1
1
0 0
0 0
0
1
0
1 0
0
1
1 0
0
1
3
1
4
2
Steps
NO
switch
Motor
Peg
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244 Chapter 12 Sequencer and Shift Register Instructions
domestic washing machine is another example of the use
of a sequencer, as are dryers and similar time-clock con-
trolled devices.
An example of the wiring and timing chart for a dish-
washer that uses a cam-operated sequencer, commonly
known as the timer, is shown in Figure 12-5. A synchro-
nous motor drives a mechanical train that, in turn, drives a
series of cam wheels. The operation of this sequencer can
• The timer motor operates continuously throughout
the cycle of operation.
• The cam advances in time increments of 45 seconds
in duration.
• The data timing chart shows the sequence of opera-
tion of the timer.
• A total of sixty 45 second steps are used to com-
plete the 45 minute operating cycle.
• Numbers in the active devices column refer to
control devices active during each step of the
cycle.
sequencer switch used in dishwashers to pilot the ma-
chinery through a wash cycle (Figure 12-4). The cycle is
always the same with a fixed routine of actions at each
step for a specific time to complete its specified task. The
Figure 12-4 Dishwasher timed sequencer switch.
Contacts
Motor
Figure 12-5 Dishwasher wiring diagram and timing chart.
Machine function
Active
devices
Timer
increment
Off
First prerinse
Drain
Fill
Rinse
Drain
1 2 4
1 3 4 5
1 4 5 6
1 2 4 5
2
3
4–5
6
Prewash
Fill
Wash
Drain
1 3 4 5
1 4 5 6
1 2 4 5
7
8–10
11
0–1
Second prerinse
Fill
Rinse
Drain
1 3 4 5
1 4 5 6
1 2 4
12
13–15
16
Wash
Fill
Wash
Drain
1 3 4
1 4 5 6
1 2 4 5
17
18–30
31
First rinse
Fill
Rinse
Drain
1 3 4 5
1 4 5 6
1 2 4 5
32
33–34
35
Second rinse
Fill
Rinse
Drain
1 3 4 5
1 4 5 6
1 2 4 5
36
37–41
42
Dry
Dry
Drain
Dry
1 4 6
1 2 4 6
1 4 6
43–58
59
60
Ground
L1
Line
switch
L2
L1
Timer
MTR
Safety water
level switch
1
Drain
pump
2
Timer
motor
4
Circulating
motor
5
Heater
6
Fill
valve
3
MTR
MTR
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Parameters that may be required to be entered in se-
quencer instructions can be summarized as follows:
File—Is the starting address for the registers in the se-
quencer file and you must use the indexed file indica-
tor (#) for this address. The file contains the data that
will be transferred to the destination address when the
instruction undergoes a false-to-true transition. Each
word in the file represents a position, starting with
position 0 and continuing to the file length.
Mask—Is the bit pattern through which the sequencer
instruction moves source data to the destination ad-
dress. Recall that in the mask bit pattern, a 1 passes
values while a 0 blocks the data flow. You use a mask
register or file name when you want to change the
mask pattern under program control. An h is placed
behind the parameter to indicate that the mask is
a hexadecimal number or a B to indicate binary
notation. Decimal notation is entered without any
indicator.
Source—Is the address of the input word or file from
which the SQC and SQL instruction obtains data for
comparison or input to its sequencer file.
Destination—Is the address of the output word or file
to which the SQO moves the data from its sequencer
file.
Control—Is the address that contains the parameters
with control information for the instruction. The con-
trol register stores the status byte of the instruction,
the length of the sequencer file, and the instantaneous
position in the file as follows:
- The enable bit (EN; bit 15) is set by a false-to-true
rung transition and indicates that the instruction is
enabled. It follows the rung condition.
- The done bit (DN; bit 13) is set after the last word
in the sequencer file is transferred. On the next
false-to-true transition of the rung with the done bit
set, the position pointer is reset to 1.
- The error bit (ER; bit 11) is set when the proces-
sor detects a negative position value, or a negative
or zero length value.
12.2 Sequencer Instructions
PLC sequencer instructions replace the mechanical drum
sequencer that is used to control machines that have a
stepped sequence of repeatable operations. Programmed
sequencers can perform the same specific on or off pat-
terns of outputs that are continuously repeated with a
drum switch, but with much more flexibility. Sequencer
instructions simplify your ladder program by allowing
you to use a single instruction or pair of instructions to
perform complex operations. For example, the on/off op-
eration of 16 discrete outputs can be controlled, using a
sequencer instruction, with only one ladder rung. By con-
trast, the equivalent contact-coil ladder control arrange-
ment would need 16 rungs in the program.
Depending on the PLC manufacturer, various sequencer
instructions can be programmed. Figure 12-6 shows the
Sequencer menu tab for the Allen-Bradley SLC 500
PLC and its associated RSLogix software. For the Allen-
Bradley line of controllers, sequencer commands may
include the following:
SQO (Sequencer Output)—Is an output instruction
that uses a file to control various output devices.
SQI (Sequencer Input)—Is an input instruction that
compares bits from an input file to corresponding bits
from a source address. The instruction is true if all
pairs of bits are the same.
SQC (Sequencer Compare)—Is an output instruc-
tion that compares bits from an input source file to
corresponding bits from data words in a sequence file.
If all pairs of bits are the same, then a bit in the con-
trol register is set to 1.
SQL (Sequencer Load)—Is an output instruction
used to capture reference conditions by manually
stepping the machine through its operating sequences.
It transfers data from the input source module to the
sequencer file. The instruction functions much like a
file-to-word transfer instruction.
Figure 12-7 shows an example of an SQO (Sequencer
Output) instruction. The SQO instruction reads data file
elements (words) one at a time, applies a mask word to
enable or disable bits from the current data file element,
and transfers the masked data file element to a designated
output.
Figure 12-6 Sequencer menu tab.
BSL BSR SQC SQL SQO FFL
File Shift / Sequencer
File/Misc Program Control
FFU LFL LFU
Figure 12-7 SQO (Sequencer Output) instruction.
SQO
SEQUENCER OUTPUT
File
Mask
Destination
Control
Length
Position
DN
EN
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Length—Is the number of steps of the sequencer file
starting at position 1. Position 0 is the start-up posi-
tion. The instruction resets (wraps) to position 1 at
each cycle completion. The actual file length will be
1 plus the file length entered in the instruction.
Position—Indicates the step that is desired to start
the sequencer instruction. The position is the word
location or step in the sequencer file from which the
instruction moves data. Any value up to the file length
may be entered, but the instruction will always reset
to 1 on the true-to-false transition after the instruc-
tion has operated on the last position. Before we start
the sequence, we need a starting point at which the
sequencer is in a neutral position. The start position is
all zeros, representing this neutral position; thus, all
outputs will be off in position 0.
To program a sequencer, binary information is first en-
tered into the sequencer file or register made up of a series
of consecutive memory words. The sequencer file is typi-
cally a bit file that contains one bit file word representing
the output action required for each step of the sequence.
Data are entered for each sequencer step according to the
requirements of the control application. As the sequencer
advances through the steps, binary information is trans-
ferred from the sequencer file to the output word.
To illustrate the purpose and function of the sequencer
file we will examine the operation of the four-step se-
quence process shown in Figure 12-8. This sequencer is to
be used to control traffic in two directions. The operation
of the process can be summarized as follows:
• Six outputs are to be energized from one 16-point
output module.
• Each light is controlled by one bit address of output
word O:2.
• The first 6 bits are programmed to execute the fol-
lowing sequence of light outputs:
- Step 1: Outputs O:2.0 (red) and O:2.5 (green)
lights will be energized.
- Step 2: Outputs O:2.0 (red) and O:2.4 (yellow)
will be energized.
- Step 3: Outputs O:2.2 (green) and O:2.3 (red)
will be energized.
- Step 4: Outputs O:2.1 (yellow) and O:2.3 (red)
will be energized.
Figure 12-8 Four-step sequencer.
Start
Positions
Step 1
Step 2
Step 3
Step 4
O:2
B3:0
B3:1
B3:2
B3:3
B3:4
Output
word
Sequencer
file
0
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
1
0
0
0
1
0
0
0
0
0
1
0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0
15 14 13 12 11 10 9 8 7 6 5 4 3 2
0
0
0
0
0
0
0
0
0
0
1
N/S
O:2.0
O:2.1
O:2.2
O:2.3
O:2.4
O:2.5
E/W
Step 1
N/S E/W
Step 2
N/S E/W
Step 3
N/S E/W
Step 4
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• Words B3:0, B3:1, B3:2, B3:3 and B3:4 make up
the sequencer file.
• Binary information (1s and 0s) that reflects the de-
sired on or off light status for each of the four steps
is entered into each word of the sequencer file.
• Before starting the sequence, you need a starting
point where the sequencer is in a neutral position.
This is provided by the start position which is all
zeros.
Due to the way in which the sequencer instruction op-
erates, all output points must be on a single output mod-
ule. When a sequencer operates on an entire output word,
there may be outputs associated with the word that do not
need to be controlled by the sequencer. In our example,
bits 6 through 15 of output word O:2 are not used by the
sequencer but could be used elsewhere in the program. To
prevent the sequencer from controlling these bits of the
output word, a mask word is used. The use of a mask word
is illustrated in Figure 12-9. The operation of the mask
• The mask word selectively screens out data from the
sequencer word file to the output word.
• The hex number 003Fh is entered as the mask
parameter.
• For each bit of output word O:2 that the sequencer
is to control, the corresponding bit of the mask word
must be set to 1.
• The arrows in the figure indicate the unmasked bits
that are passed through the mask and into the desig-
nation address.
• The dashes in the bits of the designation address
indicate that those bits remain unchanged in the des-
ignation location during the sequencing.
• These unchanged bits therefore can be used inde-
pendently of the sequencer.
Figure 12-9 Sequencer moving data through a mask word.
1
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1
Mask
003Fh (hexadecimal)
11111B (binary)
Start
Step 1
Step 2
Step 3
Step 4
B3:0
B3:1
B3:2
B3:3
B3:4
Sequencer
file
0
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
1
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
O:2 Destination 0
– – – – – – – – – – 0 0 0 0 0
0
15 14 13
EN
DN
12 11 10 9 8 7 6 5 4 3 2 1
SQO
SEQUENCER OUTPUT
File #B3:0
Mask 003Fh
Dest O:2
Control R6:0
Length 4
Position 1
Output module
0
L2
1
2
3
4
5
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The sequencer output instruction requires preceding
logic on the rung where it is located. When this logic goes
from false to true, it triggers the sequencer to perform its
functions. Only when the logic preceding the sequencer
instruction makes the transition from false to true will it
go through its functions of reading the data file, applying
the mask, and transferring the masked data file to the out-
put destination. After this cycle, it waits for another false-
to-true occurrence of the preceding logic to increment to
the next step.
Figure 12-10 illustrates how the sequencer moves data
from a file to an output. The operation of the logic rung
• Pushbutton PB is used to send false-to-true trigger
signals to the sequencer output instruction.
• The position of the sequencer instruction is incre-
mented by one for each false-to-true transition of
the sequencer rung.
• Whenever PB is momentarily closed the sequencer
is both enabled and advanced to the next position.
• When the sequencer is at step 1, the binary informa-
tion in word B3:1 (100001) of the sequencer file is
transferred into word O:2 of the output.
• As a result output O:2/0 and O:2/5 will be on and
all the rest will be off.
• Advancing the sequencer to step 2 will transfer the
data from word B3:2 (010001) into word O:2.
• When the position parameter reaches 4 (the value
in the length parameter) all words would have been
moved so the DN (done bit) in the instruction is
set to 1.
• On the next false-to-true transition of the rung, with
done bit set, the position pointer is automatically
reset to 1.
Sequencer instructions are usually retentive, and there
can be an upper limit to the number of external outputs
and steps that can be operated on by a single instruction.
Many sequencer instructions reset the sequencer automat-
ically to step 1 on completion of the last sequence step.
Other instructions provide an individual reset control line
or a combination of both.
12.3 Sequencer Programs
A sequencer program can be event-driven or time-driven.
An event-driven sequencer operates similarly to a me-
chanical stepper switch that increments by one step for
each pulse applied to it. A time-driven sequencer operates
similarly to a mechanical drum switch that increments au-
tomatically after a preset time period.
A sequencer chart, such as the one shown in Fig-
ure 12-11, is a table that lists the sequence of operation
of the outputs controlled by the sequencer instruction.
These tables use a matrix-style chart format. A matrix
is a two-dimensional, rectangular array of quantities. A
time-driven sequencer chart usually indicates outputs
on its horizontal axis and the time duration on its verti-
cal axis. An event-driven sequencer indicates outputs
on its horizontal axis and the input, or event, on its ver-
tical axis.
An example of a time-driven sequencer with timed
steps that are not all the same is shown in Figure 12-12.
This sequencer program is used for automatic traffic light
control at a four-way intersection. Output lights operate
in a sequential fashion with variably timed steps. The
system requires two SQO instructions: one for the light
outputs and the other for the timed steps. Both SQOs have
R6:0 for the control and 4 for the length. The first position
is on for 25 seconds, the second for 5 seconds, the third
for 25 seconds, and the fourth for 5 seconds.
Figure 12-10 Sequencer moving data from a ﬁle to an
output.
L1
Input
PB1
PB1
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#B3:0
001Fh
O:2
R6:0
4
1
SQO
EN
DN
L2
Output
O:2.5
O:2
File #B3:0
O:2.0
O:2.1
O:2.2
O:2.3
O:2.4
1
2
Positions
3
4
1 0 0 0 0 1
0 1 0 0 0 1
0 0 1 1 0 0
0 0 1 0 1 0
Destination
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The operation of the time-driven sequencer program
• The bits controlling the traffic light outputs are
stored in integer file #N7:0 of the first SQO instruc-
tion. The settings for the output bits for each posi-
tion are entered and stored in binary table format as
shown in Figure 12-13. Each word of the #N7:0 file
is moved from the file by the program to the desti-
nation output word O:2 as previously described.
• The second SQO instruction sequencer file, #N7:10,
contains the stored preset timer values 25, 5, 25,
5 seconds. These settings are stored in words N7:11,
N7:12, N7:13, and N7:15 as illustrated in Fig-
ure 12-14. Each word of the #N7:10 file is moved
by the program to the destination address T4:1.PRE,
which is the preset value for the timer. The program
moves information from this file to timer T4:1’s
preset. The mask allows the proper data to pass and
blocks the unnecessary data.
• The timer cycles the two SQO instructions through
their four states.
Figure 12-11 Sequencer chart.
Matrix-style chart
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
L2
Output module
at position 1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
0
1
0
1
0
0
1
0
1
0
0
0
1
0
1
1
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
1
1
0
0
0
1
0
0
1
0
1
0
1
0
0
0
1
0
0
1
Position
Outputs
Sequencer output file words
Indicates that output is energized
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7 0
Figure 12-12 Time-driven sequencer output program.
Timing chart
N/S
E/W
Red
Red
Green Yellow
Yellow
Green
25 s 25 s 5 s
5 s
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
25
0
EN
DN
O:2/5
O:2/6
T4:1/DN
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#N7:0
00FFh
O:2
R6:0
4
0
North/South
East/West
EN
DN
L2
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#N7:10
00FFh
T4:1.PRE
R6:0
4
0
EN
DN
O:2/1
O:2/2
O:2/4
T4:1/DN
O:2/0
Figure 12-13 Sequencer ﬁle #N7:0 light cycle settings.
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• Since both of the SQO instructions have R6:0 for
control and 4 for length they are stepped in unison
to provide a sequentially timed output.
An example of a time-driven sequencer program in
which the time interval between sequencer steps is always
a constant set value is shown in Figure 12-15. The opera-
• The preset time of timer T4:0 is set for 3 seconds.
• The settings of the output bits for each sequencer
position are entered and stored in bit file #B3:0.
• The timer is started by the closing switch SW and
3 seconds later the timer done bit is set to1.
• As a result the timer done bit increments the SQO
instruction to the next position and resets the timer.
• The destination is O:2 and all 16 bits of this word
are used for outputs.
• The mask is FFFF hexadecimal or
1111111111111111 binary, which allows all 16 bits
to pass through.
• As long as input SW is closed the program continues
operating with 3 seconds between sequencer steps.
With an event-driven sequencer, the SQO instruction
advances to the next step by an external pulsed input event
rather than a preset time. An example of an event-driven
sequencer is shown in Figure 12-16. The operation of the
• The sequencer SQO instruction uses two OR con-
figured sensor switches (S1 and S2).
• Any one of the two parallel paths can make the
SQO rung true.
• As each event occurs, that OR branch makes a false-
to-true transition advancing the sequencer position.
• Data are copied from file #B3:0 at the bit lo-
cations through mask word, F0FF hex or
1111000011111111 binary, to the destination O:2.
Mask bits are set to 1 to pass data and reset to 0 to
mask data.
Figure 12-14 Sequencer ﬁle #N7:10 timer settings.
Figure 12-15 Time-driven sequencer with constant time interval between steps.
SW
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
T4:0/DN
#B3:0
FFFFH
O:2
R6:0
8
0
EN
DN
SW
L2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Input
L1
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
3
0
EN
O:2
DN
T4:0/DN
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• Once the position reaches the last position on the
true-to-false transition of the instruction the position
will reset to 1.
• Note that the data in O:2 match the data in position 2
in the file, except for the data in bits 8 through 11.
• Bits 8 through 11 may be controlled from else-
where in the program; they are not affected by the
sequencer instruction because of the 0 in these bit
positions in the mask.
The sequencer input (SQI) instruction allows input
data to be compared for equality against data stored in the
sequencer file. For example, it can make comparisons be-
tween the states of input devices and their desired states:
if the conditions match, the instruction is true.
The SQI instruction is an input instruction available
in Allen-Bradley PLC-5 and ControlLogix controllers.
An example of a PLC sequencer input instruction is
shown in Figure 12-17. The entries in the instruction
are similar to those in the sequencer output instruction,
except that the destination is replaced by the source.
follows:
• The SQI instruction compares the input data in
I:3 through the mask FFF0 with the data in the se-
quencer file N7:11 through N7:15 for equality.
• The specific data in the sequencer file used in the
comparison is identified by the position parameter.
• When the unmasked source bits match those of the
corresponding sequencer file word, the instruction
goes true; otherwise, the instruction is false.
• In this example, the data at position 2 match the
unmasked input data, so the SQI instruction is true,
thus making the rung and output PL1 true.
• The input data can indicate the state of an input
device, such as the combination of input switches
shown in this example program.
• Anytime the combination of opened and closed
switches is equal to the combination of 1s and 0s on
a step in the sequencer reference file, the PL1 output
of the sequencer becomes energized.
Figure 12-16 Event-driven sequencer output program.
EN
DN
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
0
1
2
3
4
5
6
7
8
Output
0
15 1110 9 8
O:2
Mask
File
Pos
#B3:0
Destination
Pos 2
Current step
0
0
0
1
0
0
0
0
0
0
0
0
1
0
0
0
0
1
0
1
0
0
0
0
1
0
0
0
1
0
0
0
0
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
0
1
1
1
1
0
1
0
1
0
0
1
0
1
0
0
0
1
0
0
1
0
1
1
1
0
0
1
1
1
0
1
1
0
0
0
1
1
1
0
1
1
1
0
1
1
0
1
0
1
1
0
0
1
1
0
1
0
1
1
1
0 0 0 0 0 0 0 0 1 1 1 1
#B3:0
F0FF
O:2
R6:0
8
2
Pos
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
L2
Module O:2
1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1
Inputs
L1
S1
S2
S2
S1
Field triggered input events
S1 S2
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The SQI instruction uses a control register like the
SQO instruction but does not have a done bit. In addition,
the SQI instruction does not automatically increment its
position each time its control logic makes a false-to-true
transition at its input. If the SQI instruction is used alone,
the position value must be changed by another instruc-
tion (such as the move instruction) to select a new input
file value to compare against the value from the source
address.
When the SQI is paired with an SQO instruction
with identical control addresses the position is incre-
mented by the SQO instruction for both. The program of
Figure 12-18 illustrates the use of the sequencer input and
sequencer output instructions in pairs to monitor and con-
trol, respectively, a sequential operation. The operation of
• The same control address, length value, and position
value is used for each instruction.
• The sequencer input instruction is indexed by the
sequencer output instruction because both control
elements have the same address, R6:5.
• This type of programming technique allows input
and output sequences to function in unison, causing
a specific output sequence to occur when a specific
input sequence takes place.
The Allen-Bradley SLC 500’s sequencer compare
(SQC) instructions are similar but not identical to the SQI
instruction. Differences between the two include:
• The SQC instruction is an output rather than an
input instruction.
• The SQC instruction increments the position
parameter
• The SQC instruction has an additional status
bit—the found bit (FD). When the source
pattern matches the sequencer file word the
FD is set to 1. It is zero under all other
conditions.
Figure 12-17 Sequencer input (SQI) instruction.
L1
Input module
Ι:3
Inputs
SQΙ
SEQUENCER INPUT
File
Mask
Source
Control
Length
Position
#N7:11
FFF0
Ι:3
R6:21
4
2
PL1
00 00
Input word Ι:3
10 10 11 00 11 01 11 11 11 11 11 11 00 00
00 00 10 10 11 00 11 01
0
Step
1
3
2
4
#N7:11
Word
Sequencer file
12
14
13
15
Output
L2
PL1
Mask value
Figure 12-18 Sequencer input and sequencer output
instructions used in pairs.
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#N7:20
00FF
O:2
R6:5
8
0
SQI
SEQUENCER INPUT
File
Mask
Source
Control
Length
Position
#N7:1
00FF
I:3
R6:5
8
0
EN
DN
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An example of an SLC 500 sequencer compare (SQC)
instruction program is shown in Figure 12-19. The opera-
• The data in the highest 4 bits of the source (I:1) are
compared to the data in file #B3:22.
• In this example, the highest 4 bits in I:1 match
the status of the highest 4 bits in B3:25 at step
position 3.
• If the pushbutton input I:1/0 is true at this point, the
found (FD) bit is set, which turns output PL1 on.
• Whenever the combination of opened and closed
switches connected to I:1/12, I:1/13, I:1/14, and
I:1/15 is equal to the combination of 1s and 0s
on a step in the sequencer reference file and the
input I:1/0 is true, the PL1 output will become
energized.
• The mask (F000h) allows unused bits of the se-
quencer instruction to be used independently. In this
example, unused bit I:1/0 is used for the conditional
input of the sequencer compare rung.
The sequencer load (SQL) instruction is used to read
the PLC input module and store the input data in the se-
quencer file. Loading input conditions for a large number
of process steps is prone to errors. In such instances the
sequencer load instruction can be used to load data into
a sequencer file one step at a time. For example, a robot
may be jogged manually through its sequence of opera-
tion, with its input devices read at each step. At each step,
the status of the input devices is written to the data file in
the sequencer compare instruction. As a result, the file is
loaded with the desired input status at each step, and these
data are then used for comparison with the input devices
when the machine is run in automatic mode.
An example of an SLC 500 sequencer load (SQL) in-
struction program is shown in Figure 12-20. The opera-
• The sequencer load instruction is used to load the
file and does not function during the machine’s nor-
mal operation.
• It replaces the manual loading of data into the file
with the programming terminal.
Figure 12-19 Sequencer compare (SQC) instruction program.
SQO
SEQUENCER COMPARE
File
Mask
Source
Control
Length
Position
#B3:22
F000h
I:1
R6:7
5
0
EN
DN
FD
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
I:1/0
PL1
R6:7
FD
Position 3
Output
Inputs
L2
L1
PL1
Ι:0/1 Input
module Ι:1
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• The sequencer load instruction does not use a mask.
It copies data directly from the source address to the
sequencer file.
• When the instruction goes from false to true, the
instruction indexes to the next position and copies
the data.
• When the instruction has operated on the last position
and has a true-to-false transition, it resets to position 1.
• It transfers data in position 0 only if it is at position
0 and the instruction is true and the processor goes
from the program to run mode.
• By manually jogging the machine through its cycle,
the switches connected to input I:2 of the source
can be read at each position and written into the file
by momentarily pressing PB1. Otherwise, the data
would have to be entered into the file manually.
12.4 Bit Shift Registers
The PLC not only uses a fixed pattern of register (word)
bits, but also can easily manipulate and change individual
bits. A bit shift register is a register that allows the shift-
ing of bits through a single register or group of registers.
The bit shift register shifts bits serially (from bit to bit)
through an array in an orderly fashion.
A shift register can be used to simulate the movement,
or track the flow, of parts and information. We use the
shift register whenever we need to store the status of an
event so that we can act on it at a later time. Shift registers
can shift either status or values through data files. Com-
mon applications for shift registers include the following:
• Tracking of parts through an assembly line
• Controlling of machine or process operations
Figure 12-20 Sequencer load (SQL) instruction program.
SQL
SEQUENCER LOAD
File
Source
Control
Length
Position
#N7:20
Ι:2
R6:22
5
3
PB1
00 00
15
Source word Ι:2
0
10 10 11 00 11 01
00 00
15 0
10 10 11 00 11 01
0
Source
Ι:2
Current
step
2
4
3
5
N70:20
1
21
Word
Destination file #N7:20
22
24
23
25
L1
Source
word Ι:2
Inputs
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PB1
EN
DN
Cam (on machine)
Operating
force
Limit switch
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• Inventory control
• System diagnostics
Figure 12-21 illustrates the basic concept of a shift reg-
ister. A shift pulse or clock causes each bit in the shift
register to move 1 position to the right. At some point, the
number of data bits fed into the shift register will exceed
the register’s storage capacity. When this happens, the
first data bits fed into the shift register by the shift pulse
are lost at the end of the shift register. Typically, data in
the shift register could represent the following:
• Part types, quality, and size
• The presence or absence of parts
• The order in which events occur
• Identification numbers or locations
• A fault condition that caused a shutdown
You can program a shift register to shift status data ei-
ther right or left, as illustrated in Figure 12-22, by shifting
either status or values through data files. When you want
to track parts on a status basis, use bit shift registers. Bit
shift instructions will shift bit status from a source bit ad-
dress, through a data file, and out to an unload bit, one bit
at a time. There are two bit shift instructions: bit shift left
(BSL), which shifts bit status from a lower address num-
ber to a higher address number through a data file, and bit
shift right (BSR), which shifts data from a higher address
number to a lower address number through a data file.
Some PLCs provide a circulating shift register function,
which allows you to repeat a pattern again and again.
When working with a bit shift register, you can iden-
tify each bit by its position in the register. Therefore,
working with any bit in the register becomes a mat-
ter of identifying the position it occupies rather than
the conventional word number/bit number addressing
scheme.
Figure 12-23 shows the File Shift menu tab and BSL
and BSR instruction blocks that are part of the instruction
set for the Allen-Bradley SLC 500 controllers. The com-
mands can be summarized as follows:
BSL (Bit Shift Left)—Loads a bit of data into a
bit array, shifts the pattern of data through the array
Figure 12-21 Basic concept of a shift register.
1 1 0 1 1 0 0 1
New data in shift right position
0 1 1 1 0 1 0 0
1 0 1 1 0 0 1 0
Original data in initial position
1 1 1 0 1 0 0 1
Data in = 1
Shift data
(clock)
0 1 1 0 1 1 0 0
New data in shift right position
Tracking the absence of bottles
1 0 1 1 1 0 1 0
Data in = 0
Shift data
(clock)
Figure 12-22 Types of shift registers.
Bit shift right (BSR) register
1 or 0
Most significant
bit address
Bit shift left (BSL) register
1 or 0
Least significant
bit address
Wraparound or circulating shift register
Figure 12-23 Bit shift left and bit shift right instructions.
BSL
BIT SHIFT LEFT
File
Control
Bit address
Length
FFU LFL LFU
EN
DN
BSR
BIT SHIFT RIGHT
File
Control
Bit address
Length
EN
DN
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An example of a bit shift left (BSL) instruction pro-
gram is shown in Figure 12-24. The operation of the pro-
• Momentary actuation of limit switch LS causes the
BSL instruction to execute.
• When the rung goes from false to true, the enable
bit is set and the data block is shifted to the left (to a
higher bit number) one bit position.
• The specified bit, at sensor bit address I:1/1, is
shifted into the first bit position, B3:10/0.
• The last bit is shifted out of the array and stored in
the unload bit, R6:0/UL.
to the left, and unloads the last bit of data in the
array.
BSR (Bit Shift Right)—Loads a bit of data into a bit
array, shifts the pattern of data through the array to the
right, and unloads the last bit of data in the array.
Shift registers are useful for tracking the status or iden-
tification of a part as it moves down an assembly line. The
data file used for a shift register usually is the bit file be-
cause its data are displayed in binary format, making it eas-
ier to read. BSL and BSR are output instructions that load
data into a bit array one bit at a time. The data are shifted
through the array, then unloaded one bit at a time.
The BSL instruction has the same operands as the BSR
instruction. The difference is the direction in which the
bits are indexed. A bit shift instruction will execute when
its input control logic goes from false to true. To program
a bit shift instruction, you need to provide the processor
with the following information:
File—The address of the bit array you want to manip-
ulate. The address must start with the # sign and at bit
0 of the first word or element. Any remaining bits in
the last word of the array cannot be used elsewhere in
the program because the instruction invalidates them.
Control—R data-table type. The address is unique to
the instruction and cannot be used to control any other
instruction. It is a three-word element that consists of
the status word, the length, and the position.
Bit address—Is the address of the source bit. The instruc-
tion inserts the status of this bit in either the first (lowest)
bit position (for the BSL instruction) or the last (highest)
bit position (for the BSR instruction) in the array.
Length—Indicates the number of bits to be shifted,
or the file length, in bits. The status bits of the con-
trol word are the enable, done, error, and unload bits.
Their functions can be summarized as follows:
- Enable Bit (EN)—The enable bit follows the instruc-
tions status and is set to 1 when the instruction is true.
- Done Bit (DN)—The done bit is set to 1 when the
instruction has shifted all bits in the file one posi-
tion. It resets to 0 when the instruction goes false.
- Error Bit (ER)—The error bit is set to 1 when the
instruction has detected an error, which can happen
when a negative number is entered in the length.
- Unload Bit (UL)—The unload bit’s status is con-
trolled by shifting of the last bit of the file into the un-
load bit when the instruction is executed. It is the bit
location into which the status from the last bit in the
file shifts when the instruction goes from false to true.
When the next shift occurs, these data are lost, unless
additional programming is done to retain the data.
Figure 12-24 Bit shift left (BSL) instruction program.
BSL
BIT SHIFT LEFT
File
Control
Bit address
Length
EN
DN
L1
Limit switch
Sensor
LS
LS
Ι:1/1
1
15 0
1 0 0 1 1 0 0 1 1 0 1 1 0 0 0
1 1
Unload bit
R6:0/UL
B3:10
B3:11
Bit address
Ι:1/1
1 0
Shift direction
20
1
B3:Table - Before limit switch clock pulse
B3:10/
B3:11/
Invalid
1
15
0 0 1 1 0 0 1 1 0 1 1 0 0 0
1 1
Unload bit
R6:0/UL
B3:10
B3:11
Bit address
Ι:1/1
0 1
Shift direction
20
1
1
B3:Table - After limit switch clock pulse
3
15 14 13 12 11 10 7 6 4
9 8 2 1
5 0
B3:10/
B3:11/
Invalid
1
0
1 0 0 1 1 0 1 0 1
0 1 0 0
1 1
1
0 0 0 0 0 0 0 0 0
0 0 1 0
0 1
3
1
1
15
1
0
14
1
0
13
0
0
12
0
0
11
1
0
10
1
0
7
1
0
6
1
0
4
1
0
9
0
0
8
0
0
2
1
0
1
1
0
5
0
0
0
0
0
#B3:10
R6:0
Ι:1/1
20
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binary information refers to any two conditions that can
be assigned to the moving product—for example, the
presence or absence of a part. As the part moves along
the conveyer, some form of sensing device will determine
which of these two categories the passing product falls
into. Figure 12-27 illustrates cartons traveling on a con-
veyor being detected by a photoelectric sensor. The sen-
sor that drives the data line on a shift register is fixed such
that the beam detects the presence or absence of a carton.
A logic 1 sensor condition state can indicate the presence
of a carton, and a 0 the absence.
• The status that was previously in the unload bit is
lost.
• All the bits in the unused portion of the last word of
the file are invalid and should not be used elsewhere
in the program.
• For wraparound operation, set the position of the bit
address to the last bit of the array or to the UL bit,
whichever applies.
An example of a bit shift right (BSR) instruction pro-
gram is shown in Figure 12-25. The operation of the pro-
• Before the rung goes from false to true, the status of
bits in words B3:50 and B3:51 is as shown.
• The status of the bit address, I:3/5, is a 0, and the
status of the unload bit, R6:1/UL, is a 1.
• When limit switch LS closes, the status of the bit
address, I:3/5, is shifted into B3:51/7, which is the
24th bit in the file.
• The status of all the bits in the file is shifted one
position to the right, through the length of 24 bits.
• The status of B3:50/0 is shifted to the unload bit,
R6:1/UL. The status that was previously in the un-
load bit is lost.
An example of a bit BSL instruction program with
wraparound operation is shown in Figure 12-26. The clock
pulse input is a fixed regular 3 second pulse–generated on-
delay timer T4:0. The operation of the program can be
• Go to the data tables and set bit addresses B3:0/0,
B3:0/1, B3:0/2 to logic 0 and bit address R6:0/UL
to logic 1.
• When the PLC is then placed in run, bit B3:0/0 is
set to logic 1 causing PL1 to turn on.
• Closing input switch SW starts timer T4:0 timing.
• After 3 seconds the timer done bit is set to reset the
timer accumulated time to zero and shift the logic
bit 1 to the left to B3:0/1.
• This causes PL1 to turn off and PL2 to turn on.
• After another 3 seconds, the timer done bit is set
once again.
• The BSL instruction shifts the bits to the left once
more and causes PL2 to turn off and PL3 to turn on.
• The process continues with each of the pilot lights
turned on in sequence for 3 seconds.
A shift register is often used in material handling
processes where some form of binary information must
be synchronized with a moving part on a conveyor. The
Figure 12-25 Bit shift right (BSR) instruction program.
BSR
BIT SHIFT RIGHT
File
Control
Bit address
Length
EN
DN
L1
Limit switch
Sensor
LS
LS
Ι:3/5
Ι:3/5
Bit address
1
15 0
0 1 1 0 0 0 1 1
1
0 0 1 0 1 1 0
0
1 0
B3:50
B3:51
Unload bit
R6:1/UL
1 1
0 1
Ι:3/5
Bit address
Shift direction
Shift direction
24
Unload bit
R6:1/UL
1
0
B3: Table - Before limit switch clock pulse
B3:50/
B3:51/
Invalid
1 1 0 1 1 0 0 0 1 1 0 0 1 0 1 1
1
1 0 0 1
1 0
Invalid
B3:50
B3:51
24
1
0
B3: Table - After limit switch clock pulse
3
15 14 13 12 11 10 7 6 4
9 8 2 1
5 0
B3:50/
B3:51/
1
1 1 0 1 1 0 1 1 0
0 0 0 1
0 1
1
0 0 0 0 0 0 0 1 1
0 0 0 0
0 1
3
0
0
15
1
0
14
0
0
13
1
0
12
1
0
11
0
0
10
0
0
7
1
1
6
0
0
4
1
1
9
0
0
8
0
1
2
0
1
1
1
1
5
1
0
1
0
0
#B3:50
R6:1
Ι:3/5
24
0
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The program for the spray-painting operation is shown
in Figure 12-29. Its operation can be summarized as
follows:
• Limit switch LS1 is used to detect the hanger and
limit switch LS2 the part.
• The pulse generated by the hanger-operated limit
switch LS1 shifts the status of the data provided by
part-detection limit switch LS2.
• The logic of this operation is such that when a part
to be painted and a part hanger occur together at
station 1 (indicated by simultaneous closing of LS2
and by LS1), logic 1 is input into the shift register at
B3:0/0.
• This causes the SOL 1 rung to be true and the un-
dercoat spray gun to energize.
• At station 5 a 1 appears in bit B3:0/5 of the shift
register to make the SOL 2 rung true and topcoat
spray gun energize.
• Logic 0 in the shift register indicates that the con-
veyor has no parts on it to be sprayed, and it there-
fore inhibits the operation of the spray guns.
• Counter C5:1 counts the parts as they enter the pro-
cess and counter C5:2 as they exit.
• The count obtained by the two counters should be
equal when no parts are being painted.
The process of Figure 12-28 illustrates a spray-painting
operation controlled by a shift left register. As the parts
pass along the production line, the shift register bit pat-
terns represent the items on the conveyor hangers to be
painted. Each file bit location represents a station on the
line, and the status of the bit indicates whether or not a part
is present at that station.
Figure 12-27 Cartons traveling on a conveyor being
detected by a photoelectric sensor.
Source: Courtesy Banner Engineering Corp.
Figure 12-26 BSL instruction with a wraparound operation.
Inputs
L1
SW
T4:0
1.0
3
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
PL3
B3:0/2
Outputs
L2
PL3
PL2
PL1
PL2
B3:0/1
PL1
B3:0/0
B3:0/0
R6:0/UL
T4:0/DN
SW
#B3:0
R6:0
B3:0/0
3
BSL
BIT SHIFT LEFT
File
Control
Bit address
Length
T4:0/DN
EN
EN
DN
DN
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Figure 12-28 Spray-painting operation controlled by a shift left register.
Reset PL1
1
8 7 6 5 4 3 2
0 0 0 0 0 0 0
1
No
Yes
Part
present 1 1 1 1 1 1
0
1
1
Oven 2
Oven 1
B3:0/7
LS1
(Hanger)
LS2
(Part enter)
(Part exit)
LS3
Storage
Undercoat
spray gun 1
Topcoat
spray gun 2
2 3 4 5 6 7 8
B3:0/6 B3:0/5 B3:0/4 B3:0/3
Station
B3:0/2 B3:0/1 B3:0/0
File
#B3:0
Figure 12-29 Spray-painting operation program.
EN
SOL 1
SOL 2
PL1
Outputs
L2
LS1
LS2
LS3
Inputs
B3:0/0
B3:0/5
I:1/2
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:1
CTU
COUNT-UP COUNTER
Counter
Preset
Accumulated
C5:2
Reset
BSL
BIT SHIFT LEFT
File
Control
Bit address
Length
#B3:0
R6:1
I:1/2
8
C5:2
C5:1
CU
CU
DN
DN
RES
RES
EQU
EQUAL
Source A
Source B
C5:1.ACC
C5:2.ACC
PL1
DN
Reset
Clock
pulse
Spray gun 1
Spray gun 2
Data
pulse
L1
SOL 1
SOL 2
LS3
LS1
LS2
I:1/2
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• They turn off or remain off as empty carriers move
through.
• Station 5 is an inspection station where parts are
examined.
• If the part fails, the inspectors push PB1 as they re-
move the part from the system, which turns output
O:4/4 off.
• Rework parts can be added back into the system at
station 7.
• When the operator puts a part on an empty carrier,
he or she pushes PB2, turning output O:4/6 on to
resume tracking.
12.5 Word Shift Operations
The first in, first out (FIFO) instructions are word shift
operations that are similar to bit shift operations. Word
shifting provides a simpler method of loading and un-
loading data into a file, usually called the stack. It is often
used for tracking parts through an assembly line, where
parts are represented by values that have a part number or
• Whenever the two counts are equal in value the equal
instruction executes to turn on pilot light PL1. This
is an indication that the parts commencing the spray-
painting run equal the parts that have completed it.
An example for a bit shift program used to keep track
of carriers flowing through a 16-station machine is shown
Figure 12-30. The operation of the program can be sum-
marized as follows:
• Proximity switch 1 senses a carrier, and proximity
switch 2 senses a part on the carrier.
• Clock pulse generated by carrier proximity switch
I:1/1 shifts the status of the data provided by part
detection proximity switch I:1/2.
• When a part and container are sensed together, in-
dicated by simultaneous closing of I:1/2 and I:1/1;
logic 1 is input into the shift register at output
O4:0/0 to energize the pilot light connected to it.
• Remaining pilot lights connected to output module
O:4 turn on in sequence as carriers with parts move
through each station.
Figure 12-30 Program for tracking of carriers ﬂowing through a 16-station machine.
EN
O:4/4
PL4
PL6
O:4/6
I:1/1
Prox #1
Inputs
I:1/3
PB1
I:1/4
PB2
BSL
BIT SHIFT LEFT
File
Control
Bit address
Length
#O:4
R6:0
I:1/2
16
DN
U
L
O:4/6
I:1/1
I:1/4
Prox #1
(Carrier detection)
Prox #2
(Part detection)
PB2
(Reworked part)
L1
I:1/3
PB1
(Failed part)
I:1/2
Outputs
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
L2
O:4
Station
O:4
O:4/15 O:4/14 O:4/13 O:4/12 O:4/11 O:4/10 O:4/9 O:4/8 O:4/7 O:4/6 O:4/5 O:4/4
Rework Inspection
O:4/3 O:4/2 O:4/1 O:4/0
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
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be entered in the instruction block are summarized as
follows:
Source—Word address from which the data are en-
tered into the FIFO file.
FIFO—Address of the file in which the data are en-
tered. The address must start with a # sign.
Control—R data-table type and is the file address of
the control structure. The status bits, stack length, and
position are stored in this element.
Length—File length in words. Specifies the maxi-
mum number of words in the stack.
Position—Is the next available location where the
instruction loads data into the stack. The first address
in the stack is position 0. As each word is entered into
the stack, the position counter, on both the FFL and
FFU, will increment up by one. The stack is consid-
ered full when the position value equals the length.
The status bits of the control word are the enable
(EN), the done (DN), and the empty (EM) bits. Their
functions can be summarized as follows:
- Enable Bit (EN)—The enable bit follows the in-
structions status and is set to 1 when the instruction
is true.
- Done Bit (DN)—The done bit is set to 1when the in-
struction’s position equals the length. When the done
bit is set, the FIFO is full and does not accept any
more data. Also the data in the FIFO file are not over-
written when the instruction goes from false to true.
- Empty Bit (EM)—The empty bit is set to 1 when
all the data have been unloaded from the FIFO file.
Figure 12-33 shows the SLC 500 FIFO unload (FFU)
instruction. The following parameters need to be entered
in the SLC 500 FFU instruction:
FIFO—Address of the file in which the data are
entered. The address must start with a # sign. When
paired with an FFL instruction, this address is the
same as the address for the FFL.
an assembly code. Figure 12-31 shows a barcode reader
used for reading printed barcode data on boxes.
A bit shift register operates synchronously or in a serial
fashion because information is shifted one bit at a time
within a word or words. For every bit shifted in, one is
shifted out. Data entered in a bit shift register must be
shifted the length of the register (one position per shift
pulse) before they are available to shift out.
A FIFO function operates asynchronously. Rather than
shifting bits of information within a word it shifts the data
from a complete word into a file or stack. Unlike the bit
shift register, two separate shift pulses are required: one
to shift data into the file (load) and one to shift data out of
the file (unload). These two shift pulses operate indepen-
dently (asynchronously) of each other. Data loaded in a
FIFO can be immediately available for unload, regardless
of length.
The FFL and FFU instruction are used in pairs. The
FFL loads logic words into a user-created file called a
FIFO stack. The FFU instruction is used to unload the
words from the FIFO stack, in the same order as the words
were entered. The first word entered is the first word out.
The SLC 500 FIFO load (FFL) instruction is shown
in Figure 12-32. The parameters that are required to
Figure 12-31 Barcode reader.
Source: Courtesy Keyence Canada Inc.
Figure 12-32 SLC 500 FIFO load (FFL) instruction.
FFL
FIFO LOAD
Source
FIFO
Control
Length
Position
FFU LFL LFU
EN
DN
EM
Figure 12-33 SLC 500 FIFO unload (FFU) instruction.
FFU
FIFO UNLOAD
FIFO
Destination
Control
Length
Position
FFU LFL LFU
EU
DN
EM
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and FFU instruction pair. The operation of the program
• The FIFO load and FIFO unload instructions share
the same control element, R6:0, which may not be
used to control any other instructions.
• FIFO, #N7:12, is the address of the stack. The
same address is programmed for the FFL and FFU
instructions.
• Data enter the FIFO file from the source address,
N7:10, on a false-to-true transition of input A.
• Data are placed at the position indicated in
the instruction on a false-to-true transition of
the FFL instruction, after which the position
indicates the current number of data entries in
the FIFO file.
• The FIFO file fills from the beginning address of the
FIFO file and indexes to one higher address for each
false-to-true transition of input A.
Destination—Address to which the FFU unloads
data.
Control—R data-table type. It is a three-word ele-
ment that consists of the status word, the length, and
the position. When it is paired with the FFL, the con-
trol addresses are the same.
Length—File length in words. Specifies the maxi-
mum number of words in the stack.
Position—Next location from which data are un-
loaded when the instruction goes from false to true.
The status bits of the control word are the enable (EN),
the done (DN), and the empty (EM) bits. The enable bit
follows the instruction’s status, the done bit is set when
the instruction’s position equals the length, and the empty
bit is set when all the data have been unloaded from the
FIFO file.
data are indexed in and out of a FIFO file using the FFL
Figure 12-34 How data are indexed in and out of a FIFO ﬁle.
FFL
FIFO LOAD
Source
FIFO
Control
Length
Position
FFU
FIFO UNLOAD
FIFO
Dest
Control
Length
Position
N7:10
#N7:12
R6:0
10
8
#N7:12
N7:11
R6:0
10
8
Input A
Input B
L1
Inputs
Integer Table
Value
N7:10
N7:11 16
N7:12 31
N7:13 53
N7:14 146
N7:15 9875
N7:16 125
N7:17 867
N7:18 5
N7:19 11
N7:20 0
N7:21 0
Radix: Decimal
23
Input A
Input B
EN
DN
EM
EU
DN
EM
31
53
146
9875
125
867
5
11
0
0
Position
FIFO file
#N7:12
N7:12
Data index
toward the
starting
address of
the file, one
word with
each false-to-true
transition of the
FFU.
N7:21
0
1
2
3
4
5
6
7
8
9
Data exit from position 0 of the
FIFO file on a false-to-true
transition of the FFU and write
over current data in the
destination.
16
N7:11
Destination
Data enter the FIFO file on a
false-to-true transition of the FFL
at the position indicated in the
instruction.
23
N7:10
Source
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The difference between FIFO and LIFO stack opera-
tion is that the LIFO instruction removes data in the re-
verse of the order they are loaded (last in, first out). An
example of the LIFO instruction pair is shown in Fig-
ure 12-36 and the operation of this function can be sum-
marized as follows:
• The load and unload of the LIFO stack operates
similarly to that of the FIFO stack, except that the
last word in the LIFO stack is the first word that is
unloaded from the stack.
• Words can be added to the LIFO stack without dis-
turbing the words already loaded on the stack.
• Otherwise, LIFO instructions operate the same as
FIFO instructions.
• A false-to-true transition of input B causes all data
in the FIFO file to shift one position toward the
starting address of the file, with the data from the
starting address of the file shifting to the destination
address, N7:11.
The FIFO instruction is often used for inventory con-
trol. One example is where different parts need to be re-
moved from inventory to be used in production. Each part
is assigned a unique code, which is loaded into a FIFO
stack, and parts are removed in the order prescribed by
the stack. This type of control ensures that the oldest part
in the inventory is used first as the first part entered is the
first part removed.
The opposite principle—where the last data to be
stored are the first to be retrieved—is known as LIFO
(Last In, First Out). The LIFO instruction inverts the order
of the data it receives by outputting the last data received
first and the first data received last. A useful analogy is
a pile of work on your desk. As new work arrives you
drop it on the top of the stack. If your stack is LIFO, you
pick your next job from the top of the pile. If your stack
is FIFO, you pick your work from the bottom of the pile.
Figure 12-35 shows how the FIFO and LIFO operations
work for container stacking operations.
Figure 12-35 FIFO and LIFO container stacking
operations.
Vertical storage area
Out
3
3
In
LIFO stack
Horizontal storage area
3
2
1
Out
In
1
4
FIFO stack
2 1
3
Figure 12-36 LIFO instruction pair.
LFL
LIFO LOAD
Source
LIFO
Control
Length
Position
N70:1
#N70:3
R6:61
64
0
LFU
LIFO UNLOAD
LIFO
Dest
Control
Length
Position
#N70:3
N70:2
R6:61
64
0
3
4
5
6
7
8
9
10
11
63
Word
64 words allocated for
LIFO stack at #N70:3
LIFO unload
removes data
from stack in
reverse order.
Destination N70:2
Transfer of data
N70:1 Source
LIFO load enters
data into stack at
next position.
File #N70:3
FFU LFL LFU
EN
DN
EM
EU
DN
EM
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1. Describe the operation of a drum switch.
2. What type of operations are sequencers most
suitable for?
3. Why are PLC sequencers easier to program than
PLC discrete outputs?
4. Answer the following with regard to an SLC 500
PLC sequencer output instruction:
a. Where is the information for each sequencer
step entered?
b. What is the function of the output word?
c. Explain the transfer of data that occurs as the
sequencer is advanced through its various steps.
5. What is the function of the file of a sequencer?
6. What is the function of the mask in the sequencer
instruction?
7. What is the relationship between the length and the
position in a sequencer instruction?
8. What output and step programming limits may be
placed on sequencer instructions?
9. Sequencer instructions are usually retentive.
Explain what this means.
10. Compare the operation of an event-driven and a
time-driven sequencer.
11. Explain the function of a sequencer input and com-
pare instruction.
12. What is the difference between SQI and SQC
instructions?
13. What is the purpose of using the SQI and SQO
instruction in pairs?
14. What is the primary application in which an SQL
instruction is used?
15. Explain the function of a sequencer load
instruction.
16. How does a bit shift register manipulate individual
bits?
17. List four common applications for bit shift
registers.
18. When using a sensor as the input to the bit address
of a BSL instruction, what is its function?
19. Compare the operation of the BSL and BSR bit
shift instructions.
20. A bit shift register is said to operate in a synchro-
nous manner. Explain what this means.
21. What is the function of the unload bit in a BSL
instruction?
22. What is the function of the unload bit in a BSR
instruction?
23. A first in, first out word shift register operates in an
asynchronous manner. Explain what this means.
24. Why are both FFL and FFU instructions needed to
perform a FIFO function?
25. Compare the operation a FIFO register and a LIFO
register.
1. Construct an equivalent sequencer data table for
the four steps of the mechanical drum-operated
sequencer drawn in Figure 12-37.
CHAPTER 12 PROBLEMS
2. Answer the following with reference to the
sequencer file #B3:0 shown in Figure 12-38:
a. Assume that output bit addresses O:2/0 through
O:2/15 are controlling associated output pilot
Figure 12-37 Drum-operated sequencer for Problem 1.
3
1
4
2
Steps
NO
switch
Motor
Peg
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3. Answer each of the following with reference to
the timer-driven sequencer program shown in
Figure 12-39:
a. How many bit outputs are controlled by this
sequencer?
lights PL1 through PL16. State the status of each
light for steps 1 through 4.
b. What output bit addresses could be masked? Why?
c. State the status of each bit of output word O:2 for
step 3 of the sequencer cycle.
Figure 12-38 Sequencer ﬁle for Problem 2.
0
1
0
0
0
0
1
0
1
1
0
1
0
0
1
0
0
1
1
1
0
1
0
0
1
0
1
0
1
0
0
0
1
0
0
0
1
0
1
0
0
1
0
0
0
0
0
1
1
1
0
1
0
0
1
0
1
0
1
1
0
0
1
0
1
0
0
1
1
0
0
0
1
0
0
0
1
0
1
0
1 0
15 14 13 12 11 10 9 8 7 6 5 4 3 2
Start
Positions
Step 1
Step 2
Step 3
Output
O:2
B3:0
B3:1
B3:2
B3:3
Step 4
B3:4
0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Figure 12-39 Timer-driven sequencer program for Problem 3.
L2
O:2/0
O:2/1
O:2/2
T4:1/DN
T4:1/DN
North/South
East/West
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#N7:0
00FFh
O:2
R6:0
4
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:1
1.0
25
0
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
#N7:10
00FFh
T4:1.PRE
R6:0
4
0
Integer Table
Radix Decimal
Value
N7:10 0
N7:11 25
N7:12 5
N7:13 25
N7:14 5
Integer Table
Radix Binary Table: N7:Integer
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
N7:0/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N7:1/ 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1
N7:2/ 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1
N7:3/ 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0
N7:4/ 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0
EN
DN
EN
DN
EN
DN
O:2/4
O:2/5
O:2/6
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c. Assume that the sequencer is stepped to position
8; what bit outputs will be on?
d. Assume that the sequencer is at position 8 and a
true-to-false transition of one of the inputs occurs.
What happens as a result?
5. Using whatever PLC sequencer output instruction
you are most familiar with, develop a program that
will operate the cylinders in the desired sequence.
The time between each step is to be 3 seconds.
The desired sequence of operation will be as
follows:
• All cylinders to retract.
• Cylinder 1 advance.
• Cylinder 1 retract and cylinder 3 advance.
• Cylinder 2 advance and cylinder 5 advance.
• Cylinder 4 advance and cylinder 2 retract.
• Cylinder 3 retract and cylinder 5 retract.
• Cylinder 6 advance and cylinder 4 retract.
• Cylinder 6 retract.
• Sequence to repeat.
6. Using whatever PLC sequencer output instruction
you are most familiar with, develop a program to im-
plement an automatic car-wash process. The process
is to be event-driven by the vehicle, which activates
b. What is the address of the word that controls the
outputs?
c. What is the address of the sequencer file that sets
the states for the outputs?
d. What is the address of the sequencer file that con-
tains the preset timer values?
e. For what length of time is the red light
programmed to be on?
f. For what length of time is the green light
g. For what length of time is the yellow light
h. What is the time required for one complete cycle
of the sequencer?
i. Assume that the decimal value stored in N7:13
is changed to 35. Outline the changes that this
new value will have on the timing of the traffic
lights.
4. Answer each of the following with reference to
the event-driven sequencer program shown in
Figure 12-40:
a. When does the sequencer advance to the next step?
b. Assume that the sequencer is at position 2, as
shown; what bit outputs will be on?
Figure 12-40 Event-driven sequencer program for Problem 4.
L1
S1
#B3:0
F0FF
O:2
R6:0
8
2
SQO
SEQUENCER OUTPUT
File
Mask
Dest
Control
Length
Position
EN
DN
S1
S2
S2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Output
module O:2
Current
step
Pos 2
0
1
2
3
4
5
6
7
8
0
15 1110 9 8
O:2
Mask
File
Pos
#B3:0
Destination
0
0
0
1
0
0
0
0
0
0
0
0
1
0
0
0
0
1
0
1
0
0
0
0
1
0
0
0
1
0
0
0
0
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
0
1
1
1
1
0
1
0
1
0
0
1
0
1
0
0
0
1
0
0
1
0
1
1
1
0
0
0 0 0 0 0 0 0 1 1 1 1
1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1
Pos
0
0
1
1
1
0
1
1
0
0
0
1
1
1
0
1
1
1
0
1
1
0
1
0
1
1
0
0
1
1
0
1
0
1
1
1
L2
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• If the product is defective, reject status lights come
on at stations 1, 2, and 3 to tell the assembler to
ignore the part.
• When a defective part reaches station 4, a diverter
gate is activated to direct that part to a reject bin.
• Using whatever PLC bit shift register you are most
familiar with, develop a program to implement this
process.
various limit switches (LS1 through LS6) as it is
pulled by a conveyor chain through the car-wash
bay. Design the program to operate the car wash in
the following manner:
• The vehicle is connected to the conveyor chain and
pulled inside the car-wash bay.
• LS1 turns on the water input valve.
• LS2 turns on the soap release valve, which
mixes with the water input valve to provide a
wash spray.
• LS3 shuts off the soap valve, and the water input
valve remains on to rinse the vehicle.
• LS4 shuts off the water input valve and activates
the hot wax valve, if selected.
• LS5 shuts off the hot wax valve and starts the air-
blower motor.
• LS6 shuts off the air blower. The vehicle exits the
car wash.
7. A product moves continuously down an
assembly line that has four stations, as shown in
Figure 12-41.
• The product enters the inspection zone, where its
presence is sensed by the proximity switch.
• The inspector examines it and activates a reject
button if the product fails inspection.
Figure 12-41 Assembly line program for Problem 7.
Station
1
Reject
lamp
status
Product
Proximity
switch
Inspection zone Gate
Station
2
Station
3
Station
4
Reject
lamp
status
Reject
lamp
status
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268
13
PLC Installation Practices,
Editing, and Troubleshooting
Chapter Objectives
13.1 Outline and describe requirements for a
PLC enclosure
13.2 Identify and describe noise reduction techniques
13.3 Describe proper grounding practices and preventive
maintenance tasks associated with PLC systems
13.4 List and describe specific PLC troubleshooting
procedures
This chapter discusses guidelines for the instal-
lation, maintenance, and troubleshooting of a
PLC-controlled system. The chapter gives you
information on proper grounding that ensures
personal safety as well as correct operation of
equipment. Unique troubleshooting procedures
that apply speciﬁcally to PLCs are listed and
explained.
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PLC Installation Practices, Editing, and Troubleshooting Chapter 13 269
provides adequate room within the enclosure is usually
sufficient for heat dissipation. The temperature inside
the enclosure must not exceed the maximum operating
temperature of the controller (typically 60°C maximum).
Additional cooling provisions, such as a fan or blower,
may be required where high internal or ambient tem-
peratures are encountered. PLCs are always mounted
horizontally with the name of the manufacturer fac-
ing out and right-side up, as illustrated in Figure 13-2.
Vertical mounting is not recommended due to thermal
considerations.
A hardwired electromechanical master control relay
(MCR) is normally included as part of the wiring for a
13.
1 PLC Enclosures
A PLC system, if installed properly, should give years of
trouble-free service. The design of PLCs includes a num-
ber of rugged features that allow them to be installed in
almost any industrial environment. However, problems
can occur if the system is not installed properly.
Programmable logic controllers (PLCs) require protec-
tion against temperature extremes, humidity, dust, shock,
and vibration or corrosive environments. For these rea-
sons, PLCs are generally mounted within a machine or in
a separate enclosure as shown in Figure 13-1.
An enclosure is the chief protection from atmospheric
conditions. The National Electrical Manufacturers Asso-
ciation (NEMA) has defined enclosure types, based on
the degree of protection an enclosure will provide. For
most solid-state control devices, a NEMA 12 enclosure
is recommended. This type of enclosure is for general-
purpose areas and is designed to be dust-tight. Typically,
metal enclosures are used because metal enclosures pro-
vide shielding that helps minimize the effects of electro-
magnetic radiation that may be generated by surrounding
equipment.
Every PLC installation will dissipate heat from its
power supplies, local I/O racks, and processor. This heat
accumulates in the enclosure and must be dissipated
from it into the surrounding air. Excessive heat can
cause erratic operation of the PLC or PLC failure. For
many applications, normal convection cooling will keep
the controller components within the specified tempera-
ture operating range. Proper spacing of components that
Figure 13-2 PLCs are always mounted horizontally.
10.
9.
7.
8.
6.
1.
2. 3.
4.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Power supply
PLC (programmable logic controller)
Digital input cards
Digital output cards
Analog input cards
Transient surge protectors
Circuit breakers
Relay switches
Operator interface terminal
NEMA 12 enclosure
5.
Figure 13-1 Typical PLC control panel enclosure.
Source: Courtesy Aaron Associates.
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270 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting
MCR is connected to interrupt power to the I/O rack in the
event of an emergency, but still allow power to be main-
tained at the processor. Figure 13-3 shows the typical wir-
ing for an AC power distribution with a master control
PLC system. The master control relay provides a means
of de-energizing the entire circuit that is not dependent on
software. The internally programmed MCR of a PLC is
not sufficient to meet safety requirements. The hardwired
Figure 13-3 Typical wiring for an AC power distribution with a master
control relay.
Source: Courtesy Pilz GmbH Co. KG.
To V of DC
input devices
DC power
supply
To V of DC
output modules
To common
of DC input
modules
To common
of DC output
devices
To common (L1)
of 120 v AC
input devices
MCR
Multiple E-stops
Start
L1
Fuse
L1
N
Panel
ground bus
Grounding
electrode
120 V
X1
H3 H2 H4
H1
X2
Neutral
Disconnect switch
3-phase
supply
Step-down
transformer
Gnd
MCR
MCR
To L1
of 120 v AC
output modules
To neutral conn.
of 120 v AC
input modules
To neutral
of 120 v AC
output devices
L1 N
Gnd
DC common
V
Processor power supply
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• Proper routing of wiring
• Proper suppression added to noise-generating devices
Noise suppression is normally needed for inductive
loads such as relays, solenoids, and motor starters when
operated by hard contact devices such as pushbuttons or
selector switches. When inductive loads are switched off,
high transient voltages are generated that if not suppressed
can reach several thousand volts. Figure 13-4 illustrates a
typical noise suppression circuit that is used to suppress
the high voltage spikes generated when a motor starter
coil is de-energized.
Lack of surge suppression on inductive loads may con-
tribute to processor faults and sporadic operation. RAM
can be corrupted (lost), and I/O modules can appear faulty
or can reset themselves. When inductive devices are ener-
gized or de-energized, they can cause an electrical pulse
to be back-fed into the PLC system. The back-fed pulse,
when entering the PLC system, can be mistaken by the
PLC for a computer pulse. It takes only one false pulse
to create a malfunction of the orderly flow of PLC opera-
tional sequences.
Proper routing of field power and signal wiring to the
PLC enclosure as well as inside the enclosure helps to cut
down on electrical noise. The following are some general
guidelines for PLC wire routing:
• Use the shortest possible wire runs for I/O signals.
• When possible, conductors that are run from the
PLC enclosure to another location should be in a
metal conduit as the metal can serve as a shield
against EMI.
• Never run signal wiring and power wiring in the
same conduit.
• Segregate I/O wiring by signal type. Route AC and
DC I/O signal wires in separate wireways.
relay. The operation of the circuit can be summarized as
follows:
• A power disconnect switch is provided so that,
when required, the PLC can be serviced with the
power off.
• The step-down transformer provides isolation
from the main power distribution system and
decreases the voltage to the 120 volts required
for the controller power supplies and DC power
supplies.
• The momentary start button is pressed to energize
the master control relay.
• Pressing any one of the emergency-stop switches
de-energizes the master control relay and thus de-
energizes the I/O devices.
• Power to the processor of the PLC remains on so
status LEDs can continue to provide up-to-date
information.
• Emergency stop buttons use normally closed
contacts wired in series for fail-safe operation.
In the event a wire is broken or comes off a termi-
nal, the MCR relay is de-energized and power is
removed.
13.2 Electrical Noise
Electrical noise, also called electromagnetic interference,
or EMI, is unwanted electrical signals that produce unde-
sirable effects and otherwise disrupt the control system
circuits. EMI may be either radiated or conducted. Radi-
ated noise originates from a source and travels through
the air while conducted noise travels on an actual conduc-
tor, such as a power line.
When the PLC is operated in a noise-polluted indus-
trial environment, special consideration should be given
to possible electrical interference. To increase the operat-
ing noise margin, the controller should be located away
from noise-generating devices such as large AC motors
and high-frequency welders. Malfunctions resulting from
noise are temporary occurrences of operating errors that
can result in hazardous machine operation in certain ap-
plications. Noise usually enters through input, output,
and power supply lines. Noise may be coupled into these
lines by an electrostatic field or through electromagnetic
induction. The following reduce the effect of electrical
interference:
• Manufacturer design features
• Proper mounting of the controller within an
enclosure
• Proper equipment grounding
Figure 13-4 Motor starter noise suppression.
Noise
suppressor
Stop
L1 L2
Starter
coil
Start
M
OL
M
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• Low-level signal conductors such as thermocou-
ples and serial communications should be run as
shielded twisted pair and routed separately.
• A fiber optic system, which is totally immune to all
kinds of electrical interference, can also be used for
signal wiring.
An important part of a PLC installation is clearly iden-
tifying each wire to be connected and the terminal to
which it is connected. A reliable labeling method, such
as the heat-shrinkable wire identification sleeves shown
in Figure 13-5, should be used to label each wire. Wir-
ing connectors for input/output modules usually includes
spaces for labels used for identifying each I/O address and
device connected. Proper wire and terminal identification
will simplify the installation and aid in troubleshooting
and maintenance.
13.3 Leaky Inputs and Outputs
Many electronic devices with transistor or triac outputs
exhibit a small leakage current even when in the off state
that may need to be considered when they are connected
to PLC input modules. This so-called leakage is typi-
cally exhibited by two-wire proximity, photoelectric, and
other such sensors. Often, the leaky input will only cause
the module’s input indicator to flicker. However, a large
enough leakage current can activate the input circuit, cre-
ating a false input signal.
A common solution to the problem of leaky input cur-
rent is to connect a bleeder resistor across or in parallel
with the input, as shown in Figure 13-6. The bleeder re-
sistor acts as an additional lower resistance load, which
allows the leakage current to flow through the lower resis-
tance path. Typically a 10 kΩ to 20 kΩ resistor is used to
solve the problem.
Leakage current may also occur with the solid-state
switch used in many output modules. Problems similar to
that encountered with input modules can be created when
a high-impedance load device is used with these modules.
For example, a PLC output might supply a sound alert
device as illustrated in Figure 13-7. In this case the leak-
age current could cause continuous false or intermittent
operation. A resistor can be connected as shown to bleed
off this current. An isolation relay could also be used to
solve this type of problem.
13.4 Grounding
Proper grounding is an important safety measure in
all electrical installations. The authoritative source on
grounding requirements for a PLC installation is the
National Electrical Code. The NEC specifies the types
Figure 13-5 Heat-shrinkable wire identiﬁcation sleeves.
Figure 13-6 Bleeder resistor connection for input sensors.
Bleeder
resistor
Leakage
current
Input module
N
2-wire
proximity
switch
L1
Common
Figure 13-7 Bleeder resistor connection for a high-
impedance output.
Leakage
current
Bleeder
resistor
Output
module
N
High-
impedance
load
L1
L1
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Ground loops can cause problems by adding or sub-
tracting current or voltage from input signal devices.
A ground loop circuit can develop when each device’s
ground is tied to a different earth potential thereby allow-
ing current to flow between the grounds, as illustrated in
Figure 13-10. If a varying magnetic field passes through
one of these ground loops, a voltage is produced and cur-
rent flows in the loop. The receiving device is unable to
differentiate between the wanted and unwanted signals
and, thus, can’t accurately reflect actual process condi-
tions. Certain connections require shielded cables to help
reduce the effects of electrical noise coupling. Each shield
should be grounded at one end only, as a shield grounded
at both ends forms a ground loop.
of conductors, color codes, and connections neces-
sary for safe grounding of electrical components. In
addition, most manufacturers provide detailed infor-
mation on the proper grounding methods to use in an
enclosure.
Figure 13-8 illustrates a PLC grounding system. A
properly installed grounding system will provide a low-
impedance path to earth ground. The complete PLC in-
stallation, including enclosures, CPU and I/O chassis,
and power supplies are all connected to a single low-
impedance ground. These connections should exhibit low
DC resistance and low high-frequency impedance. A cen-
tral ground bus bar is provided as a single point of refer-
ence inside the enclosure to which all chassis and power
supply equipment grounding conductors are connected.
The ground bus is then connected to the building’s earth
ground.
In the event of a high value of ground current, the
temperature of the conductor could cause the solder to
melt, resulting in interruption of the ground connec-
tion. Therefore the grounding path must be permanent
(no solder), continuous, and able to conduct safely the
ground-fault current in the system with minimal imped-
ance. Paint or other nonconductive material should be
scraped away from the area where a chassis makes con-
tact with the enclosure. The minimum ground wire size
should be No. 12 AWG stranded copper for PLC equip-
ment grounds and No. 8 AWG stranded copper for en-
closure backplane grounds. Ground connections should
be made with a star washer between the grounding wire
and lug and metal enclosure surface, as illustrated in
Figure 13-9.
Figure 13-8 PLC grounding system.
CPU
or I/O
rack
CPU
or I/O
rack
Equipment
grounding
conductor
Equipment
grounding
conductor
Ground bus
Ground for slot
power supply
To grounding
electrode
system
Grounding
electrode
system
CPU
or I/O
rack
CPU
or I/O
rack
Enclosure
Figure 13-9 Make ground connections using a star
washer.
Star washer
To ground bus
Chassis
mounting tab
Equipment grounding
conductor (ground lug
with (8 AWG) wire)
Figure 13-10 Formation of ground loops.
Source
No ground loops
Source
Ground bus
Source
Ground
loop
formed
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loads. The operation of the circuit can be summarized
as follows:
• The diode is connected in reverse-bias across the
solenoid load.
• In normal operation, the electric current can’t
flow through the diode, so it flows through the
solenoid coil.
• When voltage to the solenoid is switched off a
voltage opposite in polarity to the original applied
voltage is generated by the collapsing magnetic
field.
• The induced voltage creates a current flow
through the diode bleeding off the high
voltage spike.
Figure 13-12 illustrates how an RC (resistor/capacitor)
snubber circuit is connected for suppressing AC load de-
vices. The operation of the circuit can be summarized as
follows:
• The voltage peak, which occurs at the instant the
current path to the coil is opened, is safely short-
circuited by the RC network.
• The resistor and capacitor connected in series slows
the rate of rise of the transient voltage.
• The voltage across the capacitor cannot change
instantaneously, so a decreasing transient current
will flow through it for a small fraction of a second,
allowing the voltage to increase more slowly when
the circuit is opened.
The metal oxide varistor (MOV) surge suppressor,
shown in Figure 13-13, is the most popular surge protec-
tion device. It functions in a manner similar to two zener
13.5 Voltage Variations and Surges
The power supply section of the PLC system is built to
sustain line fluctuations and still allow the system to func-
tion within its operating range. If voltage fluctuations ex-
ceed this range, then a system shutdown will occur. In
areas where excessive line voltage variation or extended
brownouts are anticipated, installing a constant voltage
(CV) transformer may be required to minimize nuisance
shutdowns of the PLC.
Isolation transformers are used in some PLC systems
to isolate the PLC from electrical disturbances generated
by other equipment connected to the distribution system.
Although the PLC is designed to operate in harsh envi-
ronments, other equipment may generate considerable
amounts of interference that may result in intermittent
disturbances in normal operation. A normal practice is
to place the PLC power supply and I/O devices on a
separate transformer that may also serve as a step-down
transformer to reduce the incoming voltage to the de-
sired level.
When current in an inductive load is interrupted or turned
off, a very high voltage spike is generated. This high volt-
age can be reduced or eliminated through suppression
techniques which absorb the inductive induced voltage.
Generally, output modules designed to drive inductive loads
include suppression networks built in as part of the module
circuit.
An additional external suppression device is recom-
mended if an output module is used to control devices
such as relays, solenoids, motor starters, or motors. The
suppression device is wired in parallel (directly across)
and as close as possible to the load device. The sup-
pression components must be rated appropriately to
suppress the switching transient characteristic of the
particular inductive device. Figure 13-11 illustrates
how a diode is connected to suppress DC inductive
Figure 13-11 Diode connected to suppress DC inductive
loads.
()
L1() L2()
Reversed-bias
diode
Solenoid coil
Output module
Figure 13-12 RC snubber circuit connected to suppress
AC loads.
Output
module
C R
L2
L1
PB
M
L1
L2
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PLC, the ladder logic program, the I/O devices, and all
associated wiring operate according to specifications. Be-
fore commissioning any control system, you should have
a good understanding of how the control system operates
and how the various components interact. The following
are general steps to be followed when commissioning a
PLC system:
• Before applying power to the PLC or the input
devices, disconnect or otherwise isolate any output
device that could potentially cause damage or injury.
Typically this precaution would pertain to outputs that
cause movement such as starting a motor or operating
a valve.
• Apply power to the PLC and input devices. Mea-
sure the voltage to verify that rated voltage is being
applied.
• Examine the PLC’s status indicator lights. If power
is properly applied, the power indicator should be
on, and there should be no fault indication. If the
PLC does not power up properly, it may be faulty.
PLCs rarely fail, but if they do fail, it usually hap-
pens immediately upon powering up.
• Verify that you have communication with the PLC
via the programming device that is running the PLC
programming software.
• Place the PLC in a mode that prevents it from ener-
gizing its output circuits. Depending on the make of
the PLC, this mode may be called disable, continu-
ous test, or single-scan mode. This mode will allow
you to monitor input devices, execute the program,
and update the output image file while keeping the
output circuits de-energized.
• Manually activate each input device, one at a
time, to verify that the PLC’s input status lights
turn on and off as expected. Monitor the associ-
ated condition instruction to verify that the input
device corresponds to the correct program address
and that the instruction turns true or false as
expected.
• Manually test each output. One way you can do this
is by applying power to the terminal where the out-
put device is wired. This test will check the output
field device and its associated wiring.
• After verifying all inputs, outputs, and program ad-
dresses, verify all preset values for counters, timers,
and so on.
• Reconnect any output devices that may have been
disconnected and place the PLC in the run mode.
Test the operation of all emergency stop buttons and
the total system operation.
diodes connected back-to-back. The operation of a MOV
• The device acts as an open circuit until the volt-
age across it in either direction exceeds its rated
value.
• Any greater voltage peak instantly makes the device
act like a short circuit that bypasses this voltage
away from the rest of the circuit.
13.6 Program Editing
and Commissioning
After you have entered the rungs for your program, you
may need to modify them. Editing is simply the ability to
make changes to an existing program through a variety of
editing functions. Using the editing function, instructions
and rungs can be added or deleted; addresses, data, and
bits can be changed. Again, the editing format varies with
different manufacturers and PLC models.
Today, most PLC programming software is Microsoft
Windows based, so if you are familiar with Windows and
know how to point and click with a mouse, you should
have no problem editing a program. In general, both in-
structions and rungs are selected simply by clicking on
them with the left mouse button. Double clicking with the
left mouse button allows you to edit an instruction’s ad-
dress, whereas right clicking displays a pop-up menu of
related editing commands. If you want to include addi-
tional explanation of a symbol or address, you can place
an address description on your ladder rung directly above
the symbol. To add a page or rung comment, right click
on the rung number to which you wish to add the page or
rung comment.
Preparing a control process for start-up, also called
commissioning, involves a series of tests to ensure that the
AC or DC
output module
VAC/VDC
OUT 0
OUT 1
OUT 2
OUT 3
OUT 4
OUT 5
OUT 6
OUT 7
COM
Inductive load
DC Com or L2
MOV
DC or L1
Figure 13-13 Metal oxide varistor (MOV) surge
suppressor.
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13.7 Programming and Monitoring
When you program a PLC, several instruction entry
modes are available, depending on the manufacturer and
the model of the unit. A personal computer, with appro-
priate software, is generally used to program and monitor
the program in the PLC. Additionally, it makes possible
offline programming, which involves writing and storing
the program in the personal computer without its being
connected to the PLC and later downloading it to the
PLC. Figure 13-14 illustrates how programs are down-
loaded and uploaded from and to the computer.
With online programming the program can be modi-
fied, the modifications can be tested, and finally they can
be accepted or rejected while the PLC is running. How-
ever, offline programming is the safest manner in which to
edit a program because additions, changes, and deletions
do not affect the operation of the system until downloaded
to the PLC.
Many manufacturers provide a continuous test mode
that causes the processor to operate from the user pro-
gram without energizing any outputs. This mode allows
the control program to be executed and debugged while
the outputs are disabled. A check of each rung can be
done by monitoring the corresponding output rung on the
programming device. A single-scan test mode may also
be available for debugging the control logic. This mode
causes the processor to complete a single scan of the user
program each time the single-scan key is pressed with no
outputs being energized.
An online programming mode permits the user to
change the program during machine operation. As the
PLC controls its equipment or process, the user can add,
change, or delete control instructions and data values as
desired. Any modification made is executed immediately
on entry of the instruction. Therefore, the user should
assess in advance all possible sequences of machine
operation that will result from the change. Online pro-
gramming should be done only by experienced person-
nel who understand fully the operation of the PLC they
are dealing with and the machinery being controlled. If
at all possible, changes should be made offline to pro-
vide a safe transition from existing programming to new
programming.
Two useful monitoring tools provided with PLC pro-
gramming packages are data monitor and cross reference.
Data monitoring functions allow you to monitor and/or
modify specified program variables. The cross reference
function allows you to search each instance of a particular
address.
The data monitor feature allows you to display data
from any place in the data table. Depending on the
PLC, the data monitor function can be used to do the
following:
• View data within an instruction
• Store data or values for an instruction prior to use
• Set or reset values and/or bits during a debug opera-
tion for control purposes
• Change the radix or data format
Figure 13-14 Downloading and uploading PLC program.
(a) Downloading a program (b) Uploading a program
Program from the computer
is loaded into the PLC.
Program that was in the
PLC is lost.
Program from the PLC
is loaded into the computer.
Program that was in the
PLC remains unchanged.
Unsaved program in the
computer is lost.
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The status of the bit(s) (on or off) and the length of time the
bit(s) remained on or off (in hours, minutes, seconds, and
hundredths of a second) are displayed. In a contact histo-
gram file, the accumulated time indicates the total time that
the histogram function was running. The delta time of the
contact histogram indicates the elapsed time between the
changes in states. Contact histograms are extremely useful
Figure 13-15 shows the data file folder and window
for the Allen-Bradley SLC 500 PLC and its associated
RSLogix software. The data file folder allows the user
to determine the status of I/O files as well as the sta-
tus file (S2), binary file (B3), timer file (T4), counter
file (C5), control file (R6), integer file (N7), and the
floating-point file (F8). Always be careful when manip-
ulating data using the data monitor function. Changing
data could affect the program and turn output devices
on or off.
When troubleshooting a PLC, it may be necessary to
locate each instance of a particular address in the ladder
program. The cross reference function searches all pro-
gram files to locate each instance of the selected address.
A user can then trace the operation by finding all the
places where a particular output coil or contact with the
same address is used in the program. Figure 13-16 shows
a sample cross reference report for the Allen-Bradley
SLC 500 PLC and its associated RSLogix software. Its
contents can be summarized as follows:
• The report contains all the addresses used in the
program.
• Addresses are displayed in the same order as the
data table files.
• The address that the search was performed for
(O:2/1) is highlighted.
• The description for each address is displayed.
• Listing includes the instruction type, program file,
and rung number for each address.
• Each occurrence of the address is displayed, starting
with program file 2 and rung 0.
The contact histogram function allows you to view the
transition history (the on and off states) of a data table value.
Figure 13-15 Data ﬁle folder and window.
Data file folders
Data Files
Cross Reference
O:0-OUTPUT
I:1-INPUT
S2-STATUS
B3-BINARY
T4-TIMER
C5-COUNTER
R6-CONTROL
N7-INTEGER
F8-FLOAT
Data file window
Input Table
15
0
0
14
0
0
11
0
0
10
0
0
9
0
0
6
0
0
5
0
0
3
0
0
13
0
0
8
0
0
7
0
0
4
0
1
2
0
0
1
0
0
12
0
0
0
0
0
I:1.0
I:2.0
Radix: Binary Table:
Address
O:0:Output
I:1:Input
S2: Status
B3: Binary
T4: Timer
C5: Counter
R6: Control
N7: Integer
F8: Float
Figure 13-16 Sample cross reference report.
Stop PB
I:1/1
Start PB Motor control relay
I:1/2 O:2/1
O:2/1
Motor control relay
O:2/1
Motor control relay
O:2/3
Run pilot light
Cross Reference Report - Sorted by Address
0:2/1 – Motor Control Realy
0:2/3 – Run Pilot Light
I:1/1 – Stop PB
I:1/2 – Start PB
OTE – File #2 MAIN_PROG – 0
OTE – File #2 MAIN_PROG – 1
XIC – File #2 MAIN_PROG – 0, 1
XIC – File #2 MAIN_PROG – 0
XIC – File #2 MAIN_PROG – 0
Sort By Symbol Refresh Help
1
0
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for detecting intermittent problems, either hardware- or
logic-related. By tracking the status and time between sta-
tus changes, you can detect different types of problems.
13.8 Preventive Maintenance
The biggest deterrent to PLC system faults is a proper pre-
ventive maintenance program. Although PLCs have been
designed to minimize maintenance and provide trouble-
free operation, there are several preventive measures that
should be looked at regularly.
Many control systems operate processes that must be
shut down for short periods for product changes. The fol-
lowing preventive maintenance tasks should be carried
out during these short shutdown periods:
• Any filters that have been installed in enclosures
should be cleaned or replaced to ensure that clear
air circulation is present inside the enclosure.
• Dust or dirt accumulated on PLC circuit boards
should be cleaned. If dust is allowed to build up on
heat sinks and electronic circuitry, an obstruction of
heat dissipation could occur and cause circuit mal-
function. Furthermore, if conductive dust reaches
the electronic boards, a short circuit could result
and cause permanent damage to the circuit board.
Ensuring that the enclosure door is kept closed will
prevent the rapid buildup of these contaminants.
• Connections to the I/O modules should be checked
for tightness to ensure that all plugs, sockets, termi-
nal strips, and module connections are making con-
nections and that the module is installed securely.
Loose connections may result not only in improper
function of the controller but also in damage to the
components of the system.
• All field I/O devices should be inspected to en-
sure that they are adjusted properly. Circuit boards
dealing with process control analogs should be
calibrated every 6 months. Other devices, such as
sensors, should be serviced on a monthly basis.
Field devices in the environment, which have to
translate mechanical signals into electrical, may
gum up, get dirty, crack, or break—and then they
will no longer trip at the correct setting.
• Care should be taken to ensure that heavy noise- or
heat-generating equipment is not moved too close to
the PLC.
• Check the condition of the battery that backs up the
RAM memory in the CPU (Figure 13-17). Most
CPUs have a status indicator that shows whether the
battery’s voltage is sufficient to back up the memory
stored in the PLC. If a battery module is to be
replaced, it must be replaced with exactly the same
type of battery module.
• Stock commonly needed spare parts. Input and output
modules are the PLC components that fail most often.
• Keep a master copy of operating programs used.
To avoid injury to personnel and to prevent equip-
ment damage, connections should always be checked
with power removed from the system. In addition to dis-
connecting electrical power, all other sources of power
(pneumatic and hydraulic) should be de-energized before
someone works on a machine or process controlled by a
PLC. Most companies use lockout and tagout procedures,
shown in Figure 13-18, to make sure that equipment does
not operate while maintenance and repairs are conducted.
A personnel protection tag is placed on the power source
for the equipment and the PLC, and it can be removed
only by the person who originally placed the tag. In ad-
dition to the tag, a lock is also attached so that equipment
cannot be energized.
Figure 13-17 CPU backup memory battery.
Bat
+
–
Bat
+
–
Figure 13-18 Lockout/tagout devices.
Source: Photo courtesy Panduit Corporation, www.panduit.com.
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13.9 Troubleshooting
In the event of a PLC fault, you should employ a care-
ful and systematic approach to troubleshoot the system to
resolve the problem. PLCs are relatively easy to trouble-
shoot because the control program can be displayed on
a monitor and watched in real time as it executes. If a
control system has been operating, you can be fairly con-
fident of the accuracy of the program logic. For a system
that has never worked or is just being commissioned, pro-
gramming errors should be considered.
When a problem occurs, the first step in the trouble-
shooting procedure is to identify the problem and its
source. The source of a problem can generally be nar-
rowed down to the processor module, I/O hardware, wir-
ing, machine inputs or outputs, or ladder logic program.
Once a problem is recognized, it is usually quite simple to
deal with. The following sections will deal with trouble-
shooting these potential problem areas.
Processor Module
The processor is responsible for the self-detection of po-
tential problems. It performs error checks during its op-
eration and sends status information to indicators that are
normally located on the front of the processor module.
You can diagnose processor faults or obtain more detailed
information about the processor by accessing the proces-
sor status through programming software. Figure 13-19
shows sample diagnostics LEDs found on a processor
module. What they indicate can be summarized as follows:
RUN (Green)
• On steady indicates that the process is in the RUN
mode.
• Flashing during operation indicates that the process
is transferring a program from RAM to the memory
module.
• Off indicates that processor is in a mode other than
RUN.
FLT (Red)
• Flashing at power-up indicates that the processor
has not been configured.
• Flashing during operation indicates a major error
either in the processor, chassis, or memory.
• On steady indicates that a fatal error is present (no
communications).
• Off indicates there are no errors.
BATT (Red)
• On steady indicates the battery voltage has fallen
below a threshold level, or the battery is missing or
not connected.
• Off indicates that the battery is functional.
The processor then monitors itself continually for any
problems that might cause the controller to execute the
user program improperly. Depending on the controller, a
set of fault relay contacts may be available. The fault relay
is controlled by the processor and is activated when one or
more specific fault conditions occur. The fault relay con-
tacts are used to disable the outputs and signal a failure.
Most PLCs incorporate a watchdog timer to moni-
tor the scan process of the system. The watchdog timer
is usually a separate timing circuit that must be set and
reset by the processor within a predetermined period. The
watchdog timer circuit monitors how long it takes the
CPU to complete a scan. If the CPU scan takes too long, a
watchdog major error will be declared. PLC user manuals
will show how to apply this function.
The PLC processor hardware is not likely to fail because
today’s microprocessors and microcomputer hardware are
very reliable when operated within the stated limits of tem-
perature, moisture, and so on. The PLC processor chassis
is typically designed to withstand harsh environments.
Input Malfunctions
If the controller is operating in the RUN mode but output
devices do not operate as programmed, the faults could be
associated with any of the following:
• Input and output wiring between field devices and
modules
• Field device or module power supplies
• Input sensing devices
• Output actuators
• PLC I/O modules
• PLC processor
Narrowing down the problem source can usually be ac-
complished by comparing the actual status of the suspect
I/O with controller status indicators. Usually each input
or output device has at least two status indicators. One of
these indicators is on the I/O module; the other indicator
is provided by the programming device monitor.
Figure 13-19 Processor diagnostics LEDs.
RUN
FLT
BATT
CPU
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The circuit of Figure 13-20 illustrates how to check for
discrete input malfunctions. The steps taken can be sum-
marized as follows:
• When input hardware is suspected to be the source
of a problem, the first check is to see if the status
indicator on the input module illuminates when it is
receiving power from its corresponding input device
(e.g., pushbutton, limit switch).
• If the status indicator on the input module does not
illuminate when the input device is on, take a volt-
age measurement across the input terminal to check
for the proper voltage level.
• If the voltage level is correct, then the input module
should be replaced.
• If the voltage level is not correct, power supply, wir-
ing, or input device may be faulty.
If the programming device monitor does not show the
correct status indication for a condition instruction, the
input module may not be converting the input signal prop-
erly to the logic level voltage required by the processor
module. In this case, the input module should be replaced.
If a replacement module does not eliminate the problem
and wiring is assumed to be correct, then the I/O rack,
communication cable, or processor should be suspected.
Figure 13-21 shows a typical input device troubleshooting
guide. This guide reviews condition instructions and how
their true/false status relates to external input devices.
Figure 13-20 Checking for input malfunctions.
IN 1
IN 2
IN 3
IN 4
IN 5
IN 6
IN 7
DC COM
Input module

LED indicator
not illuminated
Input pushbutton
contacts closed
24 V DC
Common
Check input
voltage level
?
IN 0
DC COM
Figure 13-21 Input troubleshooting guide.
Input device troubleshooting guide
None - correct indications
Input module
status indicator
Input
device condition
ON
ON
OFF
ON
OFF
Monitor display
status indicator
Possible
fault(s)
None - correct indications
Input voltage, status indicator, and ladder
instructions agree but not with sensor condition.
Short circuit in the field device or wiring.
Sensor condition, input voltage, status indicator
are correct. Ladder instructions have incorrect
indications. Input module or processor fault.
Status indicator and instructions agree but not
with the sensor condition. Open field device
or wiring.
Sensor condition, input voltage, status indicator
are correct. Ladder instructions have incorrect
indications. Input module or processor fault.
True
True
True
False
False
False
False
False
True
True
True
False
Closed — ON
24 V DC input
Closed — ON
24 V DC input
Closed — ON
0 V DC input
OFF
Open — OFF
0 V DC input
Open — OFF
0 V DC input
Open — OFF
24 V DC input
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Output Malfunctions
In addition to the logic indicator, some output modules
incorporate either a blown fuse indicator or a power indi-
cator or both. A blown fuse indicator indicates the status
of the protective fuse in the output circuit, while a power
indicator shows that power is being applied to the load.
Electronic protection, as shown in Figure 13-22, is also
used to provide protection for the modules from short-
circuit and overload current conditions. The protection
is based on a thermal cut-out principle. In the event of
a short-circuit or overload current condition on an out-
put channel, that channel will limit current within mil-
liseconds after its thermal cut-out temperature has been
reached. All other channels continue to operate as di-
rected by the processor.
When an output does not energize as expected, first
check the output module blown fuse indicator. Many out-
put modules have each output fused. This indicator will
normally illuminate only when the output circuit corre-
sponding to the blown fuse is energized. If this indicator
is illuminated, correct the cause of the malfunction and
replace the blown fuse in the module.
Figure 13-23 shows a typical discrete output module
troubleshooting guide. In general, the following items
should be noted when troubleshooting discrete output
modules:
• If the blown fuse indicator is not illuminated (fuse
OK), then check to see if the output device is re-
sponding to the LED status indicator.
• An output module’s logic status indicator func-
tions similarly to an input module’s status indica-
tor. When it is on, the status LED indicates that the
module’s logic circuitry has recognized a command
from the processor to turn on.
• If an output rung is energized, the module status
indicator is on, and the output device is not respond-
ing, then the wiring to the output device or the out-
put device itself should be suspected.
• If, according to the programming device monitor,
an output device is commanded to turn on but the
status indicator is off, then the output module or
processors may be at fault.
• Check voltage at output; if incorrect, power supply,
wiring, or output device may be faulty.
Ladder Logic Program
Many PLC software programs offer various software
checks used to verify program logic. Figure 13-24 shows
a sample of verifying program errors using RSLogix 500
software. Selecting edit then verify project will check
the program for errors. The sample shows what the error
message might look like.
The ladder logic program itself is not likely to fail, as-
suming that the program was at one time working cor-
rectly. A hardware fault in the memory IC that holds the
ladder logic program could alter the program, but this is
a PLC hardware failure. If all other possible sources of
trouble have been eliminated, the ladder logic program
should be reloaded into the PLC from the master copy of
the program. Make sure the master copy of the program is
up to date before you download it to the PLC.
Start program troubleshooting by identifying which
outputs operate properly and which outputs do not. Then
trace back from the output on the nonfunctioning rung and
examine the logic to determine what may be preventing
the output from energizing. Common logic errors include:
• Programming an examine if closed instruction
instead of an examine if open (or vice versa)
• Using an incorrect address in the program
Although the ladder logic program is not likely to fail,
the process may be in a state that was unaccounted for in
the original program and thus is not controlled properly.
In this case, the program needs to be modified to include
this new state. A careful examination of the description of
the control system and the ladder logic program can help
identify this type of fault.
The force on and force off instructions allow you to turn
specific bits on or off for testing purposes. Figure 13-25
illustrates how forces are identified as being enabled or dis-
abled in RSLogix 500 software. Forcing lets you simulate
operation or control an output device. For example, forcing
a solenoid valve on will tell you immediately whether the
solenoid is functional when the program is bypassed. If it
is, the problem must be related to the software and not the
hardware. If the output fails to respond when forced, either
the actual output module is causing the problem or the so-
lenoid itself is malfunctioning. Take all necessary precau-
tions to protect personnel and equipment during forcing.
Figure 13-22 Electronic output module protection.
E
F
U
S
E
Status
indicators
Output
Electronic
protection
LED
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Certain diagnostic instructions may be included as part
of a PLC’s instruction set for troubleshooting purposes.
The temporary end (TND) instruction, shown in Fig-
ure 13-26, is used when you want to change the amount of
logic scanned to progressively debug your program. The
Output device troubleshooting guide
None - correct indication
Output module
status indicator
Output device
condition
De-energized — OFF
Energized — ON
ON
OFF
ON
OFF
Monitor display
status indicator
Fault(s)
None - correct indication
Output instruction and status indicator
agree but the field device does not.
Open field device or wiring.
Module circuit or fuse.
Field device status and status indicator
agree but the output condition does not.
Module circuit or fuse.
Output module
status indicator
ON
VAC
OUT 0
OUT 1
OUT 2
OUT 3
OUT 4
OUT 5
OUT 6
OUT 7
AC COM
L1
L2
Open in field device
Open in wiring
Output module
False
True
True
True
Figure 13-23 Output troubleshooting guide.
Source: Photo courtesy Guardian Electric, www.guardian-electric.com.
Figure 13-25 Indication of enabled forces.
Figure 13-24 Sample of verifying program errors.
Errors
Program Files
File 2
Ring 1 Ins 3: ERROR: Undefined I/O address used
–
–
–
Verify Results
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operation of this output instruction can be summarized as
follows:
• The instruction operates only when its rung condi-
tions are true and stops the processor from scanning
any logic beyond the TND instruction.
• When the processor encounters a true TND rung,
it resets the watchdog timer (to 0), performs an I/O
update, and begins running the ladder program at
the first instruction in the main program.
• If the TND rung is false, the processor continues
the scan until the next TND instruction or the END
statement.
• By inserting the TND instruction at different loca-
tions in the program you can test parts of the program
sequentially until the entire program has been tested.
• Once the troubleshooting process has been com-
pleted, any remaining TND instructions are re-
moved from the program.
The suspend (SUS) instruction, shown in Figure 13-27,
is used to trap and identify specific conditions for program
debugging and system troubleshooting. The operation of
this output instruction can be summarized as follows:
• When the rung is true, this instruction places the
controller in the suspend or idle mode.
• The suspend ID, in this case 100, must be selected
by the programmer and entered in the instruction.
• When the SUS instruction executes, the ID number
100 is written in word 7 (S:7) of the status file.
• If multiple suspend instructions are present, then
this will indicate which SUS instruction was active.
• The suspend file (program or subroutine number
identifying where the executed SUS instruction re-
sides) is placed in word 8 (S:8) of the status file.
• All ladder logic outputs are de-energized, but other
status files have the data present when the suspend
instruction was executed.
Most PLC system faults occur in the field wiring and
devices. The wiring between the field devices and the ter-
minals of the I/O modules is a likely place for problems
to occur. Faulty wiring and mechanical connection prob-
lems can interrupt or short the signals sent to and from
the I/O modules.
The sensors and actuators connected to the I/O of the
process can also fail. Mechanical switches can wear out
or be damaged during normal operation. Motors, heaters,
lights, and sensors can also fail. Input and output field
devices must be compatible with the I/O module to ensure
proper operation.
When an instruction does not seem to be working cor-
rectly, the problem may be an addressing conflict caused
by the same address being used for two or more coil in-
structions in the same program. As a result, multiple rung
conditions can control the same output coil, making trou-
bleshooting more difficult. In the case of duplicate out-
puts, the monitored rung may be true; but if a rung farther
down in the ladder diagram is false, the PLC will keep the
output off. The program of Figure 13-28 illustrates what
Figure 13-26 TND (temporary end) diagnostic instruction.
Program Control Ascii Control Ascii String
MCR SUS
Micro
Main program
Temporary end
L1
Inputs
SW
SW
TND
Reminder of main program
Figure 13-27 SUS (suspend) diagnostic instruction.
Program Control Ascii Control Ascii String
MCR
Micro
A B C
SUS
Suspend
Suspend ID 100
L1
Inputs Trapped input conditions
Main program
A
B
C
Figure 13-28 Program with the same address used for
two coils.
Output
Same address
Inputs
L2
PL
(OFF)
L1
O:2/1
O:2/1
I:1/1
I:1/1
I:1/2
I:1/2
O:2/1
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happens when the same address is used for two coils. The
resulting problem scenario can be summarized as follows:
• The problem is turning input switch I:1/1 on will
not turn on PL output O:2/1 as it appears to be
programmed.
• The root of the problem lies in the fact that the
PLC scans the program from left to right and top to
bottom.
• Whenever input switch I:1/1 is true (closed) and
input switch I:1/2 is false (open) output O:2/1 will
be off.
• This is because when the PLC updates the outputs it
does so based on the status of input I:1/2.
• Regardless of whether input I:1/1 is open or closed
the output reacts only to the status of input switch
I:1/2.
When a problem occurs, the best way to proceed is to
try to logically identify the devices or connections that
could be causing the problem rather than arbitrarily check-
ing every connection, switch, motor, sensor, I/O module,
and so on. First, observe the system in operation and try
to describe the problem. Using these observations and the
description of the control system, you should identify the
possible sources of trouble. Compare the logic status of
the hardwired inputs and outputs to their actual state, as
illustrated in Figure 13-29. Any disagreements indicate
malfunctions as well as their approximate location.
Some of your troubleshooting can be accomplished by
interpreting the status indicators on the I/O modules. The
key is to know whether the status indicators are telling
you that there is a fault or that the system is normal. Often
PLC manufacturers supply a troubleshooting guide, map,
or tree that presents a list of observed problems and their
possible sources. Figure 13-30 shows a sample trouble-
shooting tree for a discrete output module. Figures 13-31
and 13-32 are samples of input and output troubleshoot-
ing guides.
Figure 13-29 General methods of troubleshooting.
Logic observation—determine validity of decisions made by processor
User program
True True
Hardware comparison test—compare
state of Ι/O as stored in memory to
actual state
L1
L2
Input
device
Input
module
Open-off
Status
off L1
L2
Output
module
Status
On
Load
energized
on
Memory
Input image table
Output image table
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OFF
Output
status
indicators
Check Ι/O
device wiring
• Repair
• Replace
• Modules
• Swing arm
• Power
• Rack
Find out why
Yes
Output module
OFF
Blown fuse
indicator
Display
rung on
screen
No
Yes Is output
instruction
true?
Check Ι/O
Output device
would not
ON
Output
status
indicators
Display
rung on
screen
Yes
Is output
instruction
true?
ON
ON
OFF
Turn OFF Turn ON
No
Output
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
VAC
Out 1
Out 3
Out 5
Out 7
Out 9
Out 11
Out 13
Out 15
Out 0
Out 2
Out 4
Out 6
Out 8
Out 10
Out 12
Out 14
AC
COM
L1
L2
R
G
O:2/3
O:2/8
Figure 13-30 Troubleshooting tree for a discrete output module.
If Your Input
Circuit LED Is . . .
And Your Input
Device Is . . . And Probable Cause
ON
Input
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
On/Closed/Activated Your input device will not
turn off.
Device is shorted or damaged.
Your program operates
as though it is off.
Input circuit wiring or module.
Input is forced off in program.
Off/Open/Deactivated Your program operates
as though it is on and/or
the input circuit will not
turn off.
Input device off-state leakage current exceeds
input circuit specification.
Input device is shorted or damaged.
OFF
Input
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
On/Closed/Activated
as though it is off and/or
the input circuit will not
turn on.
Input circuit is incompatible.
Low voltage across the input.
Input signal turn-on time too fast for input circuit.
Off/Open/Deactivated Your input device will not
turn on.
Input device is shorted or damaged.
as though it is on.
Input is forced on in program.
Figure 13-31 Input troubleshooting guide.
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13.
10 PLC Programming Software
You must establish a way for your personal computer
(PC) software to communicate with the programmable
logic controller (PLC) processor. Making this connection
is known as configuring the communications. The method
used to configure the communications varies with each
brand of controller. In Allen-Bradley controllers, RSLogix
software is required to develop and edit ladder programs.
A second software package, RSLinx, is needed to monitor
PLC activity, download a program from your PC to your
PLC, and upload a program from your PLC into your
PC. You cannot download multiple projects to the PLC
and then run them when required. The PLC will accept
only one program at a time, but the program can consist
of multiple subroutine files which can be conditionally
called from the main program.
RSLinx software is available in multiple packages to
meet the demand for a variety of cost and functionality
requirements. This software package is used as the driver
between your PC and PLC processor. A driver is a com-
puter program that controls a device. For example, you
must have the correct printer driver installed in your PC
Figure 13-32 Output troubleshooting guide.
If Your Output
Circuit LED Is . . .
And Your Output
Device Is . . . And Probable Cause
ON
Output
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
On/Energized
Your program indicates
that the output circuit is
off or the output circuit
will not turn off.
Programming problem:
- Check for duplicate outputs and addresses.
- If using subroutines, outputs are left in their last
state when not executing subroutines.
- Use the force function to force output off. If
this does not force the output off, output circuit
is damaged. If the output does force off, then
check again for logic/programming problem.
Output is forced on in program.
Output circuit wiring or module.
Off/De-energized
Your output device will
not turn on and the
program indicates that
it is on.
Low or no voltage across the load.
Output device is incompatible: check
specifications and sink/source compatibility
(if dc output).
OFF
Output
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
On/Energized
Your output device will
not turn off and the
program indicates that
it is off.
Output device is incompatible.
Output circuit off-state leakage current may
exceed output device specification.
Output device is shorted or damaged.
Off/De-energized
Your program indicates
that the output circuit is
on or the output circuit
will not turn on.
Programming problem:
- Check for duplicate outputs and addresses.
- If using subroutines, outputs are left in their last
state when not executing subroutines.
- Use the force function to force output on. If
this does not force the output on, output circuit
is damaged. If the output does force on, then
check again for logic/programming problem.
Output is forced off in program.
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Figure 13-33 Direct PC-to-PLC software connection.
PLC
Personal computer
(PC)
Serial cable
RSLogix
software
RSLinx
software
COM port
Processor
in order to be able to print a word-processing document
created on your PC. RSLinx works much like the printer
driver for RSLogix software. The RSLinx program must
be opened and drivers configured before communications
can be established between a PC and a PLC that is using
RSLogix software.
RSLinx allows RSLogix to communicate through an
interface cable to the PLC processor. The simplest con-
nection between a PC and a PLC is a point-to-point direct
connection through the computer serial port, as illustrated
in Figure 13-33. A serial cable is used to connect to your
PC’s COM 1 or COM 2 port and to the PLC processor’s
serial communications port. With RSLinx software you
can auto-configure the serial connection and thus auto-
matically find the proper serial port configuration.
Two important aspects of the communication link
must be considered, namely, the RS-232 standard and the
communications protocol. The RS-232 standard speci-
fies a function for each of the wires inside the standard
Figure 13-34 Serial wiring connection.
FORCE
SLC 5/04 CPU
RUN
DH
FLT
R3232
BATT
PROG
RUN REM
Send 2
Receive 3
3 Receive
Ground (common) 7
7
Send
2 Send
communications cable and their associated pins. Commu-
nications protocol is a standardized method for transmit-
ting data and/or establishing communications between
different devices.
Minimum configuration for two-way communications
requires the use of only three connected wires, as shown
in Figure 13-34. For ease of connection, the RS-232
standard specifies that computer devices have male con-
nectors and that peripheral equipment have female con-
nectors. Direct communication between two computers,
such as a PC and a PLC, does not involve intermediate pe-
ripheral equipment. Therefore, a serial null-modem type
cable must be used for the connection because both the
PC and the PLC processor use pin 2 for data output and
pin 3 for data input.
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1. Why are PLCs installed within an enclosure?
2. What methods are used to keep enclosure
temperatures within allowable limits?
3. State two ways in which electrical noise may be
coupled into a PLC control system.
4. List three potential noise-generating inductive
devices.
5. Describe four ways in which careful wire routing
can help cut down on electrical noise.
6. a. What type of input field devices and output
modules are most likely to have a small
leakage current flow when they are in the off
state? Why?
b. Explain how an input bleeder resistor reduces
leakage current.
7. Summarize the basic grounding requirements for a
PLC system.
8. Under what condition can a ground loop circuit be
developed?
9. When line voltage variations to the PLC power
supply are excessive, what can be done to solve the
problem?
10. What operating state will cause an inductive load to
generate a very high voltage spike?
11. Explain how a diode is connected to suppress a DC
inductive load.
12. Explain how a MOV suppresses an AC inductive
load.
13. What is the purpose of PLC editing functions?
14. What is involved with commissioning a PLC
system?
15. a. Compare offline and online programming.
b. Which method is safer? Why?
16. List four uses for the data monitor function.
17. What information is provided by the cross refer-
ence function?
18. What information is provided by the contact
histogram function?
19. List five preventive maintenance tasks that should
be carried out on the PLC installation regularly.
20. Outline the general procedure followed to lock out
and tag a PLC installation.
21. Typically, what does each of the following proces-
sor diagnostic light states indicate?
a. RUN light is off.
b. Fault light is off.
c. BATTERY light is on.
22. When a processor comes equipped with a fault
relay, what are the relay contacts used for?
23. Explain the function of a watchdog timer circuit.
24. A PLC is operating in the RUN mode but output
devices do not operate as programmed. List five
faults that could be responsible for this condition.
25. What is the verify results function used for?
26. A fast-acting solenoid-operated gate is suspected
of not functioning properly when energized and de-
energized by the PLC program. Explain how you
would use the force function to check its operation.
27. What happens when the processor encounters a
temporary end instruction?
28. Explain the function of the suspend instruction.
29. In what negative ways can faulty wiring and connec-
tions affect signals sent to and from the I/O modules?
30. The same address is used for two coil instructions
within the same PLC program. What will happen as
a consequence of this?
31. Compare the uses for RSLogix and RSLinx
programming software.
1. The enclosure door of a PLC installation is not kept
closed. What potential problem could this create?
2. A fuse is blown in an output module. Suggest two
possible reasons why the fuse blew.
CHAPTER 13 PROBLEMS
3. Whenever a crane located over a PLC installation is
started from a standstill, temporary malfunction of
the PLC system occurs. What is one likely cause of
the problem?
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4. During the static checkout of a PLC system, a
specific output is forced on by the programming
device. If an indicator other than the expected one
turns on, what is the probable problem?
5. The input device to a module is activated, but the
LED status indicator does not come on. A check
of the voltage to the input module indicates that no
voltage is present. Suggest two possible causes of
the problem.
6. An output is forced on. The module logic light
comes on, but the field device does not work. A
check of the voltage on the output module indi-
cates the proper voltage level. Suggest two possible
causes of the problem.
7. A specific output is forced on, but the LED module
indicator does not come on. A check of the voltage
at the output module indicates a voltage far below
the normal on level. What is the first thing to
check?
8. An electronic-based input sensor is wired to
a high-impedance PLC input and is falsely
activating the input. How can this problem be
corrected?
9. An LED logic indicator is illuminated, and
according to the programming device monitor,
the processor is not recognizing the input.
If a replacement module does not eliminate
the problem, what two other items should be
suspected?
10. a. A normally open field limit switch examined for
an on state normally cycles from on to off five
times during one machine cycle. How could you
tell by observing the LED status light that the
limit switch is functioning properly?
b. How could you tell by observing the program-
ming device monitor that the limit switch is
functioning properly?
c. How could you tell by observing the LED status
light whether the limit switch was stuck open?
d. How could you tell by observing the program-
ming device monitor whether the limit switch
was stuck open?
e. How could you tell by observing the LED status
light if the limit switch was stuck closed?
f. How could you tell by observing the program-
ming device monitor if the limit switch was
stuck closed?
11. Assume that prior to putting a PLC system into op-
eration, you want to verify that each input device is
connected to the correct input terminal and that the
input module or point is functioning properly. Out-
line a method of carrying out this test.
12. Assume that prior to putting a PLC system
into operation, you want to verify that each
output device is connected to the correct output
terminal and that the output module or point
is functioning properly. Outline a method of
carrying out this test.
13. With reference to the ladder logic program of
Figure 13-35, add instructions to modify the
program to ensure that the second pump_2
does not run while pump_1 is running. If this
condition occurs, the program should suspend
operation and enter code identification number
100 into S2:7.
14. The program of Figure 13-36 is supposed to
execute to sequentially turn PL1 off for 5 seconds
and on for 10 seconds whenever input A is
closed.
a. Examine the ladder logic and describe how the
circuit would operate as programmed.
b. Troubleshoot the program and identify what
needs to be changed to have it operate properly.
L1 L2
Pump 2
Pump 1
Pump 1
PB1
PB2
Outputs
PL1
PB2
PB1 Pump 1
Pump 2
PL1
M1
M2
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T4:2
0.1
50
0
DN
EN
DN
EN
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:3
0.1
100
0
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
PL1
T4:2
T4:2
T4:2
Input A
B3:1
L1
Input
B3:1
0
DN
DN
DN 0
L2
Input A
PL1
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291
This chapter introduces the kinds of industrial
processes that can be PLC controlled. SCADA
is such a process. Different types of control sys-
tems are used for complex processes. These
control systems may be PLCs, but other control-
lers include robots, data terminals, and comput-
ers. For these controllers to work together, they
must communicate. This chapter will discuss the
different kinds of industrial processes and the
means by which they communicate.
Chapter Objectives
14.1 Discuss the operation of continuous process, batch
production, and discrete manufacturing processes
14.2 Compare individual, centralized, and distributive
control systems
14.3 Explain the functions of the major components of a
process control system
14.4 Describe the various functions of electronic HMI
screens
14.5 Recognize and explain the functions of the control
elements of a closed-loop control system
14.6 Explain how on/off control works
14.7 Explain how PID control works
14
Process Control, Network
Systems, and SCADA
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292 Chapter 14 Process Control, Network Systems, and SCADA
car interiors, as illustrated in Figure 14-3, is one example
of discrete manufacturing.
Possible control configurations include individual,
centralized, and distributed. Individual control is used to
control a single machine. This type of control does not
normally require communication with other controllers.
Figure 14-4 shows an individual control application for
a cut to length operation. The operator enters the feed
length and batch count via the interface control panel and
then presses the start button to initiate the process. Stock
lengths vary so the operator needs to select the length and
the number of pieces to be cut.
14.
1 Types of Processes
Process control is the automated control of a process.
Such systems typically deal with analog signals from sen-
sors. The ability of a PLC to perform math functions and
utilize analog signals makes it ideally suited for this type
of control. Manufacturing is based on a series of processes
being applied to raw materials. Typical applications of
process control systems include automobile assembly,
petrochemical production, oil refining, power generation,
and food processing.
A continuous process is one in which raw materials
enter one end of the system and the finished product comes
out the other end of the system; the process itself runs con-
tinuously. Figure 14-1 shows a continuous process used
in an automotive engine assembly line. Parts are mounted
sequentially, in an assembly-line fashion, through a series
of stations. Assembly and adjustments are carried out by
both automated machine and manual operations.
In batch processing, there is no flow of product mate-
rial from one section of the process to another. Instead, a
set amount of each of the inputs to the process is received
in a batch, and then some operation is performed on the
batch to produce a product. Products produced using the
batch process include food, beverages, pharmaceutical
products, paint, and fertilizer. Figure 14-2 shows an ex-
ample of a batch process. Three ingredients are mixed
together, heated, and then stored. Recipes are the key to
producing batches as each batch may have different char-
acteristics by design.
Discrete manufacturing is characterized by individual
or separate unit production. With this manufacturing pro-
cess, a series of operations produces a useful output prod-
uct. Discrete manufacturing systems typically deal with
digital inputs to PLCs that cause motors and robotic de-
vices to be activated. The work piece is normally a discrete
part that must be handled on an individual basis. Making
Figure 14-1 Continuous process.
Automated
machine
assembly Hand assembly
Adjustments, checks
Completed
engines
Engine
block
Figure 14-2 Batch process.
Material input 2
Material input 1
Mix batch
Heat batch
Heater
Mixer
motor
Flow
meter
Store batch
Material input 3
Figure 14-3 Discrete manufacturing.
Source: Courtesy Automation IG.
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Process Control, Network Systems, and SCADA Chapter 14 293
main features of a distributive control system can be sum-
marized as follows:
• Distributive control permits the distribution of the
processing tasks among several controllers.
• Each PLC controls its associated machine or
process.
• High-speed communication among the comput-
ers is done through CAT-5 or CAT-6 twisted pair
wires, single coaxial cables, fiber optics, or the
Ethernet.
• Distributive control drastically reduces field wir-
ing and heightens performance because it places
the controller and I/O close to the machine process
being controlled.
• Depending on the process, one PLC failure would
not necessarily halt the complete process.
• DCS is supervised by a host computer that may
perform monitoring/supervising functions such as
report generation and storage of data.
Centralized control is used when several machines
or processes are controlled by one central controller.
The control layout uses a single, large control system
to control many diverse manufacturing processes and
operations, as illustrated in Figure 14-5. The main
features of centralized control can be summarized as
follows:
• Each individual step in the manufacturing process is
handled by a central control system controller.
• No exchange of controller status or data is sent to
other controllers.
• If the main controller fails, the whole process stops.
A distributive control system (DCS) is a network-based
system. Distributive control involves two or more PLCs
communicating with each other to accomplish the com-
plete control task, as illustrated in Figure 14-6. Each PLC
controls different processes locally and the PLCs are con-
stantly exchanging information through the communica-
tions link and reporting on the status of the process. The
Figure 14-4 Individual control.
Opto
module
Motor
Leadscrew
Operator interface
Cutter
Stock
PLC
Sensor
Figure 14-5 Centralized control.
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• Process actuators that include flow control valves,
pumps, positioning drives, variable speed drives,
clutches, brakes, solenoids, stepping motors, and
power relays
Controller
• Makes the system’s decisions based on the input
signals
• Generates output signals that operate actuators to
carry out the decisions
Human machine interface (HMI) equipment provides
a control and visualization interface between a human and
a process (Figure 14-7). HMIs allow operators to control,
14.2 Structure of Control Systems
Process control normally applies to the manufacturing or
processing of products in industry. In the case of a pro-
grammable controller, the process or machine is operated
and supervised under the control of the user program. The
major components of a process control system include the
following:
Sensors
• Provide inputs from the process and from the exter-
nal environment
• Convert physical information such as pressure, tem-
perature, flow rate, and position into electrical signals
Human Machine Interface (HMI)
• Allows human inputs through various types of
programmed switches, controls, and keypads to set
up the starting conditions or alter the control of a
process
Signal Conditioning
• Involves converting input and output signals to a us-
able form
• May include signal-conditioning techniques such
as amplification, attenuation, filtering, scaling, A/D
and D/A converters
Actuators
• Convert system output electrical signals into physi-
cal action
Figure 14-6 Distributive control system (DCS).
Machine
Host
computer
Machine Machine
Communications network
Human machine
interface (HMI)
PLC
PLC PLC
Figure 14-7 Human machine interface (HMI).
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• Edit and create graphical objects on the screens
• Animate the objects
Most control systems are closed loop in that they uti-
lize feedback in which the output of a process affects the
input control signal. A closed-loop system measures the
actual output of the process and compares it to the desired
output. Adjustments are made continuously by the control
system until the difference between the desired and actual
output falls within a predetermined tolerance.
Figure 14-9 illustrates an example of a closed-loop
control system. The actual output is sensed and fed back
to be subtracted from the set-point input that indicates
what output is desired. If a difference occurs, a signal to
the controller causes it to take action to change the actual
output until the difference is 0. The operation of the com-
ponent parts are as follows:
Set-point—The input that determines the desired op-
erating point for the process.
Process Variable—Refers to the feedback signal that
contains information about the current process status.
Error Amplifier—Determines whether the process
operation matches the set-point. The magnitude and
polarity of the error signal will determine how the
process will be brought back under control.
Controller—Produces the appropriate corrective out-
put signal based on the error signal input.
monitor, diagnose, and manage the application. Depend-
ing on the requirements and complexity of the process,
the operator may be required to:
• Stop and start the process.
• Operate the controls and make the adjustments
required for the process and monitor its progress.
• Detect abnormal situations and undertake corrective
action.
Graphic HMI terminals offer electronic interfacing in
a wide variety of sizes and configurations. They replace
traditional wired panels with a touch screen with graphi-
cal representations of switches and indicators. Types of
graphical display screens include the following:
Operational Summary—used to monitor the
process.
Configuration/Setup—textual in nature used to detail
process parameters.
Alarm Summary—provides a list of time-stamped
active alarms.
Event History—presents a time-stamped list of all
significant events that have occurred in the process.
Trend Values—displays information on process vari-
ables, such as flow, temperature, and production rate,
over a period of time.
Manual Control—generally available only to main-
tenance personnel and meant to bypass parts of the
automatic control system.
Diagnostics—used by maintenance personnel to diag-
nose equipment failures.
Graphic terminals come fully packaged with hardware,
software, and communications. Figure 14-8 shows the
Allen-Bradley family of PanelView graphic terminals. The
setup varies with the vendor. In general, the tasks required
to develop an HMI application include:
• Establish a communication link with the PLCs
• Create the tag addresses database
Figure 14-8 PanelView graphic terminals.
Figure 14-9 Closed-loop control system.
Set-point
Error
amplifier
Error
signal
+
–
Process variable signal
Disturbance
Feedback
path
Input
sensors
Process
Output
actuator
Controller
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that represents the weight of the container and
contents.
• The sensor signal is subtracted from the voltage sig-
nal or digital code that has been input to represent
the desired weight.
• As long as the difference between the input signal
and feedback signal is greater than 0, the controller
keeps the solenoid gate open.
• When the difference becomes 0, the controller out-
puts a signal that closes the gate.
Virtually all feedback controllers determine their output
by observing the error between the set-point and a mea-
surement of the process variable. Errors can occur when
an operator changes the set-point or when a disturbance
or a load on the process changes the process variable. The
controller’s role is to eliminate the error automatically.
14.3 On/Off Control
With on/off controllers the final control element is either
on or off—one for the occasion when the value of the
measured variable is above the set-point and the other for
the occasion when the value is below the set-point. The
controller will never keep the final control element in an
intermediate position. Controlling activity is achieved by
the period of on-off cycling action.
Figure 14-12 shows a system using on/off control in
which a liquid is heated by steam. The operation of the
process can be summarized as follows:
• If the liquid temperature goes below the set-point,
the steam valve opens and the steam is turned on.
• When the liquid temperature goes above the set-point,
the steam valve closes and the steam is shut off.
• The on/off cycle will continue as long as the system
is operating.
Output Actuator—The component that directly af-
fects a process change. Examples are motors, heaters,
fans, and solenoids.
The process shown in Figure 14-10 is an example of a
closed-loop continuous control process used to automati-
cally fill box containers to a specified weight of detergent.
An empty box is moved into position and filling begins.
The weight of the box and contents is monitored. When the
actual weight equals the desired weight, filling is halted.
Operation and block diagrams for the container-filling
process are shown in Figure 14-11. The operation of the
process can be summarized as follows:
• A sensor attached to the scale weighing the con-
tainer generates the voltage signal or digital code
Figure 14-10 Container-ﬁlling process.
Detergent
Box
Scale
Solenoid
gate
Figure 14-11 Operation and block diagrams for the
container-ﬁlling process.
Input
desired
weight
Controller
Error
amplifier
+
–
Filled
container
Filling
process
Feedback
Weight
sensor
Set-point
Container filling
Block Diagram
Operation
Solenoid Scale
Controller
Process
Final control
element
Measurement
(sensor)
Figure 14-12 On/off controlled liquid heating system.
1 3 6
Set-point
temperature
Thumbwheel
switch
On/off
controller
Temperature
sensor
Steam
Valve
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the set-point that will not produce an output as long as
the process variable is within the set limits. The inclusion
of deadband eliminates any hunting by the control device
around the set-point. Hunting occurs when minor adjust-
ments of the controlled position are continually made due
to minor fluctuations.
14.4 PID Control
Proportional controllers are designed to eliminate the
hunting or cycling associated with on/off control. They
allow the final control element to take intermediate posi-
tions between on and off. Proportioning action permits
analog control of the final control element to vary the
amount of energy to the process, depending on how much
the value of the measured variable has shifted from the
desired value.
A proportional controller allows tighter control of the
process variable because its output can take on any value
between fully on and fully off, depending on the magni-
tude of the error signal. Figure 14-14 shows an example
of a motor-driven analog proportional control valve used
as a final control element. The action of the control valve
actuator can be summarized as follows:
• The actuator receives an input current between
4 mA and 20 mA from the controller.
• In response, it provides linear control of the valve.
• A value of 4 mA corresponds to a minimum value
opening (often 0) and 20 mA corresponds to a max-
imum value opening (full scale).
• The 4 mA lower limit allows the system to detect
opens. If the circuit is open, 0 mA would result, and
the system can issue an alarm.
• Because the signal is a current, it is unaffected by
reasonable variations in connecting wire resistance
and is less susceptible to noise pickup from other
signals than is a voltage signal.
Figure 14-13 illustrates the control response for an on/
off temperature controller. The action of the control re-
sponse can be summarized as follows:
• The output turns on when the temperature falls
below the set-point and turns off when the tempera-
ture reaches the set-point.
• Control is simple, but overshoot and cycling about the
set-point can be disadvantageous in some processes.
• The measured variable will oscillate around the set-
point at an amplitude and frequency that depend on
the capacity and time response of the process.
• Oscillations may be reduced in amplitude by in-
creasing the sensitivity of the controller. This in-
crease will cause the controller to turn on and off
more often, a possibly undesirable result.
• On/off control is used when a more precise control
is unnecessary.
A deadband is usually established around the set-point.
The deadband of the controller is usually a selectable
value that determines the error range above and below
Figure 14-13 On/off control response.
Set-point
Time
Time
0
ON
OFF
Final
control
element
Process
variable
Figure 14-14 Motor-driven analog proportional control valve.
Source: Courtesy GEA Tuchenhagen.
Valve
actuator
Actuator stem
Actuator current
(mA)
Valve response
(% open)
4
6
8
10
12
14
16
18
20
0
12.5
25
37.5
50
62.5
75
87.5
100
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
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• Within the proportional band the output is turned on
and off in the ratio of the measurement difference
from the set-point.
• At the set-point (the midpoint of the proportional
band), the output on:off ratio is 1:1; that is, the on
time and off time are equal.
• If the temperature is further from the set-point, the
on and off times vary in proportion to the tempera-
ture difference.
• If the temperature is below the set-point, the output
will be on longer; if the temperature is too high, the
output will be off longer.
In theory, a proportional controller should be all that is
needed for process control. Any change in system output
is corrected by an appropriate change in controller output.
Unfortunately, the operation of a proportional controller
leads to a steady-state error known as offset, or droop.
This steady-state error is the difference between the at-
tained value of the controller and the required value that
results in an offset signal that is slightly lower than the
set-point value, as illustrated in Figure 14-17. Depending
on the PLC application, this offset may or may not be
acceptable.
The process of Figure 14-18 illustrates what effect a
proportional control steady-state error might have on a
tank-filling operation. It may require an operator to make
a small adjustment (manual reset) to bring the controlled
variable to the set-point on initial start-up, or whenever
Proportioning action can also be accomplished by turn-
ing the final control element on and off for short intervals.
This time proportioning (also known as pulse width mod-
ulation) varies the ratio of on time to off. Figure 14-15
shows an example of time proportioning used to pro-
duce varying wattage from a 200 watt heater element as
follows:
• To produce 100 watts the heater must be on 50% of
the time.
• To produce 50 watts the heater must be on 25% of
the time.
• To produce 25 watts the heater must be on 12.5% of
the time.
Proportioning action occurs within a proportional band
around the set-point. The table of Figure 14-16 is an exam-
ple of the proportional band for a heating application with a
set-point of 500°F and a proportional band of 80°F (±40°F).
Proportioning action can be summarized as follows:
• Outside proportional band, the controller functions
as an on/off unit, with the output either fully on
(below the band) or fully off (above the band).
Figure 14-15 Time proportioning of a heater element.
100% - 200 W
50% - 100 W
25% - 50 W
200 W
230 V 12.5% - 25 W
Figure 14-16 Proportional band for a heating application.
Percent
on
Percent
output
Output
level
Temp.
(°F)
On time
(seconds)
Off time
(seconds)
Time proportional
4–20 mA
proportional
0.0
0.0
12.5
25.0
37.5
50.0
62.5
75.0
87.5
100.0
100.0
0.0
0.0
12.5
25.0
37.5
50.0
62.5
75.0
87.5
100.0
100.0
0.0
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
20.0
20.0
20.0
17.5
15.0
12.5
10.0
7.5
5.0
2.5
0.0
0.0
4 mA
4 mA
6 mA
8 mA
10 mA
12 mA
14 mA
16 mA
18 mA
20 mA
20 mA
over 540
540.0
530.0
520.0
510.0
500.0
490.0
480.0
470.0
460.0
under 460
5 0 0
Figure 14-17 Proportional control steady-state error.
Set-point
Time
Offset signal
Figure 14-18 Proportional control tank-ﬁlling operation.
Valve A
Valve B
Float
Offset
Set-point
New level
Valve A
Valve B
Float
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Rate action (derivative control) acts on the error signal
just like reset does, but rate action is a function of the rate
of change rather than the magnitude of error. Rate action
is applied as a change in output for a selectable time inter-
val, usually stated in minutes. Rate-induced change in con-
troller output is calculated from the derivative of the error.
Input change, rather than proportional control error change,
is used to improve response. Rate action quickly positions
the output, whereas proportional action alone would even-
tually position the output. In effect, rate action puts the
brakes on any offset or error by quickly shifting the pro-
portional band. Proportional plus derivative (PD) control
is used in process control systems with errors that change
very rapidly. By adding derivative control to proportional
control, we obtain a controller output that responds to the
error’s rate of change as well as to its magnitude.
PID control is a feedback control method that com-
bines proportional, integral, and derivative actions. The
proportional action provides smooth control without
hunting. The integral action automatically corrects off-
set. The derivative action responds quickly to large exter-
nal disturbances. The PID controller is the most widely
used type of process controller. When combined into a
single control loop the proportional, integral and deriva-
tive modes complement each other to reduce the system
error to zero faster than any other controller. Figure 14-19
shows the block diagram of a PID control loop, the opera-
tion of which can be summarized as follows:
• During setup, the set-point, proportional band, reset
(integral), rate (derivative), and output limits are
specified.
• All these can be changed during operation to tune
the process.
• The integral term improves accuracy, and the de-
rivative reduces overshoot for transient upsets.
• The output can be used to control valve positions,
temperature, flow metering equipment, and so on.
the process conditions change significantly. The operation
• When valve B opens liquid flows out and the level
in the tank drops.
• This causes the float to lower, opening valve A and
allowing more liquid in.
• This process continues until the level drops to a point
at which the float is low enough to open valve A, thus
allowing the same input flow as output flow.
• Due to the steady-state error, the level will stabilize
at a new lower level, not at the desired set-point.
Proportional control is often used in conjunction with
integral control and/or derivative control.
• The integral action, sometimes termed reset action,
responds to the size and time duration of the error
signal. An error signal exists when there is a differ-
ence between the process variable and the set-point,
so the integral action will cause the output to change
and continue to change until the error no longer ex-
ists. Integral action eliminates steady-state error.
The amount of integral action is measured as min-
utes per repeat or repeats per minute, which is the
relationship between changes and time.
• The derivative action responds to the speed at which
the error signal is changing—that is, the greater the
error change, the greater the correcting output. The
derivative action is measured in terms of time.
Proportional plus integral (PI) control combines the
characteristics of both types of control. A step change in
the set-point causes the controller to respond proportion-
ally, followed by the integral response, which is added to
the proportional response. Because the integral mode de-
termines the output change as a function of time, the more
integral action found in the control, the faster the output
changes. This action can be summarized as follows:
• To eliminate the offset error, the controller needs to
change its output until the process variable error is
zero.
• Reset integral control action changes the controller
output by the amount needed to drive the process
variable back to the set-point value.
• The new equilibrium point after reset action is at
point “C.”
• Since the proportional controller must always oper-
ate on its proportional band, the proportional band
must be shifted to include the new point “C.”
• A controller with reset integral control does this
automatically. Figure 14-19 PID control loop.
Set-point
Integral
Proportional
Error
Derivative
PID controller
+
+ Process
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• The PLC program compares the feedback to the set-
point and generates an error signal.
• The error is examined by the PID loop calculation
in three ways: with proportional, integral, and de-
rivative methodology.
• The controller then issues an output to correct for
any measured error by adjustment of the position of
the variable flow outlet valve.
The response of a PID loop is the rate at which it com-
pensates for error by adjusting the output. The PID loop
is adjusted or tuned by changing the proportional gain,
the integral gain, and/or the derivative gain. A PID loop
is normally tested by making an abrupt change to the set-
point and observing the controller’s response rate. Adjust-
ments can then be made as follows:
• As the proportional gain is increased, the controller
responds faster.
• If the proportional gain is too high, the controller
may become unstable and oscillate.
• The integral gain acts as a stabilizer.
• Integral gain also provides power, even if the error
is zero (e.g., even when an oven reaches its set-
point, it still needs power to stay hot).
• Without this base power, the controller will droop
and hunt for the set-point.
• The derivative gain acts as an anticipator.
• Derivative gain is used to slow the controller down
when change is too fast.
Basically, PID controller tuning consists of deter-
mining the appropriate values for the gain (proportional
band), rate (derivative), and reset time (integral) tuning
• PID control allows the output power level to be
varied.
• As an example, assume that a furnace is set at 50°C.
• The heater power will increase as the temperature
falls below the 50°C set-point.
• The lower the temperature the higher the power.
• PID has the effect of gently turning the power down
as the signal gets close to the set-point.
The long-term operation of any system, large or small,
requires a mass-energy balance between input and out-
put. If a process were operated at equilibrium at all times,
control would be simple. Because change does occur, the
critical parameter in process control is time, that is, how
long it takes for a change in any input to appear in the
output. System time constants can vary from fractions of
a second to many hours. The PID controller has the ability
to tune its control action to specific process time constants
and therefore to deal with process changes over time. PID
control changes the amount of output signal in a math-
ematically specified way that accounts for the amount of
error and the rate of signal change.
Either programmable controllers can be fitted with
input/output modules that produce PID control, or they
will already have sufficient mathematical functions to
allow PID control to be carried out. PID is essentially
an equation that the controller uses to evaluate the con-
trolled variable. Figure 14-20 illustrates how a program-
mable logic controller can be used in the control of a PID
loop. The operation of the PID loop can be summarized
as follows:
• The process variable (pressure) is measured and
feedback is generated.
Figure 14-20 PLC control of a PID loop.
Process
supply
Pressure sensor
and transmitter
Process variable
Output
Process output
Variable
flow valve
Analog
output module
Analog input
module
Analog
input
module
CPU
Analog
output
module
Error
Feedback
Feedback
Set-point
PID
loop
calculation
PT
Vessel
PLC CPU
S
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Allen-Bradley SLC 500 instruction set. The PID instruc-
tion is straightforward: it takes one input and controls one
output. Normally, the PID instruction is placed on a rung
without conditional logic. The output remains at its last
value when the rung goes false. A summary of the basic in-
formation that is entered into the instruction is as follows:
Control Block—File that stores the data required to
operate the instruction.
Process Variable—The element address that stores
the process input value.
Control Variable—The element address that stores
the output of the PID instruction.
Setup Screen—Instruction on which you can double-
click to bring up a display that prompts you for other
parameters you must enter to fully program the PID
instruction.
14.5 Motion Control
A motion control system provides precise positioning,
velocity, and torque control for a wide range of motion
applications. PLCs are ideally suited for both linear and
rotary motion control applications. Pick and Place ma-
chines are used in the consumer products industry for a
wide variety of product transfer applications. The ma-
chine takes a product from one point to another. One ex-
ample is the transfer of a product to a moving conveyor
belt as illustrated in Figure 14-22.
A basic PLC motion control system consists of a con-
troller, a motion module, a servo drive, one or more mo-
tors with encoders, and the machinery being controlled.
Each motor controlled in the system is referred to as an
parameters (control constants) that will give the control
required. Depending on the characteristics of the devia-
tion of the process variable from the set-point, the tun-
ing parameters interact to alter the controller’s output and
produce changes in the value of the process variable. In
general, three methods of controller tuning are used:
Manual
• The operator estimates the tuning parameters re-
quired to give the desired controller response.
• The proportional, integral, and derivative terms
must be adjusted, or tuned, individually to a particu-
lar system using a trial-and-error method.
Semiautomatic or Autotune
• The controller takes care of calculating and setting
PID parameters.
- Measures sensor output
- Calculates error, sum of error, rate of change of
error
- Calculates desired power with PID equations
- Updates control output
Fully Automatic or Intelligent
• This method is also known in the industry as fuzzy
logic control.
• The controller uses artificial intelligence to readjust
PID tuning parameters continually as necessary.
• Rather than calculating an output with a formula,
the fuzzy logic controller evaluates rules. The first
step is to “fuzzify” the error and change-in-error
from continuous variables into linguistic variables,
like “negative large” or “positive small.” Simple if-
then rules are evaluated to develop an output. The
resulting output must be de-fuzzified into a continu-
ous variable such as valve position.
The PID programmable controller output instruction
uses closed-loop control to automatically control physi-
cal properties such as temperature, pressure, liquid level,
or flow rate of process loops. Figure 14-21 shows the PID
output instruction and setup screen associated with the
Figure 14-21 PID output instruction and setup screen.
PID
Control block
Process variable
Control variable
Control block length
Setup screen
PID
Figure 14-22 Pick and Place machine.
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• In addition it updates the controller with motor and
drive information used to monitor drive and motor
performance.
Servo Drive
• The servo drive receives the signal provided by the
motion module and translates this signal into motor
drive commands.
• These commands can include motor position, veloc-
ity, and/or torque.
• The servo drive provides power to the servo motors
in response to the motion commands.
• Motor power is supplied and controlled by the servo
drive.
• The servo drive monitors the motor’s position and
velocity by use of an encoder mounted on the motor
shaft. This feedback information is used within the
servo drive to ensure accurate motor motion.
Servo Motor
• The servo motors represent the axis being controlled.
• The servo motors receive electrical power from their
servo drive which determines the motor shaft veloc-
ity and position.
• The filler motor must accelerate the filler mecha-
nism in the direction the bottles are moving, match
their speed, and track the bottles.
• After the bottles have been filled, the filler motor
has to stop and reverse direction to return the filler
mechanism to the starting position to begin the pro-
cess again.
A robot is simply a series of mechanical links driven
by servo motors. The basic industrial robot widely used
today is an arm or manipulator that moves to perform in-
dustrial operations. Figure 14-24 illustrates the motion of
axis of motion. Figure 14-23 illustrates a bottle-filling mo-
tion control process. This application requires two axes of
motion: the motor operating the bottle filler mechanism
and the motor controlling the conveyor speed. The role of
each control component can be summarized as follows:
Programmable Logic Controller
• The controller stores and executes the user program
that controls the process.
• This program includes motion instructions that con-
trol axis movements.
• When the controller encounters a motion instruction
it calculates the motion commands for the axis.
• A motion command represents the desired position,
velocity, or torque of the servo motor at the particu-
lar time the calculations take place.
Motion Module
• The motion module receives motion commands
from the controller and transforms them into a com-
patible form the servo drive can understand.
Figure 14-23 Bottle-ﬁlling motion control process.
Motion
Bottle filler
servo motor
Communication
PLC
Conveyor servo drive
Filler servo drive
Processor
Conveyor
servo motor
Figure 14-24 Six-axis robot arm.
2
3
4
1
5
6
Shoulder
swivel
Elbow
extension
Yaw
Pitch
Roll
Arm sweep
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no more than one or two miles. Figure 14-26 illustrates
a LAN communication link. Network communications
supports communication among multiple PLCs and other
devices. PLC networks allow:
• Sharing of information such as the current state
of status bits among PLCs that may determine the
action of one another.
• Monitoring of information from a central location.
• Programs to be uploaded or downloaded from a
central location.
• Several PLCs to operate in unison to accomplish a
common goal.
Transmission media are the cable through which data
and control signals flow on a network. The transmission
media used in data communications systems include co-
axial cable, twisted pair, or fiber optics (Figure 14-27).
Each cable has different electrical capabilities and may
be more or less suitable to a specific environment or net-
work type. Not all networks transmit information through
cable. Wireless Wi-Fi Ethernet networks, such as the DF1
a six-axis robot arm. Each axis of the robot arm is funda-
mentally a closed-loop servo control system. The wrist
is the name usually given to the last three joints on the
robot’s arm. Going out along the arm, these wrist joints
are known as the pitch joint, yaw joint, and roll joint.
There are two types of controller setups that can be used
to control an industrial robot—PLC- and PC-based sys-
tems. Depending on the difficulty of the task the robotic
system will be performing, you may need a PLC or just a
robot controller.
14.6 Data Communications
Data communications refers to the different ways that
PLC microprocessor-based systems talk to each other and
to other devices. The two general types of communica-
tions links that can be established between the PLC and
other devices are point-to-point links and network links.
Figure 14-25 illustrates a point-to-point serial communi-
cations link. Serial communications is used with devices
such as printers, operator workstations, motor drives, bar
code readers, computers, or another PLC. Serial com-
munications interfaces are either built into the processor
module or come as separate modules. A serial module in-
stalled in each controller is normally all that is required
for two PLCs of the same manufacturer to establish a
point-to-point link.
As control systems become more complex, they re-
quire more effective communications schemes between
the system components. A local area network or LAN is
a system that interconnects data communications com-
ponents within a limited geographical area, typically
Figure 14-25 Point-to-point serial communications link.
Serial cable
Programmable logic controller
Printer
PLC
Point-to-point
communications
Serial
messages
Serial data
communications
Operator interface
10:31 4801
Figure 14-26 Local area network (LAN) communication
link.
Local area network (LAN)
Figure 14-27 Transmission media.
Coaxial
Twisted pair
Fiber optic
Wireless system
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Each device connected on a network is known as a
node or station. As signals travel along a network cable,
they degrade and become distorted in a process that is
called attenuation. If a cable is long enough, the attenua-
tion will finally make a signal unrecognizable. A repeater
is a device that amplifies a signal to its original strength
in order to enable its signals to travel further. Different
network types will have different specifications for cable
length and type without a repeater.
Network topology is the physical layout of devices on a
network formed by the network cables when nodes are at-
tached. The star topology illustrated in Figure 14-29 and
its operation can be summarized as follows:
• A network controller switch or hub is connected to
several PLC network nodes.
• Currently, most Ethernet networks use switches
rather than hubs. A switch performs the same
basic function as a hub but effectively increases
the speed, size, and data handling capacity of the
network.
• The configuration allows for bidirectional commu-
nication between switch/hub and each PLC.
• All transmission must be between the switch/hub
and the PLCs because the network controller hub
controls all communication.
• All transmissions must be sent to the switch/hub,
which then sends them to the correct PLC.
• One problem with the star topology is that if the
switch/hub goes down, the entire LAN is down.
• This type of system works best when information is
transmitted primarily between the main controller
and remote PLCs. However, if most communication
is to occur between PLCs, the operation speed is
affected.
• Also, the star system can use substantial amounts
of communication conductors to connect all remote
PLCs to one central location.
Radio Modem, communicate through radio waves, which
are transmitted through the air.
In industrial applications, LANs have most often been
used as the communication system for distributed control
systems (DCS). Recall that a DCS system uses individ-
ual controllers to control the subsystems of a machine or
process. This approach contrasts with centralized control
in which a single controller governs the entire operation.
A second major use of local area networks is that of su-
pervisory control and data acquisition (SCADA). A LAN
allows data collection and processing for a group of con-
trollers to be accomplished using one host computer as
the central point for collecting data.
There are three general levels of functionality of indus-
trial networks. Figure 14-28 shows an illustration of the
three levels, which can be summarized as follows:
Device Level—The device level involves various sen-
sor and actuator devices of machines and processes.
These may include devices such as sensors, switches,
drives, motors, and valves.
Control Level—The control level would be the net-
works industrial controllers are on. This level may in-
clude controllers such as PLCs and robot controllers.
Communications on the control level includes sharing
I/O and program data between controllers.
Information Level—The information level is a plant-
wide network typically composed of the company’s
business networks and computers. This level may in-
clude scheduling, sales, management, and corporate-
wide information.
Figure 14-28 Levels of functionality of industrial networks.
Information level
Control level
Device level
Figure 14-29 Star topology network.
Processor
Com
Processor
Com
Processor
Network
Switch/Hub
Com
Processor
Com
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transmit data relating to the on/off state of the device and
its operational status. Device bus networks can be fur-
ther classified as bit-wide or byte-wide buses. Device bus
networks that include discrete devices as well as small
analog devices are called byte-wide bus networks. These
networks can transfer 50 or more bytes of data at a time.
Device bus networks that interface only with discrete de-
vices are called bit-wide bus networks. Bit-wide networks
transfer less than 8 bits of information to and from simple
discrete devices.
Process bus networks are capable of communicat-
ing several hundred bytes of data per transmission. The
majority of devices used in process bus networks are
analog, whereas most devices used in device bus net-
works are discrete. Process bus networks connect with
high-level information devices such as smart process
valves and flowmeters, which are typically used in
process control applications. Process buses are slower
because of their large data packet size. Most analog
control devices are used in controlling such process
variables as flow and temperature, which are typically
slow to respond.
A protocol is a set of rules that two or more devices
must follow if they are to communicate with each other.
Protocols are to computers what language is to humans.
This book is in English, and to understand it, you must
be able to read English. Similarly, for two devices on a
network to successfully communicate, they must both un-
derstand the same protocols.
A network protocol defines how data is arranged and
coded for transmission on a network. In the past, com-
munications networks were often proprietary systems
designed to a specific vendor’s standards; users were
forced to buy all their control components from a single
supplier. This is because of the different communications
protocols, command sequences, error-checking schemes,
and communications media used by each manufacturer.
Today, the trend is toward open network systems based
on international standards developed through industry
associations.
Bus topology, illustrated in Figure 14-30, is a network
configuration in which all stations are connected in par-
allel with the communication medium and all stations
receive information from every other station on the net-
work. The operation of a bus topology network can be
• Uses a single bus trunk cable to which individual
PLC nodes are attached by a cable drop that taps off
the main cable.
• Each PLC is interfaced to the bus using a network
interface module that is attached using a drop cable
or connector.
• Due to the nature of the bus technology, and the
way the data are transmitted on the network, each
end of the bus must be terminated with a terminat-
ing resistor.
• As the data move along the total bus, each PLC
node is listening for its own node identification
address and accepts only information sent to that
address.
• Because of the simple linear layout, bus networks
require less cable than all other topologies.
• No single station controls the network and stations
can communicate freely to one another.
• Bus networks are very useful in distributive control
systems, because each station or node has equal
independent control capability and can exchange
information at any given time.
• Another advantage of the bus network is that you
can add or remove stations from the network with a
minimum amount of system reconfiguration.
• This network’s main disadvantage is that all the
nodes rely on a common bus trunk line, and a break
in that common line can affect many nodes.
I/O bus networks can be divided into two categories:
device bus networks and process bus networks. Device
bus networks interface with low-level information devices
such as pushbuttons and limit switches that primarily
Figure 14-30 Bus topology network.
Processor
Com
Network interface module
Main trunk line
Processor
Com
Processor
Com
Processor
Com
Media attachment Termination
Tee tap
Trunk line
Drop line
Connectors
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next node in the sequence, granting it the token. The token
passes sequentially from node to node, allowing each an
opportunity to transmit without interference. Tokens usu-
ally have a time limit to prevent a single node from tying
up the token for a long period of time.
Ethernet networks use a collision detection access
control scheme. With this access method, nodes listen
for activity on the network and transmit only if there are
no other messages on the network. On Ethernet networks
there is the possibility that nodes will transmit data at the
same time. When this happens a collision is detected.
Each node that had sent out a message will wait a random
amount of time and will resend its data if it does not detect
any network activity.
The access method most often used in master/slave
protocols is polling. The master/slave network is one in
which a master controller controls all communications
originating from other controllers. This configuration is
illustrated in Figure 14-33 and consists of several slave
controllers and one master controller. Its operation can be
• The master controller sends data to the slave
controllers.
• When the master needs data from a slave, it will
poll (address) the slave and wait for a response.
• No communication takes place without the master
initiating it.
• Direct communication among slave devices is not
possible.
• Information to be transferred between slaves must
be sent first to the network master unit, which will,
Gateways (Figure 14-31) make communication pos-
sible between different architectures and protocols. They
repackage and convert data going from one network
to another network so that the one can understand the
other’s application data. Gateways can change the format
of a message so that it will conform to the application
program at the receiving end of the transfer. If network-
access translation is their only function, the interfaces
are known as bridges. If the interface also adjusts data
formats or performs data transmission control, then it is
called a gateway.
A bus topology network requires some method of
controlling a particular device’s access to the bus. An ac-
cess method is the manner in which a PLC accesses the
network to transmit information. Network access control
ensures that data are transmitted in an organized manner
preventing the occurrence of more than one message on
the network at a time. Although many access methods
exist, the most common are token passing, collision de-
tection, and polling.
In a token passing network, a node can transmit data
on the network only when it has possession of a token. A
token is simply a small packet that is passed from node to
node as illustrated in Figure 14-32. When a node finishes
transmitting messages, it sends a special message to the
Figure 14-31 Translating from one network-access
scheme to another.
Modbus
Data highway
Gateway
Siemens Omron
Gateway
Gateway
Gould
Gateway
Bus network
Allen-Bradley
Figure 14-32 Example of token passing.
Node
1
1-to-2
3-to-1
2-to-3
Node
2
Node
3
Figure 14-33 Master/slave network.
Master controller
Read
command
Write
command
Slave #1 Slave #2
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simultaneously, as illustrated in Figure 14-35. Parallel
transmission of data can be summarized as follows:
• Eight transmission lines are required to transmit the
8-bit binary number.
• Each bit requires its own separate data path and all
bits of a word are transmitted at the same time.
• Parallel data transmission is less common but faster
than serial transmission.
• A common example of parallel data transmission is
the connection between a computer and a printer.
In serial transmission one bit of the binary data is
transferred at a time, as illustrated in Figure 14-36. Serial
transmission of data can be summarized as follows:
• In serial transmission, bits are sent sequentially on
the same channel (wire) which reduces costs for
wire but also slows the speed of transmission.
in turn, retransmit the message to the designated
slave device.
• Master/slave networks use two pairs of conductors.
One pair of wires is used for the master to transmit
data and the slave to receive them. On the other pair,
the slaves transmit and the master receives.
A peer-to-peer network has a distributive means of
control, as opposed to a master/slave network in which
one node controls all communications originating from
other nodes. The Allen-Bradley Data Highway, shown in
Figure 14-34, is an example of a peer-to-peer network of
programmable controllers and computers linked together
to form a data communication system. The operation of
the network can be summarized as follows:
• Peer-to-peer networks use the token passing media
access method.
• Each device has the ability to request use of, and
then take control of, the network for the purpose of
transmitting information to or requesting informa-
tion from other network devices.
• Each device is identified by an address.
• When the network is operating, the token passes
from one device to the next sequentially.
• The device that is transmitting the token also knows
the address of the next station that will receive the
token.
• Each device receives the packet information and
uses it, if needed.
• Any additional information that the node has will be
sent in a new packet.
There are two methods of transmitting PLC digital
data: parallel and serial transmission. In parallel data
transmission, all bits of the binary data are transmitted
Figure 14-34 Peer-to-peer network.
Data highway (DH) network
Figure 14-35 Parallel data transmission.
Receiving
side
1
0
1
0
1
1
0
1
Transmitting
side
Figure 14-36 Serial data transmission.
Receiving
side
Transmitting
side
1 0 1 0 1 1 0 1
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connection for a SLC 5/04 controller. The three-pin Phoe-
nix connector is used to form the network transmission
media.
Serial Communication
Serial data communication is implemented using stan-
dards such as RS-232, RS-422, and RS-485. The RS in
the standard’s name means recommended standard that
specifies the electrical, mechanical, and functional char-
acteristics for serial communications. Serial communica-
tion interfaces are either built into the processor module
or come as a separate communications interface module,
as illustrated in Figure 14-38. The simplest type of con-
nection is the RS-232 serial port. The RS interfaces are
used to connect to devices such as vision systems, barcode
readers, and operator terminals that must transfer quanti-
ties of data at a reasonably high rate between the remote
device and the PLC. The RS-232 type of serial transmis-
sion is designed to communicate between one computer
and one controller and is usually limited to lengths up
to 50 feet. RS-422 and RS-485 serial transmission types
are designed to communicate between one computer and
multiple controllers, have a high level of noise immunity,
and are usually limited to lengths of 650 feet (for RS-485)
or 1650 feet (for RS-422).
DeviceNet
DeviceNet is an open device-level network. It is rela-
tively low speed but efficient at handling the short mes-
sages to and from I/O modules. As PLCs have become
more powerful, they are being required to control an in-
creasing number of I/O field devices. Therefore, at times
it may not be practical to separately wire each sensor and
actuator directly into I/O modules. Figure 14-39 shows
a comparison between conventional and DeviceNet I/O
systems. Conventional systems have racks of inputs and
outputs with each I/O device wired back to the control-
ler. The DeviceNet protocol dramatically reduces costs
• Serial data can be transmitted effectively over much
greater distances than can parallel data.
• Each data word in the serial transmission must be
denoted with a known start bit sequence followed
by the data bits that contain the intelligence of the
data transmission and a stop bit.
• An extra bit, termed a parity bit, may be used to
provide some error-detecting ability.
A duplex communication system is a system composed
of two connected devices that can communicate with one
another in both directions at the same time. A half-duplex
system provides for communication in both directions,
but only one direction at a time (not simultaneously).
Half-duplex transmission is use for master/slave commu-
nications. Full-duplex transmission allows the transmis-
sion of data in both directions simultaneously and can be
used for peer-to-peer communications.
The different networking schemes replace traditional
point-to-point hardwiring. Network control of systems
minimizes the amount of wiring that needs to be done.
With traditional wiring multiple wires from each device,
fed through control cabinets, often result in large wire
bundles running through the system. Due to the sheer
volume of wires, installation time is considerable and
troubleshooting is complex. If a network is used all de-
vices can be directly connected to a single transmission
media cable.
High-speed industrial networking technologies offer a
variety of methods for connecting devices. PLC network
configurations may be either open or proprietary (vendor-
unique). Following is an overview of some of the indus-
trial communication technologies that play a critical role
in today’s control systems.
Data Highway
The Allen-Bradley Data Highway networks, Data High-
way Plus (DH+) and DH-485, are proprietary communi-
cations networks. They use peer-to-peer communication
implementing token passing. The medium is shielded
twisted pair cable. Figure 14-37 shows the DH+ network
Figure 14-37 Data Highway network connection.
DH⫹
network
FORCE
SLC 5/04 CPU
RUN
DH⫹
FLT
RS232
BATT
PROG
RUN REM
Communication
ports
Figure 14-38 Serial communication interface.
Vision
system
Barcode
reader
Operator
terminal
Processor
RS-232
module
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and application-specific objects. A DeviceNet network
can support up to 64 nodes and the network end-to-
end distance is variable, based on network speed. Fig-
ure 14-41 shows an example of a typical layout of the
trunk wiring for a DeviceNet network. Communications
data is carried over two wires with a second pair of wires
carrying power.
The field devices that are connected to the network
contain intelligence in the form of microprocessors or
by integrating all I/O devices on a 4-wire trunk network
with data and power conductors in the same cable. This
direct connectivity reduces costly and time-consuming
wiring.
The basic function of a DeviceNet I/O bus network
is to communicate information with, as well as supply
power to, the field devices that are connected to the bus.
The PLC drives the field devices directly with the use of
a network scanner instead of I/O modules, as illustrated
in Figure 14-40. The scanner module communicates with
DeviceNet devices over the network to:
• Read inputs from a device.
• Write outputs to a device.
• Download configuration data.
• Monitor a device’s operational status.
The scanner module communicates with the controller
to exchange information which includes:
• Device I/O data
• Status information
• Configuration data
DeviceNet also has the unique feature of having
power on the network. This allows devices with limited
power requirements to be powered directly from the
network, further reducing connection points and physi-
cal size.
DeviceNet uses the Common Industrial Protocol,
called CIP, which is strictly object oriented. Each object
has attributes (data), services (commands), and behavior
(reaction to events). Two different types of objects are
defined in the CIP specification: communication objects
Figure 14-39 Conventional and DeviceNet I/O systems.
Conventional system DeviceNet system
4-wire cable
and connector
Figure 14-40 DeviceNet network scanner.
DeviceNet
port
Power
Sensor
Data
DeviceNet I/O bus network
DeviceNet
scanner
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which may be an indication that it has reached the end of
its operating life and thus requires replacement.
ControlNet
ControlNet is positioned one level above DeviceNet. It
uses the Common Industrial Protocol (CIP) to combine
the functionality of an I/O network and a peer-to-peer
other circuits. These devices can communicate not only
the on/off status of field devices but also diagnostic infor-
mation about their operating state. For example, you can
detect via the network that a photoelectric sensor is losing
margin because of a dirty lens, and you can correct the
situation before the sensor fails to detect an object. A limit
switch can report the number of motions it has performed,
Figure 14-41 Layout of a DeviceNet network.
T-port tap
Terminator
DevicePort tap
(8-port)
DC
power
supply
PowerTap
tap
Thick cable
Generic
sealed device
Generic
sealed device
Generic
open-style device
Generic
sealed
device
DeviceBox
tap
(4-port)
Open-style
tap
Thick cable
T-port tap
Terminator
Thin cable
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complex devices from multiple vendors. Plug and
play refers to the ability of a computer system to
automatically configure devices. This allows you to
plug in a device and play (operate) it without worry-
ing about setting DIP switches, jumpers, and other
configuration elements.
• EtherNet/IP provides standardized full-duplex op-
eration which gives a single node, in a peer-to-peer
connection, full attention and therefore maximum
possible bandwidth. Bandwidth refers to the data
rate supported by a network, commonly expressed
in terms of bits per second. The greater the band-
width the greater the overall performance.
• EtherNet/IP allows interoperability of industrial
automation devices and control equipment on the
same network used for business applications and
browsing the Internet.
Modbus
Modbus is a serial communication protocol originally de-
veloped by Modicon for use with its PLCs. Basically, it is a
method used for transmitting information over serial lines
between electronic devices. The device requesting the in-
formation is called the Modbus Master and the devices
supplying information are Modbus Slaves. Modbus is an
open protocol, meaning that it’s free for manufacturers to
network providing high-speed performance for both
functions. This open high-speed network is highly de-
terministic and repeatable. Determinism is the ability to
reliably predict when data will be delivered, and repeat-
ability ensures that transmit times are constant and unaf-
fected by devices connecting to, or leaving, the network.
Electronic device data sheets (EDS-Files) are required
for each ControlNet device. During the setup phase the
ControlNet scanner must configure each device accord-
ing to the EDS-Files. The ControlNet layout shown in
Figure 14-42 has a redundant media option in which two
separate cables are installed to guard against failures such
as cut cables, loose connectors, or noise.
EtherNet/IP
EtherNet/IP (Ethernet Industrial Protocol) is an open
communications protocol based on the Common Indus-
trial Protocol (CIP) layer used in both DeviceNet and
ControlNet. It allows users to link information seamlessly
between devices running the EtherNet/IP protocol with-
out custom hardware, as illustrated in Figure 14-43.
The following are some of the important features of
EtherNet/IP:
• Sharing a common application layer between
ControlNet, DeviceNet, and Ethernet/IP will make
plug-and-play interoperability possible among
Figure 14-42 ControlNet network with redundant media installed.
ControlNet
scanner
B
A
Redundant media
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Fieldbus
Fieldbus is an open, serial, two-way communications sys-
tem that interconnects measurement and control equip-
ment such as sensors, actuators, and controllers. At the
base level in the hierarchy of plant networks, it serves
as a network for field devices used in process control
applications.
There are several possible topologies for fieldbus net-
works. Figure 14-45 illustrates the daisy-chain topology.
With this topology, the fieldbus cable is routed from de-
vice to device. Installations using this topology require
build into their equipment without having to pay royalties.
It has become a standard communications protocol in in-
dustry, and is one of the most commonly available means
of connecting industrial electronic devices. Figure 14-44
shows an Omron PLC with Modbus-RTU network com-
munication capabilities via RS-232C and RS-422/485
serial ports.
Figure 14-44 Omron PLC with Modbus-RTU network
communication capabilities.
Figure 14-43 EtherNet/IP information links.
Robotics
Sensors and other
input/output devices
I/O
Controller
Back-office mainframes and
servers (ERP, MES, etc.)
Camera
Safety
controller
Phone
Safety
I/O
Supervisory
control
Corporate network
Industrial network
Motors, drives
actuators
HMI
Office
applications,
internetworking,
data servers,
storage
Figure 14-45 Fieldbus implemented using daisy-chain
topology.
Connectors
Field
device
Fieldbus
interface
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efficiency. In general, unlike distributive control systems,
a SCADA system usually refers to a system that coordi-
nates but does not control processes in real time.
In a typical SCADA system, independent PLCs per-
form I/O control functions on field devices while being
supervised by a SCADA/HMI software package running
connectors or wiring practices such that disconnection of
a single device is possible without disrupting the continu-
ity of the whole segment.
PROFIBUS-DP
PROFIBUS-DP (where DP stands for Decentralized
Periphery) is an open, international fieldbus communi-
cation standard that supports both analog and discrete
signals. It is functionally comparable to DeviceNet.
The physical media are defined via the RS-485 or fiber
optic transmission technologies. PROFIBUS-DP com-
municates at speeds up to 12 Mbps over distances up to
1200 meters. Figure 14-46 illustrates a Siemens S7-200
Micro PLC system connection to a PROFIBUS-DP
network.
14.7 Supervisory Control
and Data Acquisition (SCADA)
In some applications, in addition to its normal control
functions, the PLC is responsible for collecting data,
performing the necessary processing, and structuring the
data for generating reports. As an example, you could
have a PLC count parts and automatically send the data to
a spreadsheet on your desktop computer.
Data collection is simplified by using a SCADA (su-
pervisory control and data acquisition) system, shown in
Figure 14-47. Exchanging data from the plant floor to a
supervisory computer allows data logging, data display,
trending, downloading of recipes, setting of selected pa-
rameters, and availability of general production data. The
additional supervisory control output capabilities allow
you to tweak your processes accurately for maximum
Figure 14-46 Micro PLC system connection to a PROFIBUS-DP network.
PROFIBUS-DP
module
PROFIBUS-DP
Figure 14-47 Supervisory control and data acquisition
(SCADA).
Alarms
PLC relay
controls
Analog
outputs
PID
process
control
Printer
Tables and
graphs
Hardcopy
Spreadsheet
Temperature
thermocouples
and resistance
temperature detectors
Pressure
Position
Force
Strain
Speed
Signal sources
Interface
system
Output loads
Computer
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Figure 14-48 Typical SCADA system.
Host computer
SCADA/HMI
software
Data
transfer
I/O
control Field
devices
I/O
control Field
devices
I/O
control Field
devices
PLC
PLC
PLC
on a host computer, as illustrated in Figure 14-48. Pro-
cess control operators monitor PLC operation on the
host computer and send control commands to the PLCs
if required. The great advantage of a SCADA system is
that data are stored automatically in a form that can be
retrieved for later analysis without error or additional
work. Measurements are made under processor control
and then displayed onscreen and stored to a hardcopy. Ac-
curate measurements are easy to obtain, and there are no
mechanical limitations to measurement speed.
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1. Compare continuous and batch processes.
2. Compare centralized and distributive control
systems.
3. State the basic function of each of the following as
part of a process control system:
a. Sensors
b. Human-machine interface
c. Signal conditioning
d. Actuators
e. Controller
4. State the purpose of each of the following types of
screens associated with HMIs:
a. Trend values
b. Operational summary
c. Alarm summary
5. What is the main characteristic of a closed-loop
control system?
6. State the function of each of the following parts of
a closed-loop control system:
a. Set-point
b. Process variable
c. Error amplifier
d. Controller
e. Output actuator
7. Explain how on/off control works.
8. How does the proportional controller eliminate the
cycling associated with on/off control?
9. Explain how a motor-driven control valve action
can provide analog control.
10. How does time proportioning provide analog
control?
11. What process error or deviation is produced by a
proportional controller?
12. What term of a PID control is designed to eliminate
offset?
13. What does the derivative action of a controller
respond to?
14. List the three gain adjustments used in tuning the
response of a PID control loop.
15. Compare manual, autotune, and intelligent tuning
of a PID controller.
16. How many input and output values are normally
referenced in a PLC PID instruction?
17. What information is contained in the process
variable and control variable elements of a PID
instruction?
18. State the function of each of the following elements
of a PLC motion control system:
a. Programmable controller
b. Motion module
c. Servo drive
d. Servo motor
19. What does each axis of a robot arm function as?
20. List four types of communication tasks provided by
local area networks.
21. Name three common types of transmission media.
22. What are the three general levels of functionality of
industrial networks?
23. Define the term node as it applies to a network.
24. Explain the physical layout of devices on a network
for each of the following network topologies:
a. Star
b. Bus
25. Compare device and process bus networks.
26. Define the term protocol as it applies to a network.
27. What is the function of a network gateway?
28. Define the term access method as it applies to a
network.
29. Summarize the token passing network access method.
30. Summarize the collision detection network access
method.
31. Summarize the polling network access method.
32. Compare parallel and serial data transmission.
33. Compare half-duplex and full-duplex data
transmission.
34. Explain how networking schemes minimize the
amount of wiring required.
35. What type of access control is used with DH+?
36. Compare the transmitting distances of RS-232
and RS-422/485 serial types.
37. What is DeviceNet used for?
38. List three pieces of information obtained from
DeviceNet devices by the network scanner.
39. What is ControlNet used for?
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44. What is Fieldbus used for?
45. Summarize the two main functions of a SCADA
system.
46. In what way does distributive control differ from
the supervisory control of a SCADA system?
40. Explain how redundant media works.
41. Define the term bandwidth as it applies to a
network.
42. What is Ethernet/IP used for?
43. What type of protocol does Modbus use?
1. Distributive control systems have to be network
based. Why?
2. Assume an alarm is sounded in a control system
with an electronic HMI interface. How would you
proceed to identify and solve the problem?
3. How would an on/off controller respond if the dead-
band were too narrow?
4. In a home heating system with on/off control, what
will be the effect of widening the deadband?
5. a. Calculate the proportional band of a temperature
controller with a 5% bandwidth and a set-point of
500°F.
CHAPTER 14 PROBLEMS
b. Calculate the upper and lower limits beyond
which the controller functions as an on/off
unit.
6. Explain the advantage of using a 4- to 20-mA
current loop as an input signal compared to a 0- to
5-V input signal.
7. What does the term deterministic mean, and why is
it important in industrial communications?
8. How might a SCADA system be applied to deter-
mine the production rate of a bottled product over a
two-week period?
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317
system, CompactLogix system, FlexLogix sys-
tem, SoftLogix 5800 controller, and DriveLogix
system. Software is the essential difference
between PACs and PLCs. Basically, the ladder
logic conﬁguration does not change but the
addressing of the instructions changes. Applica-
tion of the software that pertains to the Logix
control platform of controllers will be covered in
the various sections of this chapter. Knowledge
of basic ladder logic instructions and functions
(bit, timer, counter, etc.) covered in previous
chapters of the text is assumed and is thus not
repeated in this chapter.
Programmable logic controllers continue to
evolve as new technologies are added to their
capabilities. The PLC started out as a replace-
ment for banks of relays used to turn outputs
on and off as well as for timing and counting
functions. Gradually, various math and logic
manipulation functions were added. In order to
serve today’s expanding industrial control system
needs, leading automation companies have cre-
ated a new class of industrial controllers called
programmable automation controllers or PACs
(Figure 15-1). They look like PLCs in their physi-
cal appearance but incorporate advanced control
of communication, data logging, and signal pro-
cessing, motion, process control, and machine
vision in a single programming environment.
The Allen-Bradley programmable automa-
tion controller family includes the ControlLogix
15
ControlLogix Controllers
Figure 15-1 Programmable automation controllers (PACs).
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318
RSLogix 5000 programming software is used to set
up or configure the memory organization of an Allen-
Bradley ControlLogix controller. RSLinx communication
software is used to set up a communications link between
RSLogix 5000 programming software and the Control-
Logix hardware as illustrated in Figure 15-3. To estab-
lish communications with a controller, a driver must be
created in RSLinx software. This driver functions as the
software interface to a hardware device. The RSWho is the
network browse interface that provides a single window
to view all configured network drivers.
Figure 15-4 shows an example of the ControlLogix’s
controllers properties and modules properties dialog
boxes used as part of the configuration process. The
parameters shown are typical of what general informa-
tion is required. After first configuring the controller,
Memory Layout
ControlLogix processors provide a flexible memory struc-
ture. There are no fixed areas of memory allocated for
specific types of data or for I/O. The internal memory orga-
nization of a ControlLogix controller is configured by the
user when creating a project with RSLogix 5000 software
(Figure 15-2). This feature allows the program data to be
constructed to meet the needs of your applications rather
than requiring your application to fit a particular memory
structure. A ControlLogix (CLX) system can consist of
anything from a stand-alone controller and I/O modules in
a single chassis, to a highly distributed system consisting
of multiple chassis and networks working together.
Conﬁguration
Configuration of a modular CLX system involves es-
tablishing a communications link between the control-
ler and the process. The programming software needs to
know what CLX hardware is being used in order to be
able to send or receive data. Configuration information
includes information about the type of processor and I/O
modules used.
Part Objectives
After completing this part, you will be able to:
• Outline project organization
• Define tasks, programs, and routines
• Identify data file types
• Organize and apply the various data file types
Part 1 Memory and
Project Organization
Figure 15-2 RSLogix 5000 screen.
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Memory and Project Organization Part 1 319
the I/O modules are configured using RSLogix 5000
software. Modules will not work unless they have been
properly configured. The software contains all the hard-
ware information needed to configure any ControlLogix
module.
Project
RSLogix software stores a controller’s programming and
configuration information in a file called a project. The
block diagram of the processor’s project file is shown in
Figure 15-5. A project file contains all information relat-
ing to the project. The main components of the project file
are tasks, programs, and routines. A controller can hold
and execute only one project at a time.
Figure 15-5 ControlLogix processor program ﬁle.
Project
Controller tags
(global data)
Other routines
Program tags
(local data)
I/O data
System-shared data
Task
Main routine
Program
Figure 15-3 RSLinx and RSLogix software.
RSLogix
RSLinx
File View Communications
RSLinx
RSLogix
RSWho
RSWho
Configure Drivers...
5000
ControlLogix
Controller
Figure 15-4 Controllers properties and modules properties dialog boxes.
Controllers properties
General
Vendor:
Type:
Revision:
Allen-Bradley
1756-L55ControlLogix5555Controller
10.24
Name:
Description:
Chassis Type:
Slot: 1
Controller 1
Prime Controller
1756-A7 7-Slot Chassis
Modules properties
General
Type:
Vendor:
Parent:
1756-IB16 16 Point 10V-31.2V DC Input
Allen-Bradley
Local
Name:
Description:
Comm Form:
Revision: Electronic Keying Exact Match
1
Digital_Input_16pt
Optional
Input Data
Slot: 0
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320 Part 1 Memory and Project Organization
The RSLogix 5000 controller organizer (Figure 15-6)
displays the project organization in a tree format showing
tasks, programs, routines, data types, trends, I/O configu-
ration and tags. Each folder groups common functions
together. This structure simplifies the navigation and the
overall view of the whole project.
In front of each folder, there is an icon containing a
1 sign or a 2 sign. The 1 sign indicates that the folder
is closed. Click it to expand the tree display and display
the files in the folder. The 2 sign indicates that the folder
is already open and its contents are visible. Clicking on
the right mouse button brings up many different, context-
sensitive popup menus. Often, you find that this is a short-
cut to access the property window or menu options from
the menu bar.
Tasks
Tasks are the first level of scheduling within a project. A
task is a collection of scheduled programs. When a task
is executed, the associated programs are executed in the
order listed. This list of programs is known as the program
schedule. Tasks provide scheduling based on specific con-
ditions and do not contain any executable code. Only one
task may be executing at any given time. The number of
tasks a controller can support depends on the specific con-
troller. The main types of tasks (Figure 15-7) include:
• Continuous tasks execute nonstop but are always
interrupted by a periodic task. Continuous tasks
have the lowest priority. A ControlLogix continuous
task is similar to the File 2 in the SLC 500 platform.
Here the continuous task is named Main Task.
• Periodic tasks function as timed interrupts. They
interrupt the continuous task and execute for a fixed
length of time at specific time intervals.
• Event tasks also function as interrupts. Rather than
being an interrupt on a timed basis, an event task
is triggered by an event that happened or failed to
happen.
Programs
Programs are the second level of scheduling within a
project. The function of the folders under Main Task is
to determine and specify the order in which the programs
Figure 15-6 Controller organizer tree.
Controller RSLogix
Tasks
Controller Tags
Program Tags
MainRoutine
Continuous
Program_01
Controller Fault Handler
Power-Up Handler
Program Tags
MainRoutine
Periodic
Unscheduled Programs
Data Types
Motion Groups
Trends
User-Defined
Program_02
Program Tags
Main
HMI
Strings
Predefined
Module-Defined
I/O Configuration
Figure 15-7 Continuous and periodic tasks.
Program 1
Program 2
Program 32
Continuous
task
Continuous task scan
Task
auto-restart
Periodic task scan
5 ms
10 ms
15 ms
...
Program 1
Program 2
Program 32
Periodic
task
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execute. There is no executable code within a program.
Routines within programs will execute in the order listed
below their associated task in the controller organizer as
shown in Figure 15-8. In this example, according to the
listed order, the Main Program is scheduled to execute
first, Program_A second, and Progam_B third. Programs
that are not assigned to a task are unscheduled. Unsched-
uled programs are downloaded to the controller but do
not execute. These programs remain unscheduled until
needed. Depending on the RSLogix 5000 software ver-
sion as many as 100 programs could be scheduled within
each task.
Routines
Routines are the third level of scheduling within a proj-
ect and provide the executable code for the project. Each
routine contains a set of logic elements for a specific
programming language. When a routine is created it is
specified as ladder logic, sequential function chart, func-
tion block diagram or structured text (Figure 15-9). Any
one routine must be completely in the same language.
The number of routines per project is limited only by the
amount of controller memory. Libraries of standard rou-
tines can be created that can be reused on multiple ma-
chines or applications. A routine can be assigned as one
of the following types:
• A main routine is one configured to execute first
when the program runs. Each program will have one
main routine typically followed by several or many
subroutines.
• A subroutine is one that is called by another routine.
Subroutines are used for large or complex program-
ming tasks or tasks that require more than one pro-
gramming language.
• A fault routine is one that executes if the controller
finds a program fault. Each program can have one
fault routine, if desired.
Tags
Unlike conventional controllers, ControlLogix uses a tag-
based addressing structure. Tags are meaningful names,
descriptive of your application and not merely generic
Figure 15-8 Order of execution of programs.
Main Task
Main Program
Program_A
Program_B
Unscheduled Programs
Figure 15-9 Each routine contains a set of logic elements for a speciﬁc programming language.
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addresses. A tag is created to represent the data and iden-
tify areas in the controller’s memory where these data are
stored. In applications developed using Logix 5000 soft-
ware, there are no predefined data tables such as in an
SLC 500. When you want to use or monitor data in a pro-
gram you use tag names to refer to the memory locations,
as illustrated in Figure 15-10. This functionality allows you
to name your data specifically for their functions within the
control program while providing self-documented logic.
Whenever you wish to group data, you create an array,
which is a grouping of tags of similar types.
Scope refers to which programs have access to a tag.
The scope of a tag must be specified when you create the
tag. There are two scopes for tags: program scope and
controller scope. A program tag consists of data that can
be accessed only by routines within a specific program
(local data). The routines in other programs cannot access
program scoped tags of another program. A controller tag
consists of data that are accessible by all routines within
a controller (global data). Figure 15-11 shows two pro-
grams, A and B, within a project. Note that each program
has program scope tags with identical names (Tag_1,
Tag_2, and Tag_3). Because they are program scoped,
there is no relationship between them, even though they
have the same name. The program scope data are acces-
sible only to the routines within a single program. The
same tag name may appear in different programs as local
variables because you can select the scope in which to
create the tag.
The scope of a tag must be declared when you cre-
ate the tag. Figure 15-12 shows program and controller
scoped tags as listed in the controller organizer under the
program they are assigned to. I/O tags are automatically
created as controller scoped tags.
There are four different tag types: base, alias, produced,
and consumed tags. The tag type defines how the tag op-
erates within the project. A base tag stores various types
of data for use by logic in the project. This tag defines a
memory location where data are stored. Base tag memory
use depends on the type of data the tag represents. An
example of the base tag Local:2:O.Data.4 is shown in
Figure 15-13 and is based on the following format:
Location Network location
LOCAL 5 same chassis as the controller
Slot Slot number of I/O module in its chassis
Type Type of data
I 5 input
O 5 output
C 5 configuration
S 5 status
Member Specific data from the I/O module; depends
on what type of data the module can store.
SubMember Specific data related to a Member.
Bit Specific point on a digital I/O module;
depends on the size of the I/O module
(0-31 for a 32-point module)
Figure 15-11 Program scoped and controller scoped tags.
Other routines
Main routine
Program
scoped tags
Tag_1
Tag_2
Tag_3
Program A Program B
Program
scoped tags
Tag_1
Tag_2
Tag_3
Controller Scope Tags
Sensor_1
Temp_1
Other routines
Main routine
Figure 15-10 Tags used to assign memory locations.
Program Tags
Tag 1
Tag 2
Tag 3
Tag 1 Data
Tag 2 Data Memory
locations
Tag 3 Data
Controller memory
Figure 15-12 Listing of program and controller scoped tags.
Controller
scoped
Controller RSLogix
Tasks
Controller Tags
ProgramTags
MainRoutine
Continuous
Program_01
Controller FaultHandler
Power-UpHandler
ProgramTags
MainRoutine
Periodic
Program_02
Program
scoped
Program
scoped
An alias tag is used to create an alternate name (alias)
for a tag. The alias tag is simply another name for an al-
ready named memory location. An alias tag can refer to
a base, alias, consumed, or produced tag. The alias tag is
often used to create a tag name to represent a real-word
input or output. Figure 15-14 shows an example of the
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or consume the data. The producing controller will have
a tag that is of the produced type, whereas the consuming
controllers will have a tag with the exact same name that
is of the consumed type.
When you design your application, you configure it
to both produce globally to other controllers in the sys-
tem via the backplane and to consume tags from other
controllers. This feature allows you to be selective about
which data are sent and received by any controller. Like-
wise, multiple controllers can connect to any data being
produced, thereby preventing the need to send multiple
messages containing the same data.
Logix controllers are based on 32-bit operations. The
types of data that can be a base tag are BOOL, SINT,
INT, DINT, and REAL, as illustrated in Figure 15-16 and
listed below. The controller stores all data in a minimum
of 4 bytes or 32 bits of data.
• A BOOL or Boolean base tag is 1 bit of data stored
in bit 0 of a 4 byte memory location. The other bits,
1 to 31, are unused. BOOLs have a range of 0 to 1,
off or on respectively.
use of an alias tag. The alias tag (Fan_Motor) is linked to
the base tag (local:2:O.Data.5) so that any action to the
base also happens to the alias and vice versa. The alias
name is easier to understand and easier to relate to the ap-
plication, while the base tag contains the physical location
of the output point in the ControlLogix chassis.
Produced/consumed tags are used to share tag informa-
tion over a network between two or more devices. A pro-
duced tag sends data while a consumed tag receives data.
Produced tags are always controller scoped. Figure 15-15
shows an example of how a controller can produce data
and send them over the network to two controllers that use
Figure 15-14 Alias tag linked to a base tag.
Fan_Motor Local:2:O.Data.5 Controller memory
Tag data
Base tag
Alias tag
Fan_Motor
Local:2:O.Data.5
Figure 15-13 Base tag.
Optional
Optional
Location
Base tag
Controller
memory
Tag data
Local:2:O.Data.4
:Slot :Type .Member .SubMember .Bit
Format
Local :2 :O .Data .4
Optional
Tag Name
Controller Scoped Tags
Data Type
Local:2:O.Data BOOL
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31
Figure 15-15 Produced/consumed tags used to share information.
Consumed tag
Produced tag
Producer/consumer I/O model
Network
Output modules
Logix5500
controllers
Commonly shared data
Input modules
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• A SINT or Single Integer base tag uses 8 bits of
memory and stores the data in bits 0 to 7. These
bits are sometimes called the low byte. The other
3 bytes, bits 8 to 31, are unused. SINTs have a
range of 2128 (negative values) to 127 (positive
values).
• An INT or Integer base tag is 16 bits, bits 0 to 15,
sometimes called the lower bytes. Bits 16 to 31 are
unused. INTs have a range between 231,768 and
32,767.
• A DINT or Double Integer base tag uses 32 bits,
or all 4 bytes, and has the following range: 2231
to
231
21 (22,147,483,648 to 2,147,483,647).
• A REAL base tag also uses 32 bits of a memory
location and has a range of values based on the
IEEE Standard for Floating-Point Arithmetic.
Structures
There is another class of data types called structures. A
structure-type tag is a grouping of different data types
that function as a single unit and serve a specific purpose.
An example of an RSLogix structure is shown in Fig-
ure 15-17. Each element of a structure is referred to as a
member and each member of a structure can be a different
data type.
Figure 15-16 Types of base tag data.
BOOL
0
31
Unused
SINT
0
31 7
Unused
INT
0
31 15
Unused
DINT
0
31
REAL
0
31
Figure 15-19 Module-deﬁned structure for a digital input
module.
Figure 15-17 Structure-type tag.
Name
PRE
ACC
EN
TT
DN
FS
LS
OV
ER
DINT Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
DINT
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
Data Types Style Description
Data Types
Members
Figure 15-18 Predeﬁned structure.
Name
Description
Counter
Data type : COUNTER
Data type size : 12 byte(s)
Members
PRE
ACC
CU
CD
DN
OV
UN
DINT
DINT
BOOL
BOOL
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Data Type Style
Name Description
There are three different types of structures in a Control-
Logix controller: predefined, module-defined, and user-
defined. The controller creates predefined structures for
you that include timers, counters, messages and PID
types. An example of a predefined counter instruction
structure is shown in Figure 15-18. It is made up of the
preset value, the accumulated value, and the instruction’s
status bits.
Module-defined structures are automatically created
when the I/O modules are configured for the system.
When you add input or output modules a number of
defined tags are automatically added to the controller
tags. Figure 15-19 shows the two tags (Local:1:C and
Local:1:I) created after a digital input module has been
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in rpm. Installation and maintenance personnel can easily
locate all data associated with the operation of the tank
since all the information is stored together.
Creating Tags
There is more than one way to create tags.You may create
tags in the tag editor before your program is entered, enter
tag names as you program, or use question marks [?] in
place of tag names and assign the tags later. Figure 15-21
shows an example of a controller scope base tag created in
the new tag dialog box. When defining tags, the following
information has to be specified:
• A tag name, which must begin with an alphabetic
character or an underscore (_). Names can contain
only alphabetic characters, numeric characters,
or underscores and may be up to 40 characters in
length. They may not have consecutive or trailing
underscore characters, are not case sensitive and
cannot have spaces in the tag name.
• An optional tag description, which may be up to
120 characters in length.
• The tag type: base, alias, or consumed.
• The data type, which is obtained from the list of
predefined or user-defined data types.
• The scope in which to create the tag. Your options
are the controller scope or any one of the existing
program scopes.
• The display style to be used when monitoring the
tag in the programming software. The software will
display the choices of available styles.
• Whether or not you want to make this tag available
to other controllers and the number of other control-
lers that can consume the tag.
added. Tags of these types are created to store input, out-
put, and configuration data for the module. Input tags
labeled Data contain the actual input bits from the mod-
ule. Configuration tags determine the characteristics and
operation of the module. The name Local indicates that
these tags are in the same rack as the processor. The 1
indicates that the module occupies slot 1 in the chassis.
The letters I and C indicate whether the data are input
data or configuration data.
A user-defined structure supplements the predefined
structures by providing the ability to create custom-
defined structures to store and handle data as a group.
Figure 15-20 illustrates a user-defined structure that con-
tains data for a storage tank. All data relative to the tank
are stored together. In the design stage the programmer
creates a generic user-defined memory structure that con-
tains all the different aspects of the storage tank. Each
member has a meaningful name and is created in the ap-
propriate data type and style like REAL (floating point)
for temperature and DINT (decimal) for agitator speed
Figure 15-21 Controller scope base tag.
New Tag
OK
All_Stop
Name:
BOOL
Data Type:
Base
Tag Type:
Consumed
Produced
Alias
Controller1(controller)
Scope:
Controller controller1
Binary
Style:
This is the Line Stop pushbutton
input.
Description: Cancel
Help
Configure
consumers
1
···
Controller tags
New Tag... Ctrl+W
Print Ctrl+P
Monitor Tags
Edit Tags
Verify
Export Tags...
···
Figure 15-20 User-deﬁned storage tank structure.
Tank
Level
Pressure
Temp
Agitator_Speed
INT
DINT
REAL
DINT
Decimal
Decimal
Float
Decimal
Stores the Level in Inches
Stores the Pressure in PSIG
The Temperature in F
Speed in RPM
Name:
Description:
Size: byte(s)
16
Data Type Style
Name Description
Generic Storage Tank Data Type
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is selected new tags may be created, and existing tag
properties may be modified.
Array
Many control programs require the ability to store blocks
of information in memory in the form of tables that can
be accessed at runtime. An array is a tag type that con-
tains a block of multiple pieces of data. Each element
Monitoring and Editing Tags
After tags have been created they can be monitored using
the Monitor Tags window displayed in Figure 15-22.
When Monitor Tags is selected the actual value(s) for the
tags will be shown. The Force Mask column is used to
force inputs and outputs when troubleshooting. You can
also create new tags or edit existing tags using the Edit
Tags window displayed in Figure 15-23. When Edit Tags
Figure 15-22 Monitor Tags window.
Scope:
All_Stop Decimal
Binary
-Local:2:C
-Local:2:I
Show: Sort:
Controller1(controller) Show All Base Tag
+
+
Tag Name Value
0
{ . . . }
{ . . . }
{ . . . }
{ . . . }
2#0000_0000
Section_3_Run
Force Mask Style
Monitor Tags Edit Tags
Figure 15-23 Edit Tags window.
All_stop BOOL
AB:1756_DI:C:0
AB:1756_DI:I:0
BOOL
-Local:2:C
-Local:2:I
Controller1(controller) Show All Base Tag
+
+
Tag Name Alias For
Local:2:I.Data.0 Local:2:I.Data.0
Section_3_Run
Base Tag Type
P
Monitor Tags Edit Tags
Scope: Show: Sort:
Figure 15-24 Types of arrays.
OK
OK
Cancel
Help
Find match
Selection
DINT[6,3,2]
Array Dimensions
Dim 0
6
Dim 1
3
Dim 2
2
BOOL
CONTROL
COUNTER
DINT
INT
Load
Load_info
MESSAGE
Data Types
Select Data Type
3-dimensional
Block [6,3,2]
2-dimensional
Grid [6,3]
1-dimensional
Table [6]
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Figure 15-25 Memory layout for a 1-dimensional array.
Array - Temp
Data Type - INT[5]
Temp[0] 297
Temp[1] 200
Temp[2] 180
Temp[3] 120
Temp[4] 100
of an array must be of the same data type (e.g., BOOL,
SINT, or INT). An array occupies a contiguous block
of controller memory. Arrays are similar to tables of
values. The use of arrayed data types offers the fastest
data throughput (output) from a ControlLogix processor.
Because arrays are numerically sequenced tags of the
same data that occupy a contiguous memory location,
large amounts of data can be retrieved efficiently. Arrays
can be built using 1, 2 or 3 dimensions, as illustrated in
Figure 15-24, to represent the data they are intended to
contain.
A single tag within the array is one element. The ele-
ment may be a basic data type or a structure. The ele-
ments start with 0 and extend to the number of elements
minus 1. Figure 15-25 is an example of the memory
layout for a 1-dimensional (one column of values)
array created to hold five temperatures. The tag name
is Temp and the array consists of 5 elements numbered
0 through 4.
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1. Compare the memory configuration of a Logix
5000 controller with that of an SLC 500 controller.
2. What does a project contain?
3. List four programming functions that can be car-
ried out using the program organizer.
4. Explain the function of tasks within the project.
5. State the three main types of tasks.
6. What type of tasks function as timed interrupts?
7. Explain the function of programs within the
project.
8. Explain the function of routines within the project.
9. Which routine is configured to execute first?
10. Name the four types of programming languages
that can be used to program Logix 5000 controllers.
11. What are tags used for?
12. Compare the accessibility of program scope and
controller scope tags.
13. Name the tag type used for each of the following:
a. Create an alternate name for a tag.
b. Share information over a network.
c. Store various types of data.
PART 1 REVIEW QUESTIONS
14. What is the difference between a produced tag and
a consumed tag?
15. List the five types of base tag data.
16. State the data type used for each of the following:
a. 32-bit memory storage
b. On/Off toggle switch
c. 16-bit memory storage
d. 8-bit memory storage
17. Describe the make-up of a predefined structure.
18. Describe the make-up of a module-defined
structure.
19. Describe the make-up of a user-defined structure.
20. Explain two ways of creating tags.
21. When defining tags what limitations are placed on
the entering of a tag name?
22. What is meant by the tag display style?
23. Write an example of an array tag used to hold
4 speeds.
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329
• The status signals from the inputs are sent to the
input tags where they are stored.
• As the program is scanned by the processor, inputs
are checked for True or False conditions and the
ladder logic is evaluated based on these values.
• The resulting ON or OFF action, as a result of
evaluating each rung, is then sent to the output tags
for storage.
• During the output update portion of the scan, cor-
responding output values are sent to the process or
machine by way of the output module.
Program Scan
When a CLX controller executes a program, it must
know—in real time—when external devices controlling
a process are changing. During each operating cycle,
the processor reads all the inputs, takes these values,
and energizes or de-energizes the outputs according to
the user program. This process is known as the program
scan.
Figure 15-26 illustrates the signal flow into and out of
a Logix controller during a controller’s operating cycle
when ladder logic is executing. During the program scan,
the controller reads rungs and branches from left to right
and top to bottom as follows:
• Only one rung at a time is scanned.
• As the program is scanned, the status of inputs are
checked for True (1 or ON) or False (0 of OFF)
conditions.
Part 2 Bit-Level
Programming
Part Objectives
• Know what happens during the program scan
• Demonstrate an understanding of input, output, and
internal relay addressing format for a tag-based Logix
controller
• Develop ladder logic programs with input instructions
and output coil combinations
• Develop ladder logic programs with latched outputs
Figure 15-26 Logix controller operating cycle.
PLC
processor
Input tags
Ladder logic
program
Field output
devices
Field input
devices
Output
tags
PLC
input
module
Field
power
supply
Field power
supply
PLC
output
module
M
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• When a rung has only one output instruction it will
always be true.
• The last instruction on a rung must always be an
output instruction.
• The XIC, or Examine If Closed contact instruction,
checks to see if the input has a value of one. If the
input is one, the XIC instruction returns a true value.
• The XIO, or Examine If Open contact instruction,
checks to see if the input has a value of zero. If the
input is zero, the XIO instruction returns a true value.
• The OTE or Output Energize coil instruction sets
the tag associated with it to true or one when the
rung has logic continuity. When true it can be used
to energize an output device or simply set a value in
memory to one.
ControlLogix PLCs support multiple outputs on one
rung. CLX controllers allow the use of serial logic that
does not conform to traditional electrical hardwired cir-
cuits or ladder logic. For example, both of the rungs shown
in Figure 15-28 are valid in RSLogix 5000. However the
series connection of outputs would not work if wired that
way in an equivalent electrical circuit or programmed that
way in RSLogix 500. In both instances in RSLogix 5000,
instructions tagA and tagB must be true to energize output
tag1 and tag2.
In ControlLogix output instructions can be placed be-
tween input instructions as illustrated in Figure 15-29. In
this example instructions tagA and tagB must be true to
energize output tag1. Instructions tagA and tagB and tagC
must all be true before output tag2 is set to energize.
• I/O updates occur asynchronously to the scan
of the logic. With a ControlLogix processor two
separate 32-bit unsynchronized processes gone
on simultaneously—that is, asynchronously.
This means that the module can update the input
tag from the field and write the output tag to the
field at any point (or at several points) during the
processor’s execution of the ladder rungs. The
result is more efficiency and control over when the
input field device data are updated in the input tag
and when the output data resulting from the solved
logic are sent to the output modules and their
respective field devices.
Creating Ladder Logic
Although other programming languages are available,
ladder logic is the most common programming language
for PLCs. The instructions in ladder logic programming
can be divided into two broad categories: input and out-
put instructions. The most common input instruction is
equivalent to a relay contact and the most common output
instruction is the equivalent of a relay coil (Figure 15-27).
When creating ladder I/O bit instructions, the following
rules apply:
• All input instructions must be to the left of an out-
put instruction.
• A rung cannot begin with an output instruction if it
also contains an input instruction. This is because
the controller tests all inputs for true or false before
deciding what value the output instruction should be.
• A rung does not need to contain any input
instructions, but it must contain at least one output
instruction.
Figure 15-27 Contacts and coil instructions.
If the Data
Bit is:
Logic 0
Logic 1
False
True
XIC
instruction is:
XIC Inputs
If the Data
Bit is:
Logic 0
Logic 1
True
False
XIO
instruction is:
XIO Output
OTE
If the Data
Bit is:
Logic 0
Logic 1
False
True
OTE
instruction is:
Figure 15-28 Parallel and series outputs.
tag1
tag2
tagB
tagA
Parallel outputs
tag1 tag2
tagB
tagA
Series outputs
Figure 15-29 Output instruction placed between input
instructions.
tag1 tag2
tagB
tagA tagC
330 Part 2 Bit-Level Programming
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Tag-Based Addressing
Logix 5000 controllers use a tag-based addressing struc-
ture. A tag is a text-based name for an area of the control-
ler where data is stored. An example of how a tag-based
address is implemented using a ControlLogix controller
is shown in Figure 15-30. Tag names use a meaningful
description of the variable. In this application when the
normally closed high limit switch is activated the program
will switch the high limit output light on. The addressing
format can be summarized as follows:
• The physical address for the tag Limit_switch is
Local:1:I.Data.2(C). Local indicates that the module
is in the same rack as the processor, 1 indicates that
the module is in slot 1 in the rack, I indicates that
the module is an input type, Data indicates that it
is a digital input, 2 indicates that the limit switch is
connected to terminal 2 on the module, and C indi-
cates that it is a controller tag with global access.
• The physical address for the tag High_limit_light is
Local:2:O.Data.4(C). Local indicates that the module
is in the same rack as the processor, 2 indicates that
the module is in slot 2 in the rack, O indicates that
the module is an output type, Data indicates that it is
a digital input, 4 indicates that the high limit light is
connected to terminal 4 on the module, and C indicates
that it is a controller tag with global access.
One advantage of the use of tag-based addressing is
that the allocation of variable names for program values
is not tied to specific memory locations in the memory
structure, as is the case with rack/slot and rack/group type
Figure 15-30 Tag-based address implementation.
Power
supply
0
TAG EDIT
1 2 3 4 5 6
8-point
discrete
(digital)
input
module
Limit_switch
High_limit_light
Switch - closed
Processor
module
Program
7-slot
ControlLogix
chassis
8-point
discrete
(digital)
onput
module
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
Tag Name
Limit_switch
High_limit_light
Alias For
Local:1:I.data.2(C)
Local:2:O.data.4(C)
Base Tag
Local:1:I.data.2(C)
Local:2:O.data.4(C)
Type
BOOL
BOOL
Style
Decimal
Decimal
Limit_switch
Local:1:I.data.2(C)
High_limit_light
Local:2:O.data.4(C)
TAG MONITOR
Tag Name
Limit_switch
High_limit_light
Style
Decimal
Decimal
Type
BOOL
BOOL
Value
1
0
Light-OFF
Bit-Level Programming Part 2 331
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before adding the next instruction. In this example we have
chosen to use question marks [?] in place of tag names and
assign the tags later. Field device wiring for the two push-
button inputs and the single contactor coil output are as
illustrated. The stop button is connected to terminal 3 and
the start button to terminal 4 of the DC input module lo-
cated in slot 1 of the rack. The contactor coil is connected
to terminal 4 of the DC output module located in slot 2
of the rack. Both the start and stop buttons are examined
for a closed condition (XIC) because both buttons must be
closed to cause the motor starter to operate.
systems. Initially, all program development can proceed
with just the tag names and data types assigned. Using tag
aliases, programmers can write code independent of elec-
trical connection assignments. At a later date, input and
output field devices are easily matched to the pin numbers
on the respective module they are connected to.
Adding Ladder Logic to the
Main Routine
Figure 15-31 shows the diagram for a hardwired contac-
tor operated motor start/stop control circuit. The normally
open start button is momentarily closed to energize the
contactor coil and close its main contacts to start the motor.
The seal-in auxiliary contact of the contactor is connected
in parallel with the start button to keep the starter coil
energized when the start button is released. The normally
closed start button is momentarily opened to de-energize
the contactor coil and stop the motor.
Figure 15-32 shows the ladder logic program for the
motor start/stop control circuit and the RSLogix 5000 tool-
bar used to create it. Free form editing found in RSLogix
5000 helps speed development in that you do not have to
place an instruction and tie an address to the instruction
Contactor auxiliary
contact
Contactor coil
Motor
run
Motor
stop
Motor
start
L1 L2
M
Figure 15-31 Hardwired motor start/stop control circuit.
Figure 15-32 Programmed motor start/stop control circuit.
ⴙ ⴙ
Common Common
Rung
Branch
Bit element toolbar
M
3 4
Field
device
power
4
Terminal Terminal
Contactor
Stop
Start
?
?
? ?
Input module
Slot 1
Output module
Slot 2
ONS OSR
Bit
L U
XIC XIO OTE OTL OTU
Field
device
power
ST 0 1 2 3 4 5 6 7 ST 0 1 2 3 4 5 6 7
DC OUTPUT
DC INPUT
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With text-based Logix systems you can use the name
of the tag to document your ladder code and organize
your data to mirror your application. For the programmed
motor start/stop control circuit three tags Motor_Start,
Motor_Stop, and Motor_Run are created. Figure 15-33
illustrates how the Motor_Start tag is created in the New
Tag window. This window can be accessed by right
clicking the ? mark above the XIC instruction in the lad-
der logic program. Since this tag represents a value from
an input field device a link through the module to the
field device must be created. When Local:1:I.Data is se-
lected a dialog box for all of the terminal numbers on the
input module appears. The tag name (Motor_Start) used
in the program is then linked to input terminal number 3
where the field device represented by the tag name is
connected.
Figure 15-34 shows what the ladder logic program
would look like after all three tags have been created.
Users have the ability to reference data via multiple names
using Aliases. This allows the flexibility to name data dif-
ferently depending on their use. The tag description pro-
vides for a more meaningful description of the tag name.
Tag names are downloaded and stored in the controller
but the description is not as it is part of the documentation
of the project.
Figure 15-35 shows the state of the tags created for
the motor start/stop program as seen in the program and
Figure 15-33 Creating the Motor_Start tag.
New Tag
Alias
Local:1:I.Data
Motor_Start
Name:
Start button for
motor
Description:
Name
Local:1:C
Local:1:I
Local:1:I.Fault
Local:1:I.Data
DINT
AB:1756_DO:C:0
AB:1756_DO:I:0
Data Type
+
–
0
8
16
24
1
9
17
25
2
10
18
26
3
11
19
27
5
13
21
29
6
14
22
30
7
15
23
31
4
12
20
28
Figure 15-34 Ladder logic program after all tags have been created.
Motor contactor coil
Motor_Run
Local:2:O.Data.4
Motor_Run
Local:2:O.Data.4
Stop button for
motor
Motor_Stop
Local:1:I.Data.4
Start button for
motor
Motor_Start
Local:1:I.Data.3
Description
Tag Name
Alias
Figure 15-35 Ladder logic program and Monitor Tags window with motor operating.
Motor_Run
Local:2:O.Data.4
Motor_Run
Local:2:O.Data.4
Stop button for
motor
Motor_Stop
Local:1:I.Data.4
Inputs Output
Contactor
Start button for
motor
Motor_Start
Local:1:I.Data.3
Stop
Start Monitor Tags Window
Tag Name Value Style Data Type
Motor_Start 0 Decimal BOOL
Description
Start button for motor
Motor_Stop 1 Decimal BOOL Stop button for motor
Motor_Run 1 Decimal BOOL Motor contactor coil
M
L2
L1
Motor_Stop
Motor_Start
Motor_Run
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an equivalent hardwired control circuit. The operation of
• An internal relay is used to execute the logic of
the circuit without having to use a real-world
output.
• The status value stored in memory for all tags, when
all input switches are open, is 0 and so the room
light will be off.
• Closing Position_1_Switch changes the status of its
XIC instruction from false to true thereby establish-
ing logic continuity for Rung 1.
• As a result, the status of the internal relay coil and
its XIC contact change from false to true.
• This establishes logic continuity for Rung 2 and
switches the room light on.
• A change in the state of any of input switches will
change the current state of the light.
Latch and Unlatch Instructions
The output latch (OTL) instruction is a retentive output
instruction that is used to maintain, or latch, an output. If
this output is turned on, it will stay on even if the status
Monitor Tags window, when the motor is operating. When
the motor is operating:
• The XIC Motor_Start instruction is false because
the NO start button is open; therefore its value is 0.
• The XIC Motor_Stop instruction is true because the
NC stop button is closed; therefore its value is 1.
• The OTE Motor_Run instruction is true because the
rung has logic continuity; therefore its value is 1.
Internal Relay Instructions
Internal relay instructions are used when other than real-
world field devices are needed as input or output reference
instructions. For example, an internal relay bit is used as
an output when the logical resultant of a rung is used to
control other internal logic. An internal control relay is
programmed in the ControlLogix system by creating a tag
(either program or controller type) and assigning a Boolean
type to the tag.
Figure 15-36 shows a ControlLogix program that uses
an internal relay to implement on/off control of a room
light from three different entrances or positions. Three
single pole switches are used for inputs in place of the
two 3-way and one 4-way switches normally required for
Figure 15-36 Internal relay to implement on/off control of a room light from three different entrances.
Name Value Force Mask
Monitor Tags Window
Style Data Type
Internal_Relay 0 Decimal BOOL
Position_1_Sw... 0 Decimal BOOL
Position_2_Sw... 0 Decimal
Decimal
BOOL
0 BOOL
Position_3_Sw...
Decimal
0 BOOL
Room_Light
Position_1_Switch
Local:1:I.Data.1
L
Position_2_Switch
Local:1:I.Data.2 Internal_Relay
Position_2_Switch
Local:1:I.Data.2
Position_1_Switch
Local:1:I.Data.1
Internal_Relay
Position_3_Switch
Local:1:I.Data.3
Room_Light
Local:2:O.Data.5
Position_3_Switch
Local:1:I.Data.3 Internal_Relay
Output
Inputs
L2
L1
Room_Light
Position_1_Switch
Position_2_Switch
Position_3_Switch
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of the input logic that caused the output to energize be-
comes false. The OTL instruction will remain in a latched
on condition until an unlatch instruction (OTU) with the
same referenced tag is energized. The OTL instruction is
often used in programs where the value of a variable must
be maintained in instances where there is a shutdown due
to a power failure or system fault. Retentive memory per-
mits the system to be restarted with memory locations
holding the values that were present when the program
execution was halted.
Figure 15-37 shows a ControlLogix program that uses
an output latch and unlatch instruction pair to implement
the control of a vent fan motor. The operation of the pro-
• The OTL instruction will write a 1 to its address
when true.
• When the OTL goes false, the output address will
remain a 1.
• This is true even if the processor powers down and
then back up.
• The output address will remain a 1 until reset to 0
by the unlatch instruction.
• If the output address is off, both the latch and un-
latch instructions are not intensified, but once the bit
is turned on, you will see both the latch and unlatch
intensified even though both inputs are shut off.
One-Shot Instruction
The CLX One-Shot (ONS) instruction is an input instruc-
tion used to turn an output on for one program scan only.
The program of Figure 15-38 uses the ONS instruction
with a math instruction to perform a calculation once per
scan. This program is used to execute theADD math func-
tion only once per actuation of the limit switch, no matter
how long the limit switch is held closed. The operation of
• On any scan for which limit_switch_1 is cleared or
storage_1 is set, this rung has no effect.
• On any scan for which limit_switch_1 is set and stor-
age_1 is cleared, the ONS instruction sets storage_1
and the ADD instruction increments sum by 1.
• As long as limit_switch_1 stays set, sum stays
the same value. The limit_switch_1 must go from
cleared to set again for sum to be incremented again.
Figure 15-37 Output latch and unlatch instructions used to control a vent
fan motor.
Vent_Fan
Local:2:O.Data.4
Fan_OFF_Button
Local:1:I.Data.3
Inputs Output
Vent_Fan
Local:2:O.Data.4
L
U
Fan_ON_Button
Local:1:I.Data.2
ON
OFF
Monitor Tags Window
Fan_ON_Button 0 Decimal BOOL
Fan_OFF_Button 0 Decimal BOOL
Vent_Fan 1 Decimal BOOL
M
L2
L1
Fan_ON_Button
Fan_OFF_Button
Vent_Fan
Figure 15-38 ONS instruction used to perform a calculation once per scan.
Input
L1
Limit_Switch_1
Limit_Switch_1
Local:1:I.Data.6 Storage_1
ONS
ADD
Add
Source A
Source B
Sum
0
1
Dest Sum
0
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1. What operations are performed by the processor
during the program scan?
2. With a ControlLogix processor I/O updates occur
asynchronously. Explain what this means.
3. In ladder logic programming into what two broad
categories can instruction types be classified?
4. A field input switch is examined using an XIC
instruction.
a. What is the value (0 or 1) stored in its memory
bit when the switch is opened and closed?
b. What is the state of the instruction (true or false)
when the switch is opened and closed?
5. A field input switch is examined using an XIO
instruction.
a. What is the value (0 or 1) stored in its memory
bit when the switch is opened and closed?
b. What is the state of the instruction (true or false)
when the switch is opened and closed?
6. The value of an OTE instruction as it appears in
the Monitor Tags window is 1. Explain what this
means as far as the status of a real-world field out-
put and programmed XIC and XIO instructions
associated with this tag are concerned.
7. Define a tag in the ControlLogix system.
8. What advantage do tag-based addressing systems
have over rack/slot and rack/group types?
9. How is an internal relay programmed in the
ControlLogix system?
10. The output latch instruction is a retentive output
instruction. Explain what retentive means.
11. The ControlLogix ONS instruction is a one-shot
instruction. Explain what this means.
1. Modify the original ControlLogix start/stop motor
control program with a second start and stop button
added to the program. The additional start button is
to be connected to pin 1 and the stop button to pin 2
of the digital input module.
2. Extend control of the original ControlLogix inter-
nal relay program used to control a room light from
3 entrances to 4. The additional single-pole switch
is to be connected to pin 4 of the digital input
module.
3. Implement the hardwired latching relay alarm circuit
of Figure 15-39 in Logix format. The alarm will be
latched on anytime:
• The normally open temperature switch closes.
• Both normally open float switches 1 and
2 close.
• Either normally open sensor switch 1 or 2 closes
while the normally closed pressure switch is closed.
4. Implement the hardwired tank filling and emptying
operation shown in Figure 15-40 in Logix format.
PART 2 PROBLEMS
Figure 15-39 Hardwired latching relay alarm circuit
for Problem 3.
Relay
contact
Latch
coil
120 VAC
Reset button
24 VDC
Temp Sw
Float Sw 1
Sensor Sw 1
Sensor Sw 2
Float Sw 2
Pressure
Sw
Alarm
L
Unlatch
coil
U
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Solenoid B
Stop
Fill
1CR1
2CR1
2CR2
1CR2
Full tank
sensor
Empty tank
sensor
L1 L2
Empty
1CR
2CR
Solenoid A
Full tank
sensor
Empty tank
sensor
Control panel
Stop
Fill
Empty
Solenoid B
Solenoid A
Figure 15-40 Hardwired tank ﬁlling and emptying operation for Problem 4.
Source: Photo courtesy ASCO Valve Inc., www.ascovalve.com.
• Anytime the liquid level of the tank is above
the empty-level mark, momentarily pressing
the EMPTY pushbutton will energize control
relay 2CR.
• Contacts 2CR1
and 2CR2
will both close to seal
in the 2CR coil and energize normally closed
solenoid valve B to start emptying the tank.
• When the liquid reaches the empty level, the
normally open empty-level sensor switch opens to
open the circuit to the 2CR relay coil and switch
solenoid valve B to its de-energized closed state.
• The stop button may be pressed at any time to halt
the process.
The operation of the control circuit can be summa-
rized as follows:
• Assuming the liquid level of the tank is at or below
the empty level mark, momentarily pressing the
FILL pushbutton will energize control relay 1CR.
• Contacts 1CR1
and 1CR2
will both close to seal
in the 1CR coil and energize normally closed
solenoid valve A to start filling the tank.
• As the tank fills, the normally open empty-level
sensor switch closes.
• When the liquid reaches the full level, the normally
closed full-level sensor switch opens to open the
circuit to the 1CR relay coil and switch solenoid
valve A to its de-energized closed state.
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338
number (DINT). The time base is always 1 msec.
For example, for a 3 second timer, enter 3000 for
the PRE value.
• Accumulator (ACC)—The accumulator value is
the number of milliseconds the instruction has been
enabled. The accumulator value stops changing
when ACC value 5 PRE value.
• Enable Bit (EN)—The enable bit indicates the
TON instruction is enabled. The EN bit is true when
the rung input logic is true, and false when the rung
input logic is false.
• Timer Timing Bit (TT)—The timing bit indicates
that a timing operation is in process. The TT bit is
true only when the accumulator is incrementing.
TT remains true until the accumulator reaches the
preset value.
• Done Bit (DN)—The done bit indicates that ac-
cumulated value (ACC) is equal to the preset (PRE)
Timer Predeﬁned Structure
Timers are used to turn outputs on and off after a time
delay, turn outputs on or off for a set amount of time, and
keep track of the time an output is on or off. The timer
address in the SLC 500 controller is a data table address
or symbol, whereas the timer address in the ControlLogix
controller is a predefined structure of the TIMER data
type. The TIMER structure is shown in Figure 15-41.
Timer parameters and status bits include:
• Tag Name—User-friendly tag name for the timer
(e.g., Pump_Timer). If you want to use a timer, you
must create a tag of type timer.
• Preset (PRE)—The number of time increments that
the timer must accumulate to reach the desired time
delay. Specifies the value (in milliseconds) which
the timer must reach before the done bit (DN)
changes state. The preset value is stored as a binary
Part Objectives
• Understand ControlLogix timer tags and their members
• Utilize status bits from timers in logic
• Develop ladder logic programs using ControlLogix
timers
Part 3 Programming Timers
Figure 15-41 TIMER predeﬁned structure.
Data Type: TIMER
Name
Members: Data Type Size: 12 byte(s)
Data Type Style Description
PRE
ACC
EN
TT
DN
DINT Decimal
DINT
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
Name:
Description:
Pump_Timer
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value. The DN bit signals the end of the timing pro-
cess by changing states from false-to-true or from
true-to-false depending on the type of time contact
instruction used. The DN bit is the most commonly
used timer status bit.
On-Delay Timer (TON)
The on-delay timer (TON) is a nonretentive output in-
struction used when the application requires an action to
occur at some time after the rung conditions for the timer
become true. The ControlLogix TON on-delay instruction
and timer selection toolbar are shown in Figure 15-42.
When you want to use a timer, you must create a tag of
type TIMER (it is a predefined data type) and enter the
preset and the accumulated value. The tag must be defined
before the preset and accumulated values can be entered.
A value can be entered for the accumulator while pro-
gramming. When the program is downloaded this value
will be in the timer for the first scan. If the TON timer is
not enabled the value will be set back to zero. Normally
zero will be entered for the accumulator value.
The timer tag name is declared using the new tag prop-
erties dialog box shown in Figure 15-43. Tag name, de-
scription (optional), tag type, data type, and scope are
selected or typed to complete the validation. A descriptive
tag name, such as Solenoid_Delay, makes it easier to know
what function the timer serves in the control system.
The program of Figure 15-44 is an example of a 10000 ms
(10 s) TON timer. Timers generate both word level (DINT)
and bit level (BOOL) data and status. The operation of the
program can be summarized with reference to the Moni-
tor Tags window.
• The status of all instruction is shown after the timer
input switch has been switched from off to on (1)
and accumulated 5000 ms (5 s) of time.
• At this halfway point the EN bit is 1 since the rung
is true, the TT bit is 1 since the accumulated value is
changing, and the DN bit is 0 since the accumulated
value does not yet equal the preset value.
• When the ACC equals PRE, the accumulated value
stops incrementing, EN stays on for as long as the
rung remains true, TT equals 0 since the accumu-
lated value is not changing, and DN equals 1 since
ACC 5 PRE.
• This will result in the DN pilot light switching on at
the same time as the TT pilot light switches off.
• The EN pilot light remains on as long as the input
switch is closed.
• Opening the input switch at any time causes the
TON instruction to go false resetting the counter
ACC value to 0 and EN, TT, and DN bits to 0. This
in turn switches off all output pilot lights.
• The TON instruction is a self-resetting timer. When
the rung goes false, the timer is automatically reset.
A reset instruction can be used, but usually is not.
Figure 15-45 shows a TON timer used to delay the op-
eration of a diverter gate solenoid for 3 seconds after a tar-
get has been sensed by the solenoid energize sensor. The
• Detection of the target causes closure of the SOL_
Energize_Sensor contacts making the timer rung
true and start timing.
• With passage of the target the SOL_Energize_
Sensor contacts open but the rung remains true
through the EN bit of the TON timer.
• After 3000 ms (3 s) delay time has elapsed, delay
timer DN bit is set to 1 to energize the SOL_Gate.
Figure 15-42 TON on-delay instruction.
TON TOF RTO
Add-On
Favorites Alarms Bit
CTU CTD RES
Timer/Counter
Tag name
Input side of rung
TIMER ON DELAY
Timer
Preset
Accum
Solenoid_Delay
3000
0
TON
EN
DN
Figure 15-43 Timer tag validation.
Diverter gate
solenoid delay timer
TIMER ON DELAY
Timer
Preset
Accum
Solenoid_Delay
3000
0
TON
EN
DN
Programming Timers Part 3 339
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Figure 15-45 TON timer used to delay the operation of a diverter gate solenoid.
SOL_Energize_Sensor 0
{…}
3000 Decimal
Decimal
DINT
DINT
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
0
0
0
0
BOOL
SOL_Deenergize_Sensor 1 Decimal BOOL
SOL_Gate 0 Decimal BOOL
TIMER
T_SOL_Delay
T_SOL_Delay.PRE
T_SOL_Delay.ACC
T_SOL_Delay.EN
T_SOL_Delay.TT
T_SOL_Delay.DN
L1
Inputs
L2
Output
SOL_Energize_Sensor
Local:1:I.Data.3
SOL_De-energize_Sensor
Local:1:I.Data.6
T_SOL_Delay.EN
T_SOL_Delay
TON
TIMER ON DELAY
Timer
Preset
Accum
3000
0
EN
DN
T_SOL_Delay.DN
SOL_Gate
Local:2:O.Data.2
SOL_Energize_Sensor
SOL_Deenergize_Sensor
SOL_Gate
Figure 15-44 Ten-second TON timer program.
DN
Timer_Sw 1
{…}
10000 Decimal
Decimal
DINT
DINT
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
500
1
1
0
BOOL
EN_PL 1 Decimal BOOL
TT_PL 1 Decimal
Decimal
BOOL
TIMER
-Status_Timer
Status_Timer.PRE
Status_Timer.ACC
Status_Timer.EN
Status_Timer.TT
Status_Timer.DN
0 BOOL
DN_PL
Timer_Sw
Local:1:I.Data.6
Status_Timer
TON
TIMER ON DELAY
Timer
Preset
Accum
10000
5000
EN_PL
Local:2:O.Data.1
Status_Timer.EN
Status_Timer.TT
Status_Timer.DN
EN EN
TT_PL
Local:2:O.Data.2
DN_PL
Local:2:O.Data.3
Outputs
Input
L2
L1
EN_PL
Timer_Sw
TT
TT_PL
DN
DN_PL
340 Part 3 Programming Timers
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• When the button is then opened the timer rung
remains true through the logic path created by the
Pilot_Light_Timer.EN bit.
• After 20000 ms (20 s) have elapsed the timer DN
bit is set to reset the timer to its original state and
unlatch the Green_PL and switch it off.
The ControlLogix program of Figure 15-47 shows
three TON timers cascaded (connected together) for traf-
fic light control. The ladder logic used is the same as that
used to program the traffic lights using the SLC 500 con-
troller. The different tags created to fit the program are
• Momentary detection of the target by the SOL_
Deenergize_Sensor causes the opening of its con-
tacts and resets the program to its original state.
Figure 15-46 shows a program that uses a TON timer to
illuminate a green pilot light for 20 seconds each time a mo-
mentary button is pressed. In addition to the TON timer this
program uses multiple outputs on one rung, output latch and
unlatch instructions, as well as a timer reset instruction. The
• Initially closing the Timer_Button sets (latches) the
Green_PL on and enables the Pilot_Light_Timer.
Figure 15-47 ControlLogix trafﬁc control program.
Amber_Light_Timer.DN
Red_Light_Timer.DN
Red_Light_Timer
Green_Light_Timer.DN
Red_Light_Timer.EN
Green_Light_Timer.EN
Amber_Light_Timer.EN
Red_Light_Timer.DN
Red_Light
Local:2:O.Data.0
Green_Light_Timer.DN
Amber_Light_Timer.DN
TON
TIMER ON DELAY
Timer
Preset
Accum
30000
0
Green_Light_Timer
TON
TIMER ON DELAY
Timer
Preset
Accum
25000
Amber_ Light
Green_ Light
Red_ Light
Outputs
L2
0
Amber_Light_Timer
TON
TIMER ON DELAY
Timer
Preset
Accum
5000
0
EN
DN
Green_Light
Local:2:O.Data.1
Amber_Light
Local:2:O.Data.2
EN
DN
EN
DN
Figure 15-46 Pilot light TON timer.
Green_ PL
Timer_Button
L2
L1
Input
Green_PL
Local:2:O.Data.3
L
Time_Button
Local:1:I.Data.0
Pilot_Light_Timer.EN
Pilot_Light_Timer.DN Pilot_Light_Timer
Green_PL
Local:2:O.Data.3
U
RES
Pilot_Light_Timer
TON
TIMER ON DELAY
Timer
Preset
Accum
20000
0
EN
DN
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shown in Figure 15-48. Operation of the program can be
• Transition from red light to green light to amber
light is accomplished by the interconnection of the
EN and DN bits of the three TON timer instructions.
• The input to the Red_Light_Timer is controlled by
the Amber_Light_Timer.DN bit.
• The input to the Green_Light_Timer is controlled
by the Red_Light_Timer.DN bit.
• The input to the Amber_Light_Timer is controlled
by the Green_Light_Timer.DN bit.
• The timed sequence of the lights is:
- Red—30 s on
- Green—25 s on
- Amber—5 s on
• The sequence then repeats itself.
Off-Delay Timer (TOF)
The off-delay timer (TOF) operates in a fashion opposite
to the TON on-delay timer. An off-delay timer will turn
on immediately when the rung of ladder logic is true, Figure 15-49 ControlLogix TOF off-delay timer instruction.
Tag name
TOF
TIMER OFF DELAY
Timer Sample_TOF
Preset
Accum
5000
0
Input side
of rung
EN
DN
Figure 15-50 Pilot light TOF timer.
Green_ PL
Timer_Button
L2
L1
Input
Timer_Button
Local:1:I.Data.0
Pilot_Light_Timer.DN
Green_PL
Local:2:O.Data.3
Pilot_Light_Timer
TOF
TIMER OFF DELAY
Timer
Preset
Accum
20000
0
EN
DN
Figure 15-48 Tags created for trafﬁc light program.
-Amber_Light_Timer {…}
{…}
{…}
30000 Decimal DINT
DINT
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
0
1
1
0
1
0
0
TIMER
TIMER
TIMER
-Green_Light_Timer
-Red_Light_Timer
Red_Light
Green_Light
Amber_Light
-Red_Light_Timer.PRE
-Red_Light_Timer.ACC
Red_Light_Timer.EN
Red_Light_Timer.TT
Red_Light_Timer.DN
but it will delay before turning off after the rung goes
false. The ControlLogix TOF off-delay timer instruction
is shown in Figure 15-49. The description of the function
block fields and tag references are the same as for that of
a TON timer.
Figure 15-50 shows a program that uses a TOF timer
to illuminate a green pilot light for 20 seconds each time
a momentary button is pressed. The program code is sim-
pler than that used to accomplish the same task using a
TON timer. The operation of the program can be sum-
marized as follows:
• When the Timer_Button is initially closed the
timer rung and instruction and DN bit all become
true.
• The DN bit switches on the Green_PL and the
program remains in this state as long as the button is
held closed.
• When the button is released the Timer_Button
instruction goes false and starts the timing cycle.
• The light remains on and the timer begins accumu-
lating time.
• When the accumulator reaches 20000 ms (20 s)
the timer DN bit becomes false and the light is
switched off.
The program of Figure 15-51 uses both on-delay and
off-delay timers for control of a heating oven process.
The different tags created to fit the program are shown
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• The Timer_Cooling.DN bit of the TOF timer
becomes true which energizes the Fan_Motor.
• After 10 s (10000 ms) have elapsed the Timer_Heat.
TT bit becomes false to turn off the Warning_Horn
and the Timer_Heat.DN bit becomes true to
energize the Heater_Contactor and turn on the
heating coils.
• When the Oven_Off_Button is momentarily actu-
ated the Oven_On_PL output goes false which turns
the pilot light off and opens the continuity of its
seal-in logic path.
• The Timer_Heat timer instruction and its DN
bit instruction become false which de-energizes
the Heater_Contactor and turns off the heating
coils.
• The Timer_Cooling timer begins accumulating time
and the fan continues to operate for the 5 minute
(300000 ms) delay period after which the Timer_
Cooling.DN bit becomes false to turn the fan off.
Figure 15-51 Timer control of a heating oven process.
L1
Inputs
Oven_On_PL
Local:2:O.Data.2
Oven_On_Bautton
Oven_Off_Button
Timer_Heat
10000←
0←
TON
TIMER ON DELAY
Timer
Preset
Accum
Warning_Horn
Local:2:O.data.3
Oven_On_PL
Timer_Heat.TT
Timer_Cooling
300000←
0←
TOF
TIMER OFF DELAY
Timer
Preset
Accum
EN
Heater_Contactor
Local:2:O.data.4
Warning_Horn
Timer_Heat.DN
Fan_Motor
Local:2:O.data.5
L2
Outputs
Heater_Contactor
Fan_Motor
Timer_Cooling.DN
Oven_On_PL
Local:2:O.Data.2
Oven_On_Button
Local:1:I.Data.1
Oven_Off_Button
Local:1:I.Data.2
Oven_On_PL
Local:2:O.Data.2
DN
DN
EN
ON
OFF
Figure 15-52 Tags created for heating oven process.
Tag Name
Warning_Horn
Heater_Contactor
Fan_Motor
Oven_On_PL
Oven_On_Button
Oven_Off_Button
Local:2:O.Data.3
Local:2:O.Data.4
Local:2:O.Data.5
Local:2:O.Data.2
Local:1:I.Data.1
Local:1:I.Data.2
Local:2:O.Data.3
Local:2:O.Data.4
Local:2:O.Data.5
Local:2:O.Data.2
Local:1:I.Data.1
Local:1:I.Data.2
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
TIMER
TIMER
-Timer_Heat
-Timer_Cooling
Alias For Base Tag Data Type Style
in Figure 15-52. Operation of the program can be sum-
marized as follows:
• Pressing the Oven_On_Button energizes the Oven_
On_PL output which seals itself in and enables the
TON and TOF timer instructions.
• The Timer_Heat.TT bit of the TON timer becomes
true which sounds the Warning_Horn to warn that
the oven is about to come on.
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Retentive Timer On (RTO)
A retentive on-delay timer (RTO) operates the same
as a TON timer, except that the retentive timer retains
(remembers) its ACC value even if:
• The rung goes false.
• The processor is placed in the program mode.
• The processor faults.
• Power to the processor is temporarily interrupted
and the processor battery is functioning properly.
The ControlLogix RTO retentive on-delay timer in-
struction is shown in Figure 15-53. The description of the
function block fields and tag references are the same as
for that of a TON timer; however, a RES reset instruction
must be used to reset the accumulated value of a retentive
timer. The RES instruction must have the same tag name
as the timer you want to reset.
An example application of a limit switch 2 minute
(120000 ms) RTO timer program is shown in Figure 15-54.
The different tags created to fit the program are shown in
marized as follows:
• The status and value of all instructions, with the
timer initially reset, are as shown in the monitor
tags window.
• When the Limit_Switch has been closed for 1 min-
ute, the status and value of the instructions would be:
- PRE – 120000
- ACC – 60000
- LS_Timer.EN – 1
- LS_Timer.TT – 1
- LS_Timer.DN – 0
- LS_EN_PL – 1
- LS_TT_PL – 1
- LS_Alarm – 0
• When the Limit_Switch is opened after 1.5 minutes,
the status and value of the instructions would be:
- PRE – 120000
- ACC – 90000
- LS_Timer.EN – 0
- LS_Timer.TT – 0
- LS_Timer.DN – 0
- LS_EN_PL – 0
- LS_TT_PL – 0
- LS_Alarm – 0
Figure 15-53 RTO retentive on-delay timer instruction.
EN
DN
RES
SOL_On_Timer
Input side of rung
SOL_On_Timer
10000
0
RTO
RETENTIVE TIMER ON
Timer
Preset
Accum
Figure 15-54 Limit switch RTO timer program.
Limit_Switch
Local:1:I.Data.7
LS_Timer
RTO
RETENTIVE TIMER ON
Timer
Preset
Accum
120000
0
LS_EN_PL
Local:2:O.Data.0
LS_Timer.EN
LS_Timer.TT
LS_Timer.DN
EN
DN
LS_TT_PL
Local:2:O.Data.1
LS_Alarm
Local:2:O.Data.2
Reset_LS_Timer
Local:1:I.Data.2 LS_Timer
RES
Outputs
Input
L2
L1
LS_EN_PL
Limit_Switch
Reset_LS_Timer
LS_TT_PL
Alarm
LS_Alarm
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• When the Limit_Switch is closed and stays closed
until the timer times out, the status and value of the
instructions would be:
- PRE – 120000
- ACC –120000
- LS_Timer.EN – 1
- LS_Timer.TT – 0
- LS_Timer.DN – 1
- LS_EN_PL – 1
- LS_TT_PL – 0
- LS_Alarm – 1
• When the Limit_Switch is opened after the timer
times out, the status and value of the instructions
would be:
- PRE – 120000
- ACC –120000
- LS_Timer.EN – 0
- LS_Timer.TT – 0
- LS_Timer.DN – 1
- LS_EN_PL – 0
- LS_TT_PL – 0
- LS_Alarm – 1
• When the Reset_LS_Timer is closed, the status and
value of the instructions are reset to their original
values.
Figure 15-55 Tags created for the RTO retentive on-delay
timer program.
{…}
120000
0
0
0
0
DINT
DINT
BOOL
BOOL
BOOL
BOOL
TIMER
BOOL
Limit_Switch
LS_EN_PL
0
0
0
0
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
BOOL
BOOL
LS_TT_PL
LS_Alarm
-LS_Timer.PRE
-LS_Timer.ACC
LS_Timer.EN
LS_Timer.TT
LS_Timer.DN
-LS_Timer
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1. Compare the methods used to address timers in an
SLC 500 and a ControlLogix controller.
2. List the five different members of a TIMER
structure.
3. What type of timing application may require you to
use a TON on-delay timer?
4. What PRE value is used for a timer?
5. To what value is the accumulated value of a timer
normally set?
6. What timer status bit is set to 1 when the TON
timer times out?
7. The TON instruction is self-resetting. Explain what
this means.
8. What number would be entered into the PRE value
of a ControlLogix timer for a timing period of
4.5 minutes?
9. Compare the operation a TOF and a TON timer.
10. When does the rung of a TOF timer begin accumu-
lating time?
11. The RTO timer is a retentive timer. Explain what
this means.
12. How are the retentive timer and reset instruction
related?
1. Modify the original CLX ten-second TON timer pro-
gram with an additional rung added to the program
that will energize a solenoid whenever the timer is
enabled and timing. The solenoid is to be connected
to pin 6 of the digital output module.
2. With reference to the ladder logic of the CLX di-
verter gate program, assume the solenoid gate fails
to energize as programmed. You suspect the problem
is due to an open in the solenoid coil or wiring to it.
How might observation of the solenoid output status
light help confirm this?
3. You are required to extend the Green light-on
time of the CLX traffic control program to 40 sec-
onds. What changes would have to be made to the
program?
4. With reference to the CLX heating oven process
program, assume the oven-on pilot light burns out.
In what way would the operation of the program be
affected?
5. With reference to the CLX limit switch RTO pro-
gram, in addition to the alarm you are required to
install a warning pilot light to indicate that the timer
has timed out. How would you proceed?
6. Implement the hardwired TON alarm circuit of
Figure 15-56 in Logix format.
PART 3 PROBLEMS
Figure 15-56 Hardwired TON alarm circuit for Problem 6.
L1 L2
CR
TR
10 s
ON/OFF
switch
Low pressure
switch On-delay timer
TR
CR
Alarm
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347
counter must be used to reset the accumulated value of
the counter to zero.
All counters are retentive in that the accumulated value
of any counter is retained, even during a power failure,
until reset. The on/off status of the counter done, over-
flow, and underflow bits are retentive as well. Control-
Logix counter parameters and status bits are shown in the
edit tags window of Figure 15-58 and can be summarized
as follows:
• Preset (PRE) Value—Specifies the value the
counter must reach before the done (DN) bit turns
on (1).
• Accumulated (ACC) Value—Is the number of
false-to-true transitions of the counter run. ACC is
reset to zero when a reset (RES) instruction (of the
same counter address) is executed.
• CU (Count-Up Enable Bit)—The count-up enable
bit indicates the CTU instruction is enabled.
Counters
Counters are similar to timers, except that a counter ac-
cumulates (counts) the changes in state of an external
trigger signal whereas timers increment using an internal
clock. PLC counters are generally triggered by a change
in an input field device that causes a false-to-true transi-
tion of the counter ladder rung. It does not matter how
long the rung stays true or false—it is only the transition
that counts.
There are two basic counter types: count-up (CTU)
and count-down (CTD). The ControlLogix CTU in-
struction and counter selection toolbar are shown in
Figure 15-57. When you want to use a timer, you must
create a tag of type COUNTER (it is a predefined data
type) and enter the preset and the accumulated value.
When entering the instruction, this tag must be defined
before the preset and accumulated values can be entered.
A RES reset instruction that has the same tag name as the
Part Objectives
• Understand ControlLogix counter tags and their
members
• Utilize status bits from counters in logic
• Develop ladder logic programs using ControlLogix
counters
Part 4 Programming
Counters
Figure 15-57 CTU count-up counter instruction.
CU
DN
Package_Counter
Input side of rung
False
True
RES
24←
0←
Tag name
CTU
Count Up
Counter Package_Counter
Preset
Accum
TON
Favorites
TOF RTO CTU CTD RES
Add-On Alarms Bit Timer/Counter
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348 Part 4 Programming Counters
• CD (Count-Down Enable Bit)—The count-down
enable bit indicates the CTD instruction is
enabled.
• DN (Count-Up Done Bit)—Is set (1) when ACC
value is equal to or greater than the PRE value. Is
reset by the RES instruction.
• OV (Overflow Bit)—The overflow bit indicates the
counter exceeded the upper limit. Is set when the
ACC value is greater than +2,147,483,647 and reset
when the reset instruction is executed. Note that the
accumulated value keeps incrementing even after
the ACC value equals the PRE value.
• UN (Underflow Bit)—Indicates that the counter
exceeded the lower limit of 22,147,483,648.
The counter tag name is declared using the new tag
properties dialog box shown in Figure 15-59. Tag name,
description (optional), tag type, data type (base type is
used most often), and scope are selected or typed to com-
plete the validation.
Count-Up (CTU) Counter
Count-up (CTU) counters will cause the accumulated count
to increase by 1 every time there is a false-to-true transi-
tion of the counter ladder rung. An example application
of a count-up counter program used to count packets of
bottles is shown in Figure 15-60. The operation of the
• Each open-to-close transition of the Bottle_Sensor
proximity switch causes the counter to increment by 1.
Figure 15-58 ControlLogix counter parameters and
status bits.
DINT
DINT
BOOL
BOOL
BOOL
BOOL
COUNTER
BOOL
Part_Counter.OV
Part_Counter.UN
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
-Part_Counter.PRE
-Part_Counter.ACC
Part_Counter.CU
Part_Counter.CD
Part_Counter.DN
-Part_Counter
Tag Name Data Type Style
Figure 15-59 Counter tag validation.
Main program
Scope
COUNTER
Data Type
Base
Tag Type
24 bottle counter
Description
Package_Counter
Name
CV
DN
Package_Counter
24←
0←
CTU
Count Up
Counter
Preset
Accum
New Tag
24 bottle counter
Figure 15-60 Count-up counter program used to count packets of bottles.
24 bottle counter
Bottle_Sensor
Local:1:I.Data.1
Increment_PL
Local:2:O.Data.4
24 bottle counter
Package_Counter.CU
24 bottle counter
Package_Counter.DN
CU
DN
Preset_Reached_PL
Local:2:O.Data.5
Reset_Button
Local:1:I.Data.3
24 bottle counter
Package_Counter
Outputs
Inputs
L2
L1
Increment_PL
Bottle_Sensor
Reset_Button
Preset_Reached_PL
CTU
Count Up
Counter
Preset
Accum
Package_Counter
24*
0*
RES
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• The Increment_PL controlled by the Package_
Counter.CU status bit turns on and off as
each bottle passes to show that the counter is
incrementing.
• When the accumulated value of the counter is 24
the DN bit of the counter is set and switches on the
Preset_Reached_PL.
• The counter is reset by momentarily closing the
Reset_Button.
The program shown in Figure 15-61 uses two CTU in-
structions as part of a program to remove 5 out of every
10 containers from a conveyor line using an electric so-
lenoid. The different tags created to fit the program are
shown in Figure 15-62. The operation of the program can
• The preset for the Container_Counter_Counts is set
for 6 and that for the Container_Counter_Max is
set to 11.
Figure 15-62 Tags created for the CTU program used to
remove containers from a conveyor line.
Container_Counter_Counts .CU
Container_Counter_Counts .CD
Container_Counter_Counts .DN
Container_Counter_Counts .OV
Container_Counter_Counts .UN
Container_Counter_Counts
Container_Counter_Counts .PRE
Container_Counter_Counts .ACC
0
0
0
0
0
{...}
6
0
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
BOOL
BOOL
BOOL
BOOL
BOOL
COUNTER
DINT
DINT
Container_Counter_Max .CU
Container_Counter_Max .CD
Container_Counter_Max .DN
Container_Counter_Max .OV
Container_Counter_Max .UN
Container_Counter_Max
Container_Counter_Max .PRE
Container_Counter_Max .ACC
0
0
0
0
0
{...}
11
0
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
BOOL
BOOL
BOOL
BOOL
BOOL
0 Decimal BOOL
0
Container_Sensor
SOL Decimal BOOL
COUNTER
DINT
DINT
Figure 15-61 CTU program used to remove containers from a conveyor line.
L1 L2
Input
Container_Sensor
Local:1:I.Data.2
Container_Sensor
Local:1:I.Data.2
SOL
Local:2:O.Data.2
Container_Counter_Max.DN Container_Counter_Counts
Container_Counter_Counts.DN
CTU
Count Up
Counter
Preset
Accum
Container_Counter_Counts
6←
0←
Output
Sensor
Solenoid
SOL
Container_Sensor CU
DN
CTU
Count Up
Counter
Preset
Accum
11←
0←
CU
DN
RES
RES
Programming Counters Part 4 349
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• When the container is detected both counters will
increase their accumulated values by 1.
• When the sixth part arrives the Container_Counter_
Counts counter will then be done, thereby allow-
ing the solenoid to actuate for any container after
the fifth.
• The Container_Counter_Max counter will continue
until the eleventh part is detected and then both of
the counters will be reset.
Count-Down (CTD) Counter
The count-down (CTD) counter operates in a fashion op-
posite to the count-up CTU counter. CTD counters will
cause the accumulated count to decrease instead of in-
crease by one every time there is a false-to-true transi-
tion of the counter ladder rung. The ControlLogix CTD
down-counter instruction is shown in Figure 15-63. The
descriptions of the function block fields and the tag ref-
erences are the same as those associated with the CTU
function block. The CTD instruction is typically used
with a CTU instruction that references the same counter
structure.
The application program shown in Figure 15-64 is
used to limit the number of parts that can be stored in
the buffer zone to a maximum of 50. A CTU counter
and a CTD counter are used together with the same
Figure 15-63 Count-down CTD counter instruction.
Counter_1.CU
Counter_1.CD
Counter_1.DN
Counter_1.OV
Counter_1.UN
Counter_1
Counter_1.PRE
Counter_1.ACC
Tag Name
Count Down
Counter
Preset
Accum
CTD
Counter_1
Tag name
Input
side of
rung
Counter_1
CD
DN
RES
Figure 15-64 CTU counter and CTD counter used together to form an Up/Down counter.
Restart_Button
Local:1:I.Data.1 Counter_1
CTU
Restart_Button
Count Up
Counter Counter_1
Preset
Accum
50←
0←
CU
DN
Enter_Limit_Sw
Local:1:I.Data.3
CTD
Count Up
Counter Counter_1
Preset
Accum
50←
0←
CD
DN
Enter_Limit_Sw
Local:1:I.Data.4
Counter_1.DN
Conveyor_Contactor
Local:2:O.Data.2
RES
L1
Inputs
L2
Output
C
Enter_Limit_Sw
Exit_Limit_Sw
Conveyor_Contactor
address to form an Up/Down counter. This is the most
common type of application of the CTD counter. The
different tags created to fit the program are shown in
marized as follows:
• The Restart_Button is momentarily actuated at any
time to reset the accumulated value of the counter
to zero.
• Conveyor brings parts into a buffer zone.
• Each time a part enters the buffer zone, the
Enter_Limit_Sw is actuated and Counter_1
increments by 1.
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• Each time a part leaves the buffer zone, the
Exit_Limit_Sw is actuated and Counter_1
decrements by 1.
• When the number of parts in the buffer zone, at
any one time, reaches 50, the Counter_1.DN bit
is set.
• As a result the Conveyor_Contactor rung goes
false to de-energize the conveyor contactor, auto-
matically stopping the conveyor from bringing in
any more parts until the accumulated count drops
below 50.
Figure 15-65 Tags created for the Up/Down counter
program.
Counter_1.CU
Counter_1.CD
Counter_1.DN
Counter_1.OV
Counter_1.UN
Counter_1
Counter_1.PRE
Counter_1.ACC
0
0
0
0
0
0
{ . . . }
50
0
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
BOOL
BOOL
BOOL
BOOL
BOOL
COUNTER
DINT
DINT
Conveyor_Contactor
Restart_Button
Enter_Limit_Sw
Exit_Limit_Sw
1
0
0
Decimal
Decimal
Decimal
BOOL
BOOL
BOOL
BOOL
Programming Counters Part 4 351
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1. In what way are timers and counters similar?
2. Outline the procedure followed to create a tag when
you want to use a counter.
3. All counters are retentive. In what way does this af-
fect their operation?
4. What is specified by the preset value of a counter?
5. When is each of the following counter bits set?
a. CU
b. DN
c. CD
6. Compare the operations of a CTU and a CTD
counter.
7. What is an Up/Down counter?
8. Explain how you go about creating tags for an Up/
Down counter that uses a CTU and CTD instruction.
1. With reference to the CTU packets of bottles pro-
gram, what changes to the program would be re-
quired to count 6 bottle packets?
2. With reference to the CTU program used to remove
containers from a conveyor line, assume the output
solenoid coil failed open. In what way would the
operation of the program be affected?
3. Modify the original Up/Down counter program to
include:
a. A red pilot light to indicate entry of a part into the
buffer zone. Light to be connected to pin 4 of the
digital output module.
b. A green pilot light to indicate exit of a part from
the buffer zone. Light to be connected to pin 3 of
the digital output module.
4. Write a ControlLogix program, complete with tags,
for an Up/Down counter used to keep track of cars
entering and exiting a parking lot. The program re-
quirements for this application can be summarized
as follows:
• The parking lot holds 30 vehicles.
• There is an entrance vehicle sensor and an exit
vehicle sensor.
• When the parking lot is full a Lot Full sign is
illuminated.
• Whenever a car exits the lot, a Caution Buzzer/
Light is activated to warn pedestrians.
PART 4 PROBLEMS
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353
Math Instructions
ControlLogix basic math instructions include addition,
subtraction, multiplication, division, square root, and
clear. Figure 15-66 shows the Compute/Math toolbar for
the ControlLogix controller.
The ADD instruction is used to add two numbers. This
instruction adds these values from Source A and Source B.
The source can be a constant value or a tag. The result of
the ADD instruction is put in the destination (Dest) tag.
Figure 15-67 shows an example of an ADD instruction
rung along with its Monitor Tags window. The operation
of the rung can be summarized as follows:
• When the ADD_Sw is closed the rung will be true.
• The ADD instruction will execute to add the num-
ber from Source A (Value_A) and the value from
Source B (Value_B).
Part Objectives
• Utilize ControlLogix math instructions in programs
• Utilize ControlLogix comparison instructions in
programs
• Utilize ControlLogix move instructions in programs
• Develop and follow the operation of programs that use
math, comparison, and move instructions
Part 5 Math, Comparison,
and Move Instructions
Figure 15-66 Compute/Math toolbar for the ControlLogix
controller.
CPT
Compare
ADD SUB MUL DIV MOD SQR
Move/Logical File/Misc.
Compute/Math
Figure 15-67 ADD instruction rung and its Monitor Tags
window.
Input
L1
ADD
Add
Source A
Source B
Dest
Value_A
25
Value_B
50
Total_Value
75
Add_Sw
Local:1:I.Data.1
Add_Sw
Value_B
ADD_Sw
Total_Value
Value_A
50
1
75
25
Decimal
Decimal
Decimal
Decimal
DINT
BOOL
DINT
DINT
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354 Part 5 Math, Comparison, and Move Instructions
• The result will be stored in the Dest tag
(Total_Value).
• In this example, the 25 was added to 50 and the
result (75) was stored in Total_Value.
The SUB instruction is used to subtract two numbers.
Figure 15-68 shows an example of a SUB instruction rung
along with its Monitor Tags window. The operation of the
• When the SUB_Sw or Calculate tag is true the SUB
• Source B (Shipped_Parts) is subtracted from Source
A (Parts_Stock) and the result is stored in the Dest
tag named Current_Inventory.
• In this example, the 200 was subtracted from 900
and the result (700) was stored in Current_Inventory.
• Source A and Source B can be constants (numbers)
or tags.
The MUL instruction is used to multiply two numbers.
Figure 15-69 shows an example of a MUL instruction
rung along with its Monitor Tags window. When multiple
bottles are packed in cases, the number of bottles per case,
the number of cases, and the multiply instruction will give
you the total number of bottles. The operation of the rung
• When the Sw_1 and Sw_2 are both true the MUL
Figure 15-68 SUB instruction rung and its Monitor Tags window.
Input
L1
SUB
Subtract
Source A
Source B
Dest
Parts_Stock
900
Shipped_Parts
200
Current_Inventory
700
Sub_Sw
Local:1:I.Data.4
Calculate
Add_Sw
Current_Inventory
Sub_Sw
Parts_Stock
Shipped_Parts
700
1
900
200
Decimal
Decimal
Decimal
Decimal
DINT
BOOL
DINT
DINT
Calculate 0 Decimal BOOL
Figure 15-69 MUL instruction rung and its Monitor Tags window.
Input
L1
MUL
Multiply
Source A
Source B
Dest
Cases_Produced
60
Bottles_Per_Case
12
Bottles_Produced
720
Sw_2
Local:1:I.Data.2
Sw_1
Local:1:I.Data.1
Sw_1
Sw_2
Cases_Produced
Bottles_Per_Case
Sw_1
Sw_2
60
12
1
1
Decimal
Decimal
Decimal
Decimal
DINT
DINT
BOOL
BOOL
Bottles_Produced 720 Decimal DINT
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Math, Comparison, and Move Instructions Part 5 355
• The addition in the ADD instruction places the sum
of the accumulated values of the two counters in the
Conveyor_3_Parts tag.
• When the accumulated value for either counter is
equal to 150 the reset (RES) instructions for both
counters are enabled to automatically reset both
counter ACC values to zero.
• Both counters can also be reset manually at any time
by actuation of the Manual_Conveyor_Reset button.
Comparison Instructions
Compare instructions are used to compare two val-
ues. They can be used to see if two values are equal, if
one value is greater or less than the other, and so on. In
ControlLogix controllers compare instructions are input
instructions that do comparisons by either using an ex-
pression or doing the comparison indicated by the specific
instruction. Figure 15-72 shows the Compare toolbar for
the ControlLogix controller.
The equal (EQU) instruction is used to test if two val-
ues are equal. Values compared can be actual values or
tags that contain values. Figure 15-73 shows an example
of an EQU instruction rung along with its Monitor Tags
window. The operation of the rung can be summarized as
follows:
• The value stored at Source A is compared to the
value stored at Source B.
• If the values are equal, the instruction is
logically true.
• If the values are unequal, the instruction is
logically false.
• In this example Source A (25) is equal to Source B
(25) so the instruction is true and output Equal_PL
is on.
• Source A and Source B may be SINT, INT, DINT,
or REAL data types.
The not equal (NEQ) instruction is used to test two
values for inequality. Figure 15-74 shows an example of
an NEQ instruction rung. When Source A is not equal to
Source B, the instruction is logically true; otherwise, it
is logically false. In this example the two values are not
equal so the Not_Equal_PL is energized.
The less than (LES) instruction is used to check if a
value from one source is less than the value from a sec-
ond source. Figure 15-75 shows an example of an LES
instruction rung. When Source A is less than Source B,
the instruction is logically true; otherwise, it is logically
false. In this example Value_1 (100) is less than Value_2
(300) so the Less_Than_PL is energized.
Figure 15-70 DIV instruction rung and its Monitor Tags
window.
DIV
Divide
Source A
Source B
Dest
5
3
Answer_Real
1.6666666
Calculate
Calculate
Answer_Real
1
1.6666666
Decimal
Float
BOOL
REAL
• Source A (the value in tag Cases_Produced) is
multiplied by Source B (the value in tag Bottles_
Per_Case) and the result is stored in the Dest tag
Bottles_Produced.
• Source A and Source B can be constants (numbers)
or tags.
The DIV instruction is used to divide two numbers.
Figure 15-70 shows an example of a DIV instruction rung
along with its Monitor Tags window. The operation of the
• A constant (5) is used for Source A and a con-
stant (3) for Source B. Note that tags could have
been used for Source A or Source B.
• When the Calculate tag is true the DIV instruction
is executed.
• Source A (5) is divided by Source B (3) and the
result (1.6666666) is stored in the Dest tag Answer_
Real. Note that in this example a Real-type tag has
been used for its destination.
The program of Figure 15-71 is used as part of a parts
tracking system with three conveyors. The number of
parts in conveyor 1 and the number of parts in conveyor
2 are added to get the number of parts on conveyor 3.
follows:
• Each time Conveyor_1_Sensor is actuated
the accumulated value of Counter_1_Parts is
incremented by 1.
• Each time Conveyor_2_Sensor is actuated
the accumulated value of Counter_2_Parts is
incremented by 1.
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The greater than (GRT) instruction is used to check
if a value from one source is greater than the value from
a second source. Figure 15-76 shows an example of a
GRT instruction rung. When Source A is greater than
Source B, the instruction is logically true; otherwise,
it is logically false. In this example Value_1 (1420) is
Figure 15-72 Compare toolbar for the ControlLogix
controller.
CMP
Bit
LIM MEQ EQU NEQ LES GRT
Input/Output Compare
Timer/Counter
Figure 15-71 Program used as part of a parts tracking system.
Conveyor_1_Sensor
Local:1:I.Data.4
Conveyor_2_Sensor
Local:1:I.Data.5
Conveyor_1_Parts
250
30
CTU
Count Up
Counter
Preset
Accum
Conveyor_1_Parts.DN
Conveyor_2_Parts
250
70
CTU
Count Up
Counter
Preset
Accum
Conveyor_1_Parts.ACC
30
70
Conveyor_3_Parts
100
Conveyor_2_Parts
ADD
Add
Source A
Source B
Dest
CU
DN
CU
DN
Conveyor_2_Parts.DN
Manual_Conveyor_Reset
Local:1:I.Data.2
Conveyor 1
Conveyor 2
Conveyor 3
RES
Conveyor_1_Parts
RES
L1
Inputs
Conveyor_1_Sensor
Conveyor_2_Sensor
Conveyor_1_Parts.CU
Conveyor_1_Parts.DN
Conveyor_1_Parts
Conveyor_1_Parts.PRE
Conveyor_3_Parts
Conveyor_2_Parts.CU
Conveyor_2_Parts.DN
Conveyor_1_Sensor
Conveyor_2_Parts
Conveyor_2_Parts.PRE
0
0
0
{ . . . }
{ . . . }
250
250
70
30
0
0
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
Decimal
BOOL
BOOL
BOOL
BOOL
BOOL
0
100
0
Decimal
Decimal
Decimal
BOOL
BOOL
Conveyor_2_Sensor
COUNTER
COUNTER
DINT
DINT
DINT
DINT
DINT
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Figure 15-73 EQU instruction rung and its Monitor Tags
window.
Value_1
25
Value_2
25
Equal_PL
Equal_PL
Local:2:O.Data.2
L2
Output
Equal_PL 1
25
25
Decimal
Decimal
Decimal
BOOL
DINT
DINT
-Value_1
-Value_2
EQU
Equal
Source A
Source B
Figure 15-74 NEQ instruction rung.
Value_1
10
Value_2
25
Not_Equal_PL
Local:2:O.Data.3
L2
Output
NEQ
Not Equal
Source A
Source B
Not_Equal_PL
Figure 15-75 LES instruction rung.
Value_1
100
Value_2
300
Less_Than_PL
Local:2:O.Data.4
L2
Output
LES
Less than (AB)
Source A
Source B
Less_Than_PL
greater than Value_2 (1200) so the Greater_Than_PL is
energized.
The compare (CMP) instruction performs a compari-
son on the arithmetic operations specified by the expres-
sion. The expression may contain arithmetic operators,
comparison operators, and tags. The execution of a CMP
instruction is slightly slower and uses more memory than
the execution of the other comparison instructions. The
advantage of the CMP instruction is that it allows you
to enter complex expressions in one instruction. Fig-
ure 15-77 shows an example of a CMP instruction rung.
In this example the comparison operator found in the
expression is the equivalent of an EQU instruction. The
comparison instruction is true because Value_1 (300) is
equal to Value_2 (300).
The program of Figure 15-78 is an example of the use
of comparison instructions used to test the accumulated
value of a counter. The operation of the program can be
• When the accumulated count is between 5 and 10
the GRT and LES instructions will both be logically
true so the PL_1 pilot light will be on.
• When the accumulated count is equal to 15, the
EQU instruction will be logically true so the PL_2
pilot light will be on.
• The PL_3 pilot light will be on at all times except
when the accumulated count is 20 at which time the
NEQ instruction is logically false.
• The counter is reset automatically when the accu-
mulated count reaches 25 or manually anytime the
Reset_PB is actuated.
Move Instructions
The move (MOV) instruction is an output instruction
that can move a constant or the contents of one mem-
ory location to another location. Figure 15-79 shows
the Move toolbar and instruction for the ControlLogix
Figure 15-76 GRT instruction rung.
Value_1
1420
Value_2
1200
Greater_Than_PL
Local:2:O.Data.5
L2
Output
GRT
Greater than (AB)
Source A
Source B
Greater_Than_PL
Figure 15-77 CMP instruction rung.
Compare
Expression Value_1 = Value_2
CMP
Equal_PL
Equal_PL
Local:2:O.Data.2
L2
Output
Equal_PL 1
300
300
Decimal
Decimal
Decimal
BOOL
DINT
DINT
-Value_1
-Value_2
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controller. The MOV instruction is used to copy data
from a source to a destination. Both the source and the
destination data type of a MOV instruction may be INT,
DINT, SINT, or REAL.
Figure 15-79 Move toolbar for the ControlLogix
controller.
MOV MVM AND OR XOR NOT SWPB
Move/Logical
Move
Source
Dest
MOV
Figure 15-78 Comparison instructions used to test the accumulated value of a counter.
Ladder Logic program
Outputs L2
PL_3
L1 Inputs
Count_PB
Reset_PB
Count Up
Counter
Preset
Accum
C1
25
0
CTU
Greater Than (AB)
Source A
Source B
C1.ACC
0
5
GRT
Less Than (AB)
Source A
Source B
C1.ACC
0
10
LES
CU
Equal
Source A
Source B
C1.ACC
0
15
EQU
Count_PB
Local:1:I.Data.1
C1.DN
PL_1
Local:2:O.Data.1
RES
C1
PL_2
Local:2:O.Data.2
Not Equal
Source A
Source B
C1.ACC
0
20
NEQ
PL_3
Local:2:O.Data.3
Reset_PB
Local:1:I.Data.2
DN
PL_2
PL_1
MOV instruction can be used to create a variable preset
as follows:
• Actuating the PB_10s button executes its MOV in-
struction to transfer 10000 to the timer preset value
setting the delay period for 10 seconds.
• Actuating the PB_15s button executes its MOV in-
struction to transfer 15000 to the timer preset value
setting the delay period for 15 seconds.
• Closing the Timer_Start switch starts the timer
timing.
• While the timer is timing, the pilot light PL_1 is on
for the duration of the timer preset period.
• When the timer times out, PL_1 turns off and PL_2
turns on.
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Figure 15-80 MOV instruction used to create a variable preset timer.
Ladder Logic program Outputs
L2
PL_2
L1
Inputs
PB_10s
PB_15s
Timer_Start
Move
Source
Dest
10000
T1.PRE
10000
MOV
PB_10s
Local:1:I.Data.1
Move
Source
Dest
15000
T1.PRE
10000
MOV
PB_15s
Local:1:I.Data.2
TIMER ON DELAY
Timer
Preset
Accum
T1
10000
0
TON
Timer_Start
Local:1:I.Data.3
EN
T1.TT
PL_1
Local:2:O.Data.1
T1.DN
PL_2
Local:2:O.Data.2
DN
PL_2
PL_1
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1. Construct a ControlLogix ladder rung with a math
instruction that executes when a toggle switch is
closed to add the tag named Pressure_A (value 680)
to the constant of 50 and store the answer in the tag
named Result.
instruction that executes when two normally open
limit switches are closed to subtract the tag named
Count_1 (value 60) from the tag named Count_2
(value 460) and store the answer in the tag named
Count_Total.
instruction that executes when either one of two nor-
mally open pushbuttons is closed to multiply the tag
named Cases (value 10) by the constant 24 and store
the answer in the tag named Cans.
4. Construct a ControlLogix ladder rung with a com-
pare instruction that will energize a pilot light output
anytime the value stored at Data_3 is 60.
5. Construct a ControlLogix ladder rung with a com-
pare instruction that will energize a pilot light output
anytime the value stored at Data_2 is not the same as
that stored at Data_6.
6. Construct a ControlLogix ladder rung with compare
instructions that will energize a pilot light output
anytime the pressure of a system goes above 300 psi
or below 100 psi.
1. While checking the operation of the parts tracking sys-
tem with the Monitor Tags window, you note that the
value of Conveyor_Sensor_1 remains at 1 with parts
passing by. What can you surmise from this? Why?
2. Three conveyors are delivering the same parts in dif-
ferent packages. A package can hold 12, 24, or 18
parts. Proximity switches installed on each of the
conveyor lines are used to advance the accumulated
PART 5 PROBLEMS
value of the three counters. Write a ControlLogix
program that uses multiply and add instructions to
calculate the sum of the parts.
3. A single pole switch is used in place of the two
pushbuttons for the variable preset timer program.
When this switch is closed the timer is to be set for
10 seconds and when open to 15 seconds. Make the
necessary changes to the program.
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361
indicates what type of data are present. A dash line indi-
cates a Boolean signal path (e.g., 0 or 1) and a solid line
indicates an integer or real value.
Function blocks are graphical representations of ex-
ecutable code. A function block can take one or more in-
puts and make decisions or calculations and then generate
one or more outputs. There are many different types of
function blocks included in the programming software to
perform various common tasks. In addition, customized
Add-On instructions can be created by the programmer
for sets of commonly used logic. Once an Add-On in-
struction is defined in a project, it appears on the instruc-
tion toolbar and behaves like the standard instructions.
Figure 15-82 shows an example of a BAND (Boolean
AND) function block. The information associated with a
function block can be summarized as follows:
• Inputs are shown entering from the left and outputs
exiting on the right.
Function Block Diagram (FBD)
A function block diagram (FBD) is a graphical depic-
tion of process flow using simple and complex intercon-
necting blocks. It is similar to a ladder logic diagram,
except that function blocks replace the interconnec-
tion of contacts and the coils. In addition, there are no
power rails.
A function block circuit is analogous to an electrical
circuit where links and wires depict signal paths between
components. The workplace is known as a sheet and con-
sists of function blocks joined together with lines called
wires. The structure of a function block program, or rou-
tine, is shown in Figure 15-81. A function block diagram
consists of four basic elements: function block, refer-
ences, wire connectors, and wires. Data flow on a wire
from wire connectors or input references, move through
the function block, and then pass on to an output refer-
ence. The line type of the link between function blocks
Part Objectives
• Describe the difference between ladder logic and
function block diagram programming
• Recognize the basic elements of a function block
diagram
• Write and read a function block diagram
Part 6 Function Block
Programming
Figure 15-81 Structure of function block or routine.
Input wire connector
Input reference
Function block
Value
Value
Value
Value
Wire
Value
Value
Output reference
Boolean
(0 or 1)
Integer
or real
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362 Part 6 Function Block Programming
• The function block type is shown within the block.
• A tag name for the block is placed above it.
• The names of the inputs and outputs are shown
within the block.
• The default view of the block has some but not all
of the input and output parameters visible when the
box is placed into the program.
• The properties box, used to set the option of input
and output parameters, is displayed by clicking
the selection button located at the upper right hand
corner of the block.
• The 1 and 0 next to the inputs and outputs identifies
the logical state of the input and output pins for the
instruction.
• The dots on the input and output pins indicate
BOOL type data is required.
References represent tags that are linked to values
stored in a controller’s memory. The two types of refer-
ences, input and output, are illustrated in Figure 15-83.An
input reference, or IREF, is used to receive a value from
an input device or tag. An output reference, or OREF, is
used to send a value to an output device or tag. When you
use an IREF or an OREF you must create a tag or assign
an existing tag to the element.You may use any of the data
types for an IREF or OREF.
Function blocks can be connected to other function
blocks by connecting their outputs to the input of an-
other function block using wires and pins (Figure 15-84).
Wires map a signal’s path and show the flow of control-
ler execution. Each element in a function block diagram
contains pins. Elements are connected by moving wires
from input pins to output pins or vice versa. The pins on
the left of a function block are input pins, and those on the
Figure 15-82 Example of a BAND (Boolean AND) function block.
Properties - Tag_Name
Vis
I
I
I
I
I
I
I
I
I
O
O
Name
Parameters Tag
Type Description
Enable Input. If false, the instru...
Boolean Input to the instruction.
Enable Output.
The result of ANDing all eight
EnableIn
In1
In2
In3
In4
In5
In6
In7
In8
EnableOut
Out
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
BOOL
1
1
1
1
1
1
1
1
1
0
0
Value
In 1
1 0
1
1
1
Out
BAND
Tag_Name
Click to open properties box
Boolean And
In 2
In 3
In 4
...
Figure 15-83 Input and output references.
Input references
IREF
IREF
OREF
IREF
OREF
Output references
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Function Block Programming Part 6 363
right are output pins. To wire two elements together, click
the output pin of the first element (A) and then click the
input pin of the other element (B). A green dot shows a
valid connection point.
Wire connectors are used to create a path without
using a wire. When there are many function blocks on a
sheet, or the function blocks are far apart, wire connectors
used in place of wires can make the logic harder to read.
Wire connectors are also used to connect function blocks
that are on a different sheet of the same function block
routine, as illustrated in Figure 15-85. The use of wire
connectors can be summarized as follows:
• An output wire connector, or OCON, sends a value
or signal to an input wire connector, or ICON.
• Each output wire connector must have at least one
corresponding input wire connector.
• Each output wire connector requires a unique tag
name and the corresponding input connector must
have the same name.
• Multiple input wire connectors can reference
the same output wire connector. This lets you
share data at several points in your function block
diagram.
Figure 15-86 illustrates the signal flow and execution
of an FBD program. The operation can be summarized as
follows:
• Each program scan sets all the FBD blocks starting
on the left side of the signal flow and continues to
evaluate all blocks according to the signal flow until
the final output is determined.
• The location of a block does not affect the order in
which the blocks execute.
• The inputs of a block require data to be available
before the controller can execute that block.
• If function blocks are not wired together, it does not
matter which block executes first as there is no data
flow between the blocks.
• The interconnected line between the blocks indi-
cates what type of signal is present.
Data latching refers to how the controller verifies
that the data present at the input to a function block are
valid. If you use an IREF to specify input data for a func-
tion block instruction, as illustrated in Figure 15-87, the
data in that IREF are latched (won’t change) for the
scan of the function block routine. The IREF latches
data from program-scoped and controller-scoped tags.
The controller updates all IREF data at the beginning
of each scan. A function block routine executes in the
following order:
• The controller latches all data values in IREFs.
Figure 15-84 Function block diagram wire and pins.
Input
pin
Wire
Output
pin Output
pin
Output
pin
Output
pin
Input
pin
Input
pins
Wire
Wire
Wiring elements
...
A B
Figure 15-85 OCON and ICON wire connectors.
Speed
Speed
OCON
ICON
Sheet 1
Output wire connector
Sheet 2
Output wire connector
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When a group of function blocks are in a feedback
loop, the controller cannot determine which block to
execute first. This problem is resolved by placing an
Assume Data Available indicator mark at the input pin
of the function block that should be executed first. In the
example shown in Figure 15-89, the input for block 1
uses the data from block 3 that were produced in the
previous scan. To place the indicator click on the inter-
connecting wire and select the Assume Data Available
choice.
• The controller executes the other function blocks
in order.
• The controller writes outputs in OREFs.
To create a feedback loop around a block, wire an out-
put pin of the block to an input pin of the same block.
The input pin will receive the value of the output that was
produced on the last scan of the function block. The loop
contains only a single block, so execution order does not
matter. Figure 15-88 shows an example of a feedback loop
used to reset an on-delay timer. When the timer finishes
timing its DN bit is used to reset the timer.
Figure 15-87 IREF is latched for the scan of the function
block routine.
...
Start_PB
IREF
Figure 15-88 Feedback loop used to reset an on-delay timer.
TONR_01
TONR
TimerEnable
PRE
ACC
DN
Reset
Feedback loop
Timer_Enable_Bit
Preset_Value
Accumulated_Time
Figure 15-86 Signal ﬂow and execution of an FBD program.
Function block
Input reference
Input reference
Output reference
Solenoid
Output wire
connector
Input wire
connector
Input reference
Function block
IREF
OREF
OCON
ICON
IREF
IREF
Function block
Figure 15-89 Assume Data Available indicator marker.
...
Feedback loop
Assume Data
Available indicator
Block 1 ...
Block 2 ...
Block 3
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FBD Programming
Figure 15-90 illustrates the setup procedure for FBD pro-
gramming. The steps to be followed can be summarized
as follows:
• Right click on the MainProgram file and select New
Routine from the pop-up menu.
• Select the Function Block diagram entry from the
Type window.
• Enter a name for the Routine (e.g., FDB_Sample).
• You will now see the new program (FDB_Sample)
listed under MainProgram.
• Left clicking the FBD_Sample twice opens the
graphic development window.
• FBD instructions selected from the Language Element
toolbar are used in the development of the program.
• Extra sheets can be added when the current sheet is
full by clicking the add sheet icon. Movement be-
tween sheets is provided by left and right arrows.
The MainRoutine is always a ladder logic program in
RSLogix 5000 software, and all other routines are called
from the MainRoutine. Therefore, the MainRoutine will
have one unconditional rung with a jump to subroutine
(JSR) calling FBD_Sample. The FBD program will ex-
ecute from the JSR instruction. No subroutine or return
subroutine instruction in the FBD is necessary.
Function block programs are similar to ladder logic
programs, except that the process is visualized in the form
of function blocks instead of ladder rungs. Figure 15-91
shows a comparison between ladder logic and the FBD
equivalent for a three-input AND ladder logic rung. The
operation of the FBD can be summarized as follows:
Figure 15-90 Setup procedure for FDB programming.
IREF OREF ICON
Add sheet
MainProgram
Move Sheet
OCON Function blocks
Other function blocks
Program Tags
MainRoutine
FBD_Sample
Figure 15-91 Comparison between ladder logic and the FBD equivalent for a three-input AND ladder logic rung.
Caution_PL
...
BAND_01
BAND
Boolean And
In1
In2
Out
In3
Sensor_1
Ladder logic
FBD equivalent
Output
L2
Caution_PL
Sensor_1
Local:1:I.Data.1
Sensor_2
Local:1:I.Data.2
Sensor_3
Local:1:I.Data.3
0
0
Sensor_2
0
Sensor_3
0
Caution_PL
Local:2:O.Data.4
L1
Inputs
Sensor_1
Sensor_2
Sensor_3
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• When the inputs represented by Sensor_1, Sensor_2,
and Sensor_3 are true (value 1) the BAND (Boolean
AND) function block will be true.
• The BAND block executes to set output Caution_PL
true and switch the pilot light on.
• The 0 to the right of the input reference and out pin
indicates its logic state. A 0 indicates the state of the
tag is false, while a 1 signifies it is true.
• The same field input sensors and output pilot
light devices and tags can be used with either
program.
• The XIC and OTE contact and coil instructions have
been replaced by the BAND function block.
Figure 15-92 shows a comparison between ladder
logic and the FBD equivalent for a two-input OR lad-
der logic rung. As with ladder OR logic, if any of the
two inputs is true the BOR function block will be true.
In this example, with the BOR function block true, the
output reference tag SOL_1 will be true energizing the
solenoid.
Figure 15-93 shows a comparison between ladder logic
and the FBD equivalent for a combination of multiple in-
puts. The operation of the FBD can be summarized as
follows:
• The alarm will be energized if either input In1 or
In2 to the BOR block is true.
• Input In2 of the BOR block will be true only when
all three of the sensor switches are closed.
• Input In1 of the BOR block will be true only when
the Temp_Sw is closed at the same time as the
Press_Sw is open.
• The BNOT function block executes similarly to an
XIO ladder logic contact instruction. When In is 0,
Out is 1 and vice versa.
and the FBD equivalent for the motor start/stop control
circuit. The logic sequence for starting and stopping the
motor can be summarized as follows:
• When Motor_Start button is closed the BOR output
will become true making the BAND output true.
• Motor_Run output energizes the contactor coil, the
contacts of which close to start the motor operating.
• When the Motor_Start button is then opened
the output of the BOR block remains true due to
the 1 status of the feedback signal from the
Motor_Run tag.
• When the Motor_Stop button is opened the output
of the BAND block turns false to de-energize the
contactor coil and stop the motor.
and the FBD equivalent for the 10 second TON (on-delay
two-input OR ladder logic rung.
SOL_1
...
BOR_01
BOR
Boolean Or
In1
In2
Out
Sw_1
FBD equivalent
0
0
Sw_2
0
Sw_1
Local:1:I.Data.3
Sw_2
Local:1:I.Data.4
SOL_1
Local:2:O.Data.4
Ladder logic
Inputs
L1
Sw_1
Sw_2
L2
Output
SOL_1
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Figure 15-93 Comparison between ladder logic and the FBD equivalent for a combination of multiple inputs.
FBD equivalent
Alarm
...
BAND_01
BAND
Boolean And
In1
In2
Out
Temp_Sw
0
0
Press_Sw
0 1
Sensor_1
0
Sensor_2
0
Sensor_3
0
...
BOR_01
BOR
Boolean Or
In1
In2
Out
0
...
BAND_02
BAND
Boolean And
In1
In2
Out
0
In3
...
BNOT_01
BNOT
Boolean Not
In Out
Temp_Sw
Local:1:I.Data.1
Sensor_1
Local:1:I.Data.3
Sensor_2
Local:1:I.Data.4
Press_Sw
Local:1:I.Data.2
Sensor_3
Local:1:I.Data.5
Alarm
Local:2:O.Data.4
Ladder logic
Inputs
Temp_Sw
L1
Sensor_1
Sensor_2
Sensor_3
Press_Sw
L2
Output
Alarm Alarm
Figure 15-94 Comparison between ladder logic and the FBD equivalent for a motor start/stop
control circuit.
...
BAND_01
BAND
Boolean And
In1
In2
Out
0
1
Start button for
motor
Motor_Start
Stop button for
motor
Motor_Stop
Motor
contactor coil
Motor_Run
0
Start button for
motor
Motor_Start
Local:1:I.Data.3
Motor_Run
Local:2:O.Data.4
Stop button for
motor
Motor_Stop
Local:1:I.Data.4
Motor_Run
Local:2:O.Data.4
Ladder logic
...
BOR_01
BOR
Boolean Or
In1
In2
Out
0
Inputs
L1
Motor_Start
Motor_Stop
Start
Stop
FBD equivalent
L2
Output
C
Motor_Run
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and the FBD equivalent for the Up/Down counter used to
limit the number of parts stored in a buffer zone to 50. The
operation of the FBD can be summarized as follows:
• The CTUD up/down counter function block accu-
mulated value is initially reset by momentary actua-
tion of the Restart_Button.
• The accumulated count is monitored by the output
reference tag named ACC.
• Each time a part enters the buffer zone, the Enter_
Limit_Sw is actuated and the CUEnable input turns
true to increment the count by 1.
• Each time a part exits the buffer zone, the Exit_
Limit_Sw is actuated and the CDEnable input turns
true to decrement the count by 1.
• Whenever the number of parts in the buffer zone
reaches 50 the DN bit is set to 1 and the output of
the BNOT block is reset to zero. This de-energizes
the Conveyor_Contactor to stop the conveyor motor
from delivering more parts to the buffer zone.
timer) and TONR (on-delay with reset). The operation of
the FBD can be summarized as follows:
• When the Timer_Sw is closed, the TONR func-
tion block timer turns true and starts accumulating
time.
• The accumulated time is monitored by the output
reference tag named ACC.
• The EN (enable bit) output changes to 1 to turn on
the EN_PL.
• The TT (timer timing bit) output changes to 1 to
turn on the TT_PL.
• The timer times out after 10 seconds to set the DN
(done bit) to 1 and turn on the DN_PL and reset the
TT bit to zero and turn off the TT_PL.
• The EN bit and EN_PL remain on as long as the
Timer_Sw stays toggled closed.
• Opening the Timer_Sw resets all outputs as well as
the accumulated value to zero.
• The timer can also be reset by way of the Reset
input.
10000
0
DN_PL
0
TT_PL
0
EN_PL
Status_Timer.DN
DN_PL
Local:2:O.Data.3
Status_Timer.TT
Input
L1
TT_PL
Local:2:O.Data.2
Status_Timer.EN
EN
Timer On Delay
Timer
Preset
Accum
Status_Timer
10000
0
EN_PL
Local:2:O.Data.1
DN
Timer_Sw
Local:1:I.Data.6
TON
Ladder logic
FBD equivalent
TONR_01
...
TONR
TimerEnable ACC
PRE
Reset
EN
TT
DN
Timer_Sw
Outputs L2
TT_PL
EN_PL
0
ACC_Value
0
10000
Timer_Sw
DN_PL
10 second TON and TONR timer.
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• Using one sheet for each device that is to be pro-
grammed helps organize your program and make it
easier to understand.
• The use of the OCON and ICON named ACC
enables the function blocks to be on different sheets
of the same function block routine.
• The numbers and letters under the ACC output
indicate the sheet number and location on the sheet
where the output is used.
and the FBD equivalent for the program used to test the
accumulated value of a counter. The operation of the FBD
• The function block routine is broken into four
sheets.
• The order of the sheets does not affect the order in
which the function blocks execute.
• When a function block routine executes, all sheets
execute.
50
50
0
Exit_Limit_Sw
0
Restart_Button
1
Conveyor_Contactor
L1 Inputs
Count Up
Counter
Preset
Accum
Counter_1
50
0
CTU
RES
Counter_1
Restart_Button
Local:1:I.Data.1
Enter_Limit_Sw
Local:1:I.Data.3
Ladder logic
CTUD_01
...
CTUD
Count Up/Down
CUEnable ACC
CDEnable
PRE
Reset
DN
L2
Output
Conveyor_Contactor C
...
BNOT
Boolean Not
In Out
0
ACC
0
0
Enter_Limit_Sw
Count Down
Counter
Preset
Accum
Counter_1
50
0
CTD
Counter_1.DN
Exit_Limit_Sw
Local:1:I.Data.4
Conveyor_Contactor
Local:2:O.Data.2
Restart_Button
Enter_Limit_Sw
Exit_Limit_Sw
FBD equivalent
BNOT_01
CU
DN
CD
DN
Figure 15-96 Comparison between ladder logic and the FBD equivalent for an Up/Down counter
application.
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25
25
0
L1 Inputs
Count_PB
Local:1:I.Data.1
Ladder logic
CTUD_01
...
CTUD
Count Up/Down
CUEnable ACC
CDEnable
PRE
Reset
DN
L2
Outputs
PL_1
PL_2
PL_3
0
ACC
2-B2 3-B2
4-B2
0
0
Count_PB
0
Reset_PB
Sheet 1 of 4 Sheet 2 of 4
Sheet 3 of 4 Sheet 4 of 4
C1_DN
PL_1
Local:2:O.Data.1
Count_PB
Reset_PB
FBD equivalent
...
BOR
Boolean Or
In2
In1
Out
BOR_01
5
5
10
10
PL_1
0
0
PL_2
GRT_02
...
GRT
Greater Than (AB)
Source A
Source B Dest
0
0
...
LES
Less Than (AB)
Source B
Source A
Dest
LES_02
...
BAND
Boolean And
In2
In1
Out
BAND_01
Count Up
Counter
Preset
Accum
C1
25
0
CTU
DN
C1
RES
Less than (AB)
Source A
Source B
C1.ACC
0
10
LES
Greater than (AB)
Source A
Source B
C1.ACC
0
5
GRT
PL_2
Local:2:O.Data.2
Equal
Source A
Source B
C1.ACC
0
15
EQU
PL_3
Local:2:O.Data.3
Reset_PB
Local:1:I.Data.2
Not Equal
Source A
Source B
C1.ACC
0
20
NEQ
ACC
0
ACC
0
...
EQU
Equal
SourceB
SourceA Dest
EQU_01
PL_3
0
ACC
20
20
15
15
1-C2 1-C2
0
...
NEQ
Not Equal
SourceB
SourceA Dest
NEQ_01
1-C2
CU
Figure 15-97 Comparison between ladder logic and the FBD equivalent for a program used to test the accumulated
value of a counter.
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1. Compare the graphical representation of a function
block diagram to that of a logic ladder diagram.
2. Name the four basic elements of an FBD.
3. What do the solid and dashed interconnecting lines
between FBD function blocks indicate?
4. What is an Add-On instruction?
5. How are the input and output parameter options for
a function block set?
6. What does the dot on an input or output pin of a
function block indicate?
7. Compare the functions of input and output refer-
ence tags.
8. Which pins of a function block are inputs and
which are outputs?
9. Explain the role of input and output wire
connectors.
10. How does the program scan function for an FBD
program?
11. Explain data latching as it applies to function block
inputs.
12. How is a function block feedback loop created?
13. What is the Assume Data Available indicator
used for?
14. Outline how an FBD program is initiated.
1. Write an FBD program that will cause the output,
solenoid SOL_1, to be energized when pushbutton
PB_1 is open and PB_2 is closed, and either limit
switch LS_1 is open or limit switch LS_2 is closed.
Assume all pushbuttons and limit switches are of the
normally open type.
2. Modify the motor start/stop FBD program to include
a second start/stop pushbutton station.
3. You are required to change the on-delay time of the
10 second timer program to 1 minute. What changes
would have to be made to the FBD program?
4. Modify the Up/Down counter FBD program to
include the following pilot lights:
• PL_1 to come on when a part enters
• PL_2 to come on when a part exits
• PL_3 to come on when the buffer zone is full
5. Modify the test accumulated value of a counter FBD
program as follows:
• PL_1 to be on for an accumulated count between
0 and 5
• PL_2 to be on for an accumulated count of 12
• PL_3 to be on at all times except for when the
accumulated count is 15
PART 6 PROBLEMS
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373
Glossary
1’s complement The system used to represent negative numbers in a
personal computer and a programmable logic controller.
2’s complement A numbering system used to express positive and
negative binary numbers.
A
Access To locate data stored in a programmable logic controller
system or in computer-related equipment.
Accumulated value The number of elapsed timed intervals or
counted events.
Actuator An output device normally connected to an output module.
Examples are an air valve and cylinder.
Address A code that indicates the location of data to be used by a
program, or the location of additional program instructions.
Algorithm Mathematical procedure for problem solving.
Alias tag References a memory location that has been defined by
another tag.
Alphanumeric Term describing character strings consisting of any
combination of letters, numerals, and/or special characters (e.g., A15$)
used for representing text, commands, numbers, and/or code groups.
Alternating current (AC) input module An input module that con-
verts various alternating current signals originating at user devices to
the appropriate logic level signal for use within the processor.
Alternating current (AC) output module An output module that
converts the logic level signal of the processor to a usable output sig-
nal to control a user alternating current device.
Ambient temperature The temperature of the air surrounding a
module or system.
American National Standard Code for Information Interchange
(ASCII) An 8-bit (7 bits plus parity) code that represents all charac-
ters of a standard typewriter keyboard, both uppercase and lowercase,
as well as a group of special characters used for control purposes.
American National Standards Institute (ANSI) A clearinghouse
and coordinating agency for voluntary standards in the United States.
American wire gauge (AWG) A standard system used for designat-
ing the size of electrical conductors. Gauge numbers have an inverse
relationship to size; larger numbers have a smaller diameter.
Analog device Apparatus that measures continuous information
(e.g., voltage or current). The measured analog signal has an infinite
number of possible values. The only limitation on resolution is the
accuracy of the measuring device.
Analog input module An input circuit that employs an analog-to-
digital converter to convert an analog value, measured by an analog
measuring device, to a digital value that can be used by the processor.
Analog output module An output circuit that employs a digital-to-
analog converter to convert a digital value, sent from the processor, to
an analog value that will control a connected analog device.
Analog signal Signal having the characteristic of being continuous
and changing smoothly over a given range rather than switching sud-
denly between certain levels, as with discrete signals.
Analog-to-digital (A/D) converter A circuit for converting a vary-
ing analog signal to a corresponding representative binary number.
AND (logic) A Boolean operation that yields a logic 1 output if all
inputs are 1, and a logic 0 if any input is 0.
Arithmetic capability The ability to do addition, subtraction,
multiplication, division, and other advanced math functions with the
processor.
Array A combination of panels, such as LEDs, coordinated in struc-
ture and function.
ASCII-input module Converts ASCII-code input information
from an external peripheral into alphanumeric information a PLC can
understand.
ASCII-output module Converts alphanumeric information from the
PLC into ASCII code to be sent to an external peripheral.
Asynchronous Recurrent or repeated operations that occur in unre-
lated patterns over time.
Automatic control A process in which the output is kept at a
desired level by using feedback from the output to control the input.
Auxiliary power supply A power supply not associated with the
processor. Auxiliary power supplies are usually required to supply
logic power to input/output racks and to other processor support
hardware and are often referred to as remote power supplies.
B
Backplane A printed circuit board, located in the back of a chassis,
that contains a data bus, power bus, and mating connectors for mod-
ules to be inserted in the chassis.
Base tag A definition of the memory location at which a data ele-
ment is stored.
BASIC A computer language that uses brief English-language state-
ments to instruct a computer or microprocessor.
Battery indicator A diagnostic aid that provides a visual indication
to the user and/or an internal processor software indication that the
memory power-fail support battery is in need of replacement.
Baud A unit of signaling speed equal to the number of discrete
conditions or signal events per second; often defined as the number of
binary digits transmitted per second.
BCD-input module Allows the processor to accept 4-bit BCD digi-
tal codes.
BCD-output module Enables a PLC to operate devices that require
BCD-coded signals to operate.
Binary A number system using 2 as a base. The binary number
system requires only two digits, zero (0) and one (1), to express any
alphanumeric quantity desired by the user.
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374 Glossary
Cell controller A specialized computer used to control a work cell
through multiple paths to the various cell devices.
Central processing unit (CPU) The electronic circuitry that con-
trols all the data activity of the PLC, performs calculations, and makes
decisions with its operation controlled by a sequence of instructions.
The central processing unit is also referred to as the processor or
the CPU.
Channel A designated path for a signal.
Character A symbol that is one of a larger group of similar symbols
and that is used to represent information on a display device. The let-
ters of the alphabet and the decimal numbers are examples of charac-
ters used to convey information.
Chassis A rack that serves as an electrical backplane for a PLC pro-
cessor and I/O modules.
Chip A tiny piece of semiconductor material on which electronic
components are formed. Chips are normally made of silicon and are
typically less than 1/4 in. square and 1/100 in. thick.
Clear An instruction or a sequence of instructions that removes all
current information from a programmable logic controller’s memory.
Clock A circuit that generates timed pulses to synchronize the tim-
ing of computer operations.
Clock rate The speed at which the microprocessor system
operates.
Closed loop A control system that uses feedback from the process to
maintain outputs at a desired level.
Coaxial cable A transmission line constructed such that an outer
conductor forms a cylinder around a central conductor. An insulating
dielectric separates the inner and outer conductors, and the complete
assembly is enclosed in a protective outer sheath. Coaxial cables are
not susceptible to external electric and magnetic fields and generate no
electric or magnetic fields of their own.
Code A system of communications that uses arbitrary groups of
symbols to represent information or instructions. A set of programmed
instructions.
Coil Represents the output of a programmable logic controller.
In the output devices, it is the electrical coil that, when energized,
changes the status of its corresponding contacts.
Coil format Format that uses coils to display instructions.
Comment Text that is included with each PLC ladder rung and is
used to help individuals understand how the program operates or how
the rung interacts with the rest of the program.
Communication module Allows the user to connect the PLC to
high-speed local networks that may differ from the network communi-
cation provided with the PLC.
Compare An instruction that compares the contents of two desig-
nated data memory locations of a programmable logic controller for
equality or inequality.
Compatibility The ability of various specified units to replace one
another with little or no reduction in capability; the ability of units to
be interconnected and used without modification.
Complement A logical operation that inverts a signal or bit.
Complementary metal-oxide semiconductor (CMOS) A logic
base that offers lower power consumption and high-speed operation.
Computer Any electronic device that can accept information, ma-
nipulate it according to a set of preprogrammed instructions, and sup-
ply the results of the manipulation.
Binary-coded decimal (BCD) A system of numbering that ex-
presses each individual decimal digit (0 through 9) of a number as a
series of 4-bit binary notations. The binary-coded decimal system is
often referred to as 8421 code.
Binary word A related group of 1s and 0s that has meaning as-
signed by position or by numerical value in the binary system of
numbers.
Bit An abbreviated term for the words binary digit. The bit is the
smallest unit of information in the binary numbering system. It repre-
sents a decision between one of two possible and equally likely values
or states. It is often used to represent an off or on state as well as a true
or false condition.
Bit manipulation instructions A family of programmable logic
controller instructions that exchange, alter, move, or otherwise modify
the individual bits or groups of processor data memory words.
Bit storage A user-defined data table area in which bits can be
set or reset without directly affecting or controlling output devices.
However, any storage bit can be monitored as necessary in the user
program.
Block diagram A method of representing the major functional sub-
divisions, conditions, or operations of an overall system, function, or
operation.
Block format Format that uses a box shape to display instructions.
Block transfer An instruction that copies the contents of one or
more contiguous data memory words to a second contiguous data
memory location; an instruction that transfers data between an intelli-
gent input/output module or card and specified processor data memory
locations.
BOOL A data type that stores the state of a single bit, where 0
equals off and 1 equals on.
Boolean algebra A mathematical shorthand notation that expresses
logic functions, such as AND, OR, EXCLUSIVE OR, NAND, NOR,
and NOT.
Branch A parallel logic path within a rung.
Buffer In software terms, a register or group of registers used for
temporary storage of data; a buffer is used to compensate for transmis-
sion rate differences between the transmitter and receiving device. In
hardware terms, a buffer is an isolating circuit used to avoid the reac-
tion of one circuit with another.
Bug A system defect or error that causes a malfunction; can be
caused by either software or hardware.
Burn The process by which information is entered into program-
mable read-only memory.
Bus A group of lines used for data transmission or control; power
distribution conductors.
Bus topology A network configuration in which all stations are con-
nected in parallel with the communication medium and all stations can
receive information from any other station on the network.
Byte A group of adjacent bits usually operated on as a unit, such as
when moving data to and from memory. There are 8 bits per byte.
C
Cascading In the programming of timers and counters, a tech-
nique used to extend the timing or counting range beyond what
would normally be available. This technique involves the driving of
one timer or counter instruction from the output of another, similar
instruction.
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Glossary 375
Cycle A sequence of operations repeated regularly; the time it takes
for one such sequence to occur.
D
Data Information encoded in a digital form, which is stored in an
assigned address of data memory for later use by the processor.
Data address A location in memory where data can be stored.
Data file A group of data memory words acted on as a group rather
than singly.
Data highway A communications network that allows devices such
as PLCs to communicate. They are normally proprietary, which means
that only like devices of the same brand can communicate over the
highway.
Data latching A technique used to read the value of the input data
that will be operated on by the instructions with a function block.
Data link The equipment that makes up a data communications
network.
Data manipulation The process of exchanging, altering, or moving
data within a programmable logic controller or between program-
mable logic controllers.
Data manipulation instructions A classification of processor
instructions that alter, exchange, move, or otherwise modify data
memory words.
Data table The part of processor memory that contains input and
output values as well as files where data are monitored, manipulated,
and changed for control purposes.
Data transfer The process of moving information from one loca-
tion to another, in other words, from register to register, from device to
device, and so forth.
Data transmission line A medium for transferring signals over a
distance.
Debouncing The act of removing intermediate noise states from a
mechanical switch.
Debug The process of locating and removing mistakes from a soft-
ware program or from hardware interconnections.
Decimal number system A number system that uses ten numeral
digits (decimal digits): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Each digit position
has a place value of 1, 10, 100, 1000, and so on, beginning with the
least significant (rightmost) digit; base 10.
Decrement The act of reducing the contents of a storage location or
value in varying increments.
Determinism The ability to reliably predict when data will be
delivered.
DeviceNet An open communication network designed to connect
factory-floor devices together without interfacing through an I/O system.
Up to 64 intelligent nodes can be connected to one DeviceNet network.
Diagnostic program A user program designed to help isolate
hardware malfunctions in the programmable logic controller and the
application equipment.
Diagnostics The detection and isolation of an error or malfunction.
Digital device One that processes discrete electric signals.
Digital gate A device that analyzes the digital states of its inputs and
outputs and an appropriate output state.
Digital signals A system of discrete states: high or low, on or off,
1 or 0.
Computer integrated manufacturing (CIM) A manufacturing sys-
tem controlled by an easily reprogrammable computer for flexibility
and speed of changeover.
Computer interface A device designed for data communication
between a programmable logic controller and a computer.
Consumed tag References data that come from another controller.
Contact The current-carrying part of an electric relay or switch.
The contact engages to permit power flow and disengages to interrupt
power flow to a load device.
Contact bounce The uncontrollable making and breaking of a con-
tact during the initial engaging or disengaging of the contact.
Contact histogram An instruction sequence that monitors a desig-
nated memory bit or a designated input or output point for a change of
state. A listing is generated by the instruction sequence that displays
how quickly the monitored point is changing state.
Contactor A special-purpose relay designed to establish and inter-
rupt the power flow of high-current electric circuits.
Contact symbology A set of symbols used to express the control
program with conventional relay symbols.
Continuous current per module The maximum current for each
module. The sum of the output current for each point should not ex-
ceed this value.
Continuous current per point The maximum current each output is
designed to supply continuously to a load.
Controlled variable The output variable that the automatic control
adjusts to keep the process value at a set-point.
Control logic The control plan for a given system; the program.
Control loop The method of adjusting the control variable in a pro-
cess control system by analyzing the process variable data and then
comparing it to the set-point to determine the amount of error in the
system.
ControlNet An open, high-speed, deterministic network that
transfers on the same network time-critical I/O updates, controller-to-
controller interlocking data, and non-time-critical data such as data
monitoring and program uploads and downloads.
Control relay A relay used to control the operation of an event or a
sequence of events.
Counter An electromechanical device in relay-based control sys-
tems that counts numbers of events for the purpose of controlling
other devices based on the current number of counts recorded; a pro-
grammable logic controller instruction that performs the functions of
its electromechanical counterpart.
Cross reference In ladder diagrams, letters or numbers to the right
of coils or functions. The letters or numbers indicate where on other
ladder lines contacts of the coil or function are located.
Crosstalk Undesired energy appearing in one signal path as a result
of coupling from other signal paths or use of a common return line.
Current The rate of electrical electron movement, measured in
amperes.
Current-carrying capacity The maximum amount of current a con-
ductor can carry without heating beyond a predetermined safe limit.
Current sinking Refers to an output device (typically an NPN transis-
tor) that allows current flow from the load through the output to ground.
Current sourcing Output device (typically a PNP transistor) that al-
lows current flow from the output through the load and then to ground.
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376 Glossary
Enclosure A steel box with a removable cover or hinged door used
to house electric equipment.
Encoder A rotary device that transmits position information; a de-
vice that transmits a fixed number of pulses for each revolution.
Energize The physical application of power to a circuit or device to
activate it; the act of setting the on, true, or 1 state of a programmable
logic controller’s relay ladder diagram output device or instruction.
Erasable programmable read-only memory (EPROM) A pro-
grammable read-only memory that can be erased with ultraviolet light,
then reprogrammed with electrical pulses.
Error signal A signal proportional to the difference between the
actual output and the desired output.
Ethernet An network type or protocol that uses the Carrier Sense
Multiple Access Collision Detection (CSMA/CD) network access
method.
Ethernet/IP An open industrial networking standard that takes ad-
vantage of commercial off-the-shelf Ethernet communication devices
and physical media; IP refers to industrial protocol.
Even parity When the sum of the number of 1s in a binary word is
always even.
Examine if closed (XIC) Refers to a normally open contact instruc-
tion in a logic ladder program. An examine if closed instruction is true
if its addressed bit is on (1). It is false if the bit is off (0).
Examine if open (XIO) Refers to a normally closed contact instruc-
tion in a logic ladder program. An examine if open instruction is true
if its addressed bit is off (0). It is false if the bit is on (1).
Exclusive-OR gate A logic device requiring one or the other, but
not both, of its inputs to be satisfied before activating its output.
Execution The performance of a specific operation accomplished
through processing one instruction, a series of instructions, or a com-
plete program.
Execution time The total time required for the execution of one
specific operation.
F
False As related to programmable logic controller instructions, a
disabling logic state.
Fault Any malfunction that interferes with normal operation.
Fault indicator A diagnostic aid that provides a visual indication
and/or an internal processor software indication that a fault is present
in the system.
Fault-routine file A special subroutine that, if assigned, executes
when the processor has a major fault.
Feedback In analog systems, a correcting signal received from the
output or an output monitor. The correcting signal is fed to the control-
ler for process correction.
Fiber optic cable Transmits information via light pulses down
optical fibers.
Fieldbus An open, all-digital, serial, two-way communications sys-
tem that interconnects measurement and control equipment such as
sensors, actuators, and controllers.
File A formatted block of data treated as a unit.
Fixed I/O Input/output terminals on a programmable logic control-
ler that are built into the unit and are not changeable. A fixed I/O PLC
has no removable modules.
Digital-to-analog converter An electrical circuit that converts
binary bits to a representative, continuous, analog signal.
DINT A data type that stores a 32-bit (4-byte) signed integer value.
DIP switch A group of small, in-line on-off switches. From dual
in-line package.
Direct addressing An addressing mode in which the memory
address of the data is supplied with the instruction.
Discrete I/O A group of input and/or output modules that operate
with on/off signals, as contrasted to analog modules that operate with
continuously variable signals.
Disk drive The device that writes or reads data from a magnetic
disk.
Diskette The flat, flexible disk on which a disk drive writes and
reads.
Display The image that appears on a cathode-ray tube screen or on
other image projection systems.
Display menu The list of displays from which the user selects spe-
cific information for viewing.
Distributed control A method of dividing process control into sev-
eral subsystems. A PLC oversees the entire operation.
Divide A programmable logic controller instruction that performs a
numerical division of one number by another.
Documentation An orderly collection of recorded hardware and
software data such as tables, listings, and diagrams to provide refer-
ence information for programmable logic controller application opera-
tion and maintenance.
Done bit (DN) Bit that is set to 1 when the instruction has com-
pleted its task, such as reaching its preset value.
Double precision The system of using two addresses or registers
to display a number too large for one address or register; allows the
display of more significant figures because twice as many bits are used.
Down-counter A counter that starts from a specified number and
increments down to zero.
Download Loading data from a master listing to a readout or
another position in a computer system.
Dry-contact-output module Enables a PLC’s processor to control
output devices by providing a contact isolated electrically from any
power source.
E
Edit The act of modifying a programmable logic controller program
to eliminate mistakes and/or simplify or change system operation.
Electrically erasable programmable read-only memory
(EEPROM) A type of programmable read-only memory that is
programmed and erased by electrical pulses.
Electrical optical isolator A device that couples input to output
using a semiconductor light source and detector in the same package.
Electromagnetic interference (EMI) A phenomenon responsible
for noise in electric circuits.
Element A single instruction of a relay ladder diagram program.
Emergency stop relay A relay used to inhibit all electric power to a
control system in an emergency or other event requiring that the con-
trolled hardware be brought to an immediate halt.
Enable To permit a particular function or operation to occur under
natural or preprogrammed conditions.
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Glossary 377
Hardwired The physical interconnection of electric and electronic
components with wire.
Hexadecimal A number system having a base of 16. This number-
ing system requires 16 elements for representation, and thus uses the
decimal digits zero (0) through nine (9) and the first six letters of the
alphabet, A through F.
High-speed counter encoder module A module that enables you to
count and encode faster than you could with a regular control program
written on a PLC in which the control program’s execution is too slow.
Histogram A graphic representation of the frequency at which an
event occurs.
Host computer A main computer that controls other computers,
PLCs, or computer peripherals.
Human machine interface (HMI) Graphical display hardware in
which machine status, alarms, messages, diagnostics, and data entry
are available to the operator in graphical display format.
I
IEC 1131 programming standard The international standard for
programmable controller programming languages.
Image table An area in programmable logic controller memory
dedicated to input/output data. Ones and zeros (1s and 0s) represent on
and off conditions, respectively. During every input/output scan, each
input controls a bit in the input image table file; each output is con-
trolled by a bit in the output image table file.
Immediate input instruction A programmable logic controller
instruction that temporarily halts the user program scan so that the
processor can update the input image table file with the current status
of one or more user-specified input points.
Immediate output instruction A programmable logic controller
instruction that temporarily halts the user program scan so that the cur-
rent status of one or more user-specified output points can be updated
to current output image table file status by the processor.
Impedance The total resistive and inductive opposition that an
electric circuit or device offers to a varying current at a specified fre-
quency. Impedance is measured in ohms (V) and is denoted by the
symbol Z.
Increment The act of increasing the contents of a storage location
or value in varying amounts.
Index A reference used to specify an element within an array.
Indirect addressing An addressing mode in which the address of
the instruction serves as a reference point instead of the actual address.
Inductance A circuit property that opposes any current change.
Inductance is measured in henrys and is represented by the letter H.
Industrial terminal The device used to enter and monitor the pro-
gram in a PLC.
Input Information transmitted from a peripheral device to the input
module and then to the data table.
Input devices Devices such as limit switches, pressure switches,
pushbuttons, and analog and/or digital devices that supply data to a
programmable logic controller.
Input/output (I/O) address A unique number assigned to each
input and output. The address number is used when programming,
monitoring, or modifying a specific input or output.
Input/output (I/O) module A plug-in assembly that contains
more than one input or output circuit. A module usually contains two
Floating-point data file Used to store integers and other numerical
values that cannot be stored in an integer file.
Flowchart A graphical representation for the definition, analysis,
or solution of a problem. Symbols are used to represent a process or
sequence of decisions and events.
Force function A mode of operation or instruction that allows an
operator to override the processor to control the state of a device.
Force off function A feature that allows the user to reset an input
image table file bit or de-energize an output independently of the pro-
grammable logic controller program.
Force on function A feature that allows the user to set an image
table file bit or energize an output independently of the programmable
logic controller program.
Full duplex A mode of data communications in which data may be
transmitted and received simultaneously.
Function block Rectangular block with inputs entering from the left
and outputs exiting on the right.
Function block diagram (FBD) Graphical language where the
basic programming elements appear as blocks.
Function keys Keys on a personal computer, electronic operator
device, or hand-held programmer keyboard that are labeled F1, F2,
and so on. The operation of each of these keys is defined on many
electronic operator interface devices.
G
Gate A circuit having two or more input terminals and one output
terminal, where an output is present when and only when the pre-
scribed inputs are present.
Gateway A device or pair of devices that connects two or more
communication networks. This device may act as a host to each net-
work and may transfer messages between the networks by translating
their protocols.
Glitch A voltage or current spike of short duration that adversely
affects the operation of a PLC.
Gray code A binary coding scheme that allows only 1 bit in the data
word to change state at each increment of the code sequence.
Gray-encoder module Converts the Gray-code signal from an input
device into straight binary.
Ground A conducting connection between an electric circuit or
equipment chassis and the earth ground.
Ground loop A condition in which two or more electrical paths
exist within a ground line.
Ground potential Zero voltage potential with respect to the ground.
H
Half-duplex A mode of data transmission that communicates in two
directions but in only one direction at a time.
Handshaking The method by which two digital machines establish
communication.
Hard contacts Any type of physical switch contacts.
Hard copy Any form of a printed document such as a ladder dia-
gram program listing, paper tape, or punched cards.
Hard drive An inflexible recording disk used as a computer disk drive.
Hardware The mechanical, electric, and electronic devices that
make up a programmable logic controller and its application.
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378 Glossary
J
Jumper A short length of conduit used to make a connection be-
tween terminals around a break in a circuit.
Jump instruction An instruction that permits the bypassing of se-
lected portions of the user program. Jump instructions are conditional
whenever their operation is determined by a set of preconditions and
unconditional whenever they are executed to occur every time they are
programmed.
K
K 210
5 1K 5 1024; used to denote size of memory and can be
expressed in bits, bytes, or words; example: 2K 5 2048.
k Kilo; a prefix used with units of measurement to designate quanti-
ties 1000 times as great.
Keyboard The alphanumeric keypad on which the user types
instructions to the PLC.
Keying Bands installed on backplane connectors to ensure that only
one type of module can be inserted into a keyed connector.
L
Label instruction A programmable logic controller instruction
that assigns an alphanumeric designation to a particular location in a
program. This location is used as the target of a jump, skip, or jump to
subroutine instruction.
Ladder diagram An industry standard for representing relay logic
control systems. The diagram resembles a ladder because the vertical
supports of the ladder appear as power feed and return buses and the
horizontal rungs of the ladder appear as series and/or parallel circuits
connected across the power lines.
Ladder diagram programming A method of writing a user pro-
gram in a format similar to a relay ladder diagram.
Ladder matrix A rectangular array of programmed contacts that de-
fines the number of contacts that can be programmed across a row and
the number of parallel branches allowed in a single ladder rung.
Language A set of symbols and rules for representing and commu-
nicating information among people or between people and machines;
the method used to instruct a programmable device to perform various
operations.
Language module Enables the user to write programs in a high-
level language. BASIC is the most popular language module. Other
language modules available include C, Forth, and PASCAL.
Latching relay A relay that maintains a given position by mechani-
cal or electrical means until released mechanically or electrically.
Latch instruction One-half of an instruction pair (the second
instruction of the pair being the unlatch instruction) that emulates the
latching action of a latching relay. The latch instruction for a program-
mable logic controller energizes a specified output point or internal
coil until it is de-energized by a corresponding unlatch instruction.
Leakage The small amount of current that flows in a semiconductor
device when it is in the off state.
Least significant bit (LSB) The bit that represents the smallest
value in a byte or word.
Least significant digit (LSD) The digit that represents the smallest
value in a byte or word.
Light-emitting diode (LED) A semiconductor junction that emits
light when biased in the forward direction.
or more identical circuits. Normally, it contains 2, 4, 8, 16 32, or
64 circuits.
Input/output (I/O) scan time The time required for the processor to
monitor inputs and control outputs.
Input/output (I/O) update The continuous process of revising each
and every bit in the input and output tables, based on the latest results
from reading the inputs and processing the outputs according to the
control program.
Input scan One of three parts of the PLC scan. During the
input scan, input terminals are read and the input table is updated
accordingly.
Instruction A command that causes a programmable logic control-
ler to perform one specific operation. The user enters a combination of
instructions into the programmable logic controller’s memory to form
a unique application program.
Instruction set The set of general-purpose instructions available
with a given controller. In general, different machines have different
instruction sets.
INT Two-byte integer.
Integer A positive or negative whole number.
Integrated circuit (IC) A circuit in which all components are inte-
grated on a single tiny silicon chip.
Intelligent field devices Microprocessor-based devices used to
provide process-variable, performance, and diagnostic information to
the PLC processor. These devices are able to execute their assigned
control functions with little interaction, except communications, with
their host processor.
Intelligent input/output module A microprocessor-based module
that performs processing or sophisticated closed-loop application
functions.
Interface A circuit that permits communication between the central
processing unit and a field input or output device. Different devices
require different interfaces.
Interlock A system for preventing one element or device from
turning on while another device is on.
Internal coil instruction A relay coil instruction used for internal
storage or buffering of an on/off logic state. An internal coil instruc-
tion differs from an output coil instruction because the on/off status of
the internal coil is not passed to the input/output hardware for control
of a field device.
Inversion Conversion of a high level to a low level, or vice versa.
Inverter The digital circuit that performs inversion.
I/O module A plug-in assembly, containing two or more identical
input or output circuits, that contain the connections between a proces-
sor and connected devices.
Interrupt The act of redirecting a program’s execution to perform a
more urgent task.
IP address A specified Internet protocol address for every Ethernet
device that is unique and is assigned by the manufacturer.
Isolated input module A module that receives dry contacts as
inputs, which the processor can recognize and change into two-state
digital signals.
Isolated input/output (I/O) circuits Input and output circuits that
are electrically isolated from any and all other circuits of a module.
Isolated input/output circuits are designed to allow field devices that
are powered from different sources to be connected to one module.
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Glossary 379
Master control relay (MCR) A mandatory hardwired relay that
can be de-energized by any series-connected emergency stop switch.
Whenever the master control relay is de-energized, its contacts open to
de-energize all application input and output devices.
Master control relay (MCR) zones User program areas in which
all nonretentive outputs can be turned off simultaneously. Each master
control relay zone must be delimited and controlled by master control
relay fence codes (master control relay instructions).
Matrix A logic network that is an intersection of input and output
connection points.
Memory That part of the programmable logic controller in which
data and instructions are stored either temporarily or semi-permanently.
The control program is stored in memory.
Memory map A diagram showing a system’s memory addresses
and what programs and data are assigned to each section of memory.
Menu A list of programming selections displayed on a program-
ming terminal.
Metal-oxide semiconductor (MOS) A semiconductor device in
which an electric field controls the conductance of a channel under a
metal electrode called a gate.
Metal oxide varistor (MOV) Used for suppressing electrical power
surges.
Microprocessor A central processing unit manufactured on a single
integrated-circuit chip (or several chips) by utilizing large-scale inte-
gration technology.
Microsecond One millionth of a second 5 1 3 1026
second 5
0.000001 second.
Millisecond One thousandth of a second 5 1 3 1023
second 5
0.001 second.
Mnemonic A term, usually an abbreviation, that is easy to remem-
ber and pronounce.
Mnemonic code A code in which information is represented by
symbols or characters.
Modbus A network that uses a master/slave communication technique.
Mode A term used to refer to the selected operating method, such as
automatic, manual, TEST, PROGRAM, or diagnostic.
Module An interchangeable, plug-in item containing electronic
components.
Module addressing A method of identifying the input/output mod-
ules installed in a chassis.
Most significant bit (MSB) The bit representing the greatest value
of a byte or word.
Most significant digit (MSD) The digit representing the greatest
value of a byte or word.
Motor controller or starter A device or group of devices that serve
to govern, in a predetermined manner, the electric power delivered to
a motor.
Motor starter A special relay designed to provide power to mo-
tors; it has both a contactor relay and an overload relay connected in
series and prewired so that, if the overload operates, the contactor is
de-energized.
Move instruction A programmable logic controller instruction that
moves data from one location to another. Although a move instruction
typically places the data in a new location, the original data still reside
in their original location.
Light-emitting diode (LED) display A display device incorporating
light-emitting diodes to form the segments of the displayed characters
and numbers.
Limit switch An electric switch actuated by some part and/or
motion of a machine or equipment.
Limit test A test that determines if a value is inside or outside a
specified range.
Line A component part of a system used to link various subsystems
located remotely from the processor; the source of power for opera-
tion; example: 120 V alternating current line.
Line-powered sensor Normally, three-wire sensors, although four-
wire sensors also exist. The line-powered sensor is powered from the
power supply. A separate wire (the third) is used for the output line.
Liquid-crystal display (LCD) A display device using reflected light
from liquid crystals to form the segments of the displayed characters
and numbers.
Load The power used by a machine or apparatus; to place data into
an internal register under program control; to place a program from an
external storage device into central memory under operator control.
Load-powered sensor A two-wire sensor. A small leakage current
flows through the sensor even when the output is off. The current is
required to operate the sensor electronics.
Local area network (LAN) A system of hardware and software de-
signed to allow a group of intelligent devices to communicate within a
fairly close proximity.
Local input/output (I/O) A programmable logic controller whose
input/output distance is physically limited. The PLC must be located
near the process; however, the PLC may still be mounted in a separate
enclosure.
Local power supply The power supply used to provide power to the
processor and a limited number of local input/output modules.
Location In reference to memory, a storage position or register iden-
tified by a unique address.
Logic A process of solving complex problems through the repeated
use of simple functions that can be either true or false. The three basic
logic functions are AND, OR, and NOT.
Logic diagram A diagram that represents the logic elements and
their interconnections.
Logic level The voltage magnitude associated with signal pulses
representing 1s and 0s in binary computation.
Loop control A control of a process or machine that uses feedback.
An output status indicator modifies the input signal effect on the pro-
cess control.
M
Machine language A programmable language using the binary
form.
Major fault A fault condition that is severe enough for the control-
ler to shut down, unless the condition is cleared.
Manufacturing automation protocol (MAP) Standard developed
to make industrial devices communicate more easily.
Masking A means of selectively screening out data. Masking allows
unused bits in a specific instruction to be used independently.
Mass storage A means of storing large amounts of data on magnetic
tape, floppy disks, and so on.
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380 Glossary
NOT A logical operation that yields a logic 1 at the output if a logic 0
is entered at the input, and a logic 0 at the output if a logic 1 is entered
at the input. The NOT, also called the inverter, is normally used in con-
junction with the AND and OR functions.
O
Octal number system A base eight numbering system that uses
numbers 0–7, 10–17, 20–27, and so on. There are no 8s or 9s in the
octal number system.
Odd parity Condition when the sum of the number of 1s in a binary
word is always odd.
Off-delay timer An electromechanical relay with contacts that
change state a predetermined time period after power is removed from
its coil; on re-energization of the coil, the contacts return to their shelf
state immediately; also, a programmable logic controller instruction
that emulates the operation of the electromechanical off-delay relay.
Offline programming and/or offline editing A method of program-
mable logic controller programming and/or editing in which the op-
eration of the processor is stopped and all output devices are switched
off. Offline programming is the safest way to develop or edit a pro-
grammable logic controller program since the entry of instructions
does not affect operating hardware until the program can be verified
for accuracy of entry.
On-delay timer An electromechanical relay with contacts that
change state a predetermined time period after the coil is energized;
the contacts return to their shelf state immediately on de-energization
of the coil; also, a programmable logic controller instruction that emu-
lates the operation of the electromechanical on-delay timer.
One-shot A programmed technique that sets a storage bit or output
for only one program scan.
Online data change Allows the user to change various data table
values using a peripheral device while the application is operating
normally.
Online programming and/or online editing The ability of a proces-
sor and programming terminal to make joint user-directed additions,
deletions, or changes to a user program while the processor is actively
solving and executing the commands of the existing user program.
Extreme care should be exercised when performing online program-
ming to ensure that erroneous system operation does not result.
Open loop A system that has no feedback or auto correction.
Operand A number used in an arithmetic operation as an input.
Operational amplifier (op-amp) A high-gain DC amplifier used to
increase signal strength for devices such as analog input modules.
Optical coupler A device that couples signals from one circuit to
another by means of electromagnetic radiation, usually infrared or
visible. A typical optical coupler uses a light-emitting diode to convert
the electric signal of the primary circuit into light and uses a photo-
transistor in the secondary circuit to reconvert the light back into an
electric signal; sometimes referred to as optical isolation.
Optical isolation Electrical separation of two circuits with the use
of an optical coupler.
OR A logical operation that yields a logic 1 output if one of any
number of inputs is 1, and a logic 0 if all inputs are 0.
Output Information sent from the processor to a connected device
via some interface. The information could be in the form of control
data that will signal some device such as a motor to switch on or off or
to vary the speed of a drive.
Multiplexing The time-shared scanning of a number of data lines
into a single channel, and only one data line is enabled at any time; the
incorporation of two or more signals into a single wave from which
the individual signals can be recovered.
Multiply instruction A programmable logic controller instruction
that provides for the mathematical multiplication of two numbers.
Multiprocessing A method of applying more than one microproces-
sor to a specific function to speed up operation time and reduce the
possibility of system failure.
N
National Electrical Code (NEC) A set of regulations developed by
the National Fire Protection Association that governs the construction
and installation of electric wiring and electric devices. The National
Electrical Code is recognized by many governmental bodies, and com-
pliance is mandatory in much of the United States.
National Electrical Manufacturers Association (NEMA) An or-
ganization of electric device and product manufacturers. The National
Electrical Manufacturers Association issues standards relating to the
design and construction of electric devices and products.
NEMA Type 12 enclosure A category of industrial enclosures
intended for indoor use and designed to provide a degree of protec-
tion against dust, falling dirt, and dripping noncorrosive liquids.
They do not provide protection against conditions such as internal
condensation.
Nested branches A branch that begins or ends within another
branch.
Network A series of stations or devices connected by some type of
communications medium.
Network access control The method of accessing the network
media (cable) to ensure that data are transmitted in an organized man-
ner in order to reduce the possibilities of data corruption.
Node In hardware, a connection point on the network; in program-
ming, the smallest possible increment in a ladder diagram.
Noise Random, unwanted electric signals, normally caused by radio
waves or electric or magnetic fields generated by one conductor and
picked up by another.
Noise filter or noise suppressor An electronic filter network used to
reduce and/or eliminate any noise that may be present on the leads to
an electric or electronic device.
Noise immunity A measure of insensitivity of an electronic system
to noise.
Noise spike A short burst of electric noise with more magnitude
than the background noise level.
Nonretentive output An output controlled continuously by a pro-
gram rung. Whenever the rung changes state (true or false), the output
turns on or off; contrasted with a retentive output, which remains in
its last state (on or off) depending on which of its two rungs, latch or
unlatch, was last true.
Nonvolatile memory A memory designed to retain its data while its
power supply is turned off.
NOR The logic gate that results in zero unless both inputs are zero.
Normally closed contact (NC) A contact that is conductive when
its operating coil is not energized.
Normally open contact (NO) A contact that is nonconductive when
its operating coil is not energized.
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Glossary 381
Polarity The directional indication of electrical flow in a circuit; the
indication of charge as either positive or negative, or the indication of
a magnetic pole as either north or south.
Polling A network access method where a master controller man-
ages the communication process by interrogating each slave controller
under it to determine whether the slave has any information to send.
Port A connector or terminal strip used to access a system or
circuit. Generally, ports are used for the connection of peripheral
equipment.
Power supply The unit that supplies the necessary voltage and cur-
rent to a system’s circuitry.
Preset value (PRE) The number of time intervals or events to be
counted.
Pressure switch A switch activated at a specified pressure.
Process A continuous manufacturing operation.
Program A sequence of instructions to be executed by the processor
to control a machine or process.
Program files The area of processor memory in which the ladder
logic programming is stored.
Programmable controller A computer that has been hardened to
work in an industrial environment and is equipped with special I/O
and a control programming language.
Programmable read-only memory (PROM) A retentive memory
used to store data. This type of memory device can be programmed
only once and cannot be altered afterward.
Programming terminal A combination of keyboard and monitor
used to insert, modify, and observe programs stored in a PLC.
Program scan One of three parts of the PLC scan. During the pro-
gram scan, the CPU scans each rung of the user program.
Project file Contains all data associated with the PLC project. A
project comprises five major pieces: help folder, controller folder, lad-
der folder, data folder, and data base folder.
Proportional-integral-derivative (PID) A mathematical formula
that provides a closed-loop control of a process. Inputs and outputs are
continuously variable and typically will be analog signals.
Protocol A formal definition of criteria for receiving and transmit-
ting data through communications channels.
Proximity switch An input device that senses the presence or ab-
sence of a target without physical contact.
Pulse A short change in the value of a voltage or current level. A
pulse has a definite rise and fall time and a finite duration.
R
Rack A housing or framework used to hold assemblies; a plastic
and/or metal assembly that supports input/output modules and pro-
vides a means of supplying power and signals to each input/output
module or card.
Random-access memory (RAM) A memory system that permits
the random accessing of any storage location for the purpose of either
storing (writing) or retrieving (reading) information. Random-access
memory systems allow the data to be retrieved and stored at speeds
independent of the storage locations being accessed.
Read The accessing of information from a memory system or data
storage device; the gathering of information from an input device or
devices or a peripheral device.
Output device Any connected equipment that will receive informa-
tion or instructions from the central processing unit, such as control
devices (e.g., motors, solenoids, alarms) or peripheral devices (e.g.,
line printers, disk drives, displays). Each type of output device has a
unique interface to the processor.
Output image table file A portion of a processor’s data memory
reserved for the storage of output device statuses. A 1, on, or true state
in an output image table file storage location is used to switch on the
corresponding output point.
Output instruction The term applied to any programmable logic
controller instruction capable of controlling the discrete or analog sta-
tus of an output device connected to the programmable logic controller.
Output register or output word A particular word in a processor’s
output image table file in which numerical data are placed for trans-
mission to a field output device.
Output scan One of three parts of the PLC scan. During the output
scan, data associated with the output status table are transferred to the
output terminals.
Overflow Exceeding the numerical capacity of a device such as a
timer or counter. The overflow can be either a positive or negative
value.
Overload A load greater than the one that a component or system is
designed to handle.
Overload relay A special-purpose relay designed so that its contacts
transfer whenever its current exceeds a predetermined value. Overload
relays are used with electric motors to prevent motor burnout due to
mechanical overload.
P
Parallel circuit A circuit in which two or more of the connected
components or contact symbols in a ladder program are connected
to the same pair of terminals so that current may flow through all the
branches; contrasted with a series connection, in which the parts are
connected end to end so that current flow has only one path.
Parallel instruction A programmable logic controller instruction
used to begin and/or end a parallel branch of instructions programmed
on a programming terminal.
Parallel operation A type of information transfer in which all bits,
bytes, or words are handled simultaneously.
Parallel transmission A computer operation in which two or more
bits of information are transmitted simultaneously.
Parity The use of a self-checking code that employs binary digits in
which the total number of 1s is always even or odd.
PC Personal computer.
Peer-to-peer network A network in which nodes are given an equal
chance of initiating and controlling communications.
Peripheral equipment Units that communicate with the program-
mable logic controller but are not part of the programmable logic con-
troller; example: a programming device or computer.
PID Proportional-integral-derivative closed-loop control that lets the
user hold a process variable at a desired set-point.
Pilot-type device Used in a circuit as a control apparatus to carry
electric signals for directing performance. This device does not carry
primary current.
PLC processor A computer designed specifically for programmable
controllers. It supervises the action of the modules attached to it.
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382 Glossary
S
SCADA An acronym for supervisory control and data acquisition.
Scan time The time required to read all inputs, execute the control
program, and update local and remote input and output statuses. Scan
time is, in effect, the time required to activate an output controlled by
programmed logic.
Schematic A diagram of graphic symbols representing the electrical
scheme of a circuit.
Search function Allows the user to display quickly any instruction
in the programmable logic controller program.
Self-diagnostic The hardware and firmware within a controller that
monitors its own operation and indicates any fault it can detect.
Sensor A device used to gather information by the conversion of a
physical occurrence to an electric signal.
Sequencer A mechanical, electric, or electronic device that can be
programmed so that a predetermined set of events occurs repeatedly.
Sequence table A table or chart indicating the sequence of opera-
tion of output devices.
Sequential control A process that dictates the correct order of events
and allows one event to occur only after the completion of another.
Sequential Function Chart (SFC) Graphical language whose basic
language elements are steps or states with associated actions and tran-
sitions with associated conditions used to move from the current state
to the next.
Serial communication A type of information transfer in which the
bits are handled sequentially; contrasted with parallel communication.
Series circuit A circuit in which the components or contact symbols
are connected end to end, and all must be closed to permit current
flow.
Servo module The device whose feedback is used to accomplish
closed-loop control. Though programmed through a PLC, once pro-
grammed it can control a device independently without interfering
with the PLC’s normal operation.
Set-point The value that the process value is to be held to by the
automatic control function.
Shield A barrier, usually conductive, that substantially reduces the
effect of electric and/or magnetic fields.
Shift To move binary data within a shift register or other storage
device.
Shift register A PLC function capable of storing and shifting binary
data.
Short circuit An undesirable path of very low resistance in a circuit
between two points.
Short-circuit protection Any fuse, circuit breaker, or electronic
hardware used to protect a circuit or device from severe overcurrent
conditions or short circuits.
Signal The event or electrical quantity that conveys information
from one point to another.
Significant digit A digit that contributes to the precision of a num-
ber. The number of significant digits is counted beginning with the
digit contributing the most value, called the most significant digit (left-
most), and ending with the digit contributing the least value, called the
least significant digit (rightmost).
Silicon-controlled rectifier (SCR) A semiconductor device that
functions as an electronic switch.
Read-only memory (ROM) A permanent memory structure in
which data are placed at time of fabrication or by the user at a speed
much slower than it will be read. Information entered in a read-only
memory is usually not changed once it is entered.
Read/write memory Memory in which data can be stored (write
mode) or accessed (read mode). The write mode replaces previously
stored data with current data; the read mode does not alter stored data.
Real numbers Numbers that have both integer and fractional parts.
Real-time clock (RTC) A device that continually measures time in
a system without respect to what tasks the system is performing.
Rectifier A solid-state device that converts alternating current to
pulsed direct current.
Register A memory word or area for the temporary storage of data
used within mathematical, logical, or transfer functions.
Relay An electrically operated device that mechanically switches
electric circuits.
Relay contacts The contacts of a relay that are either opened or
closed according to the condition of the relay coil. Relay contacts are
designated as either normally open or normally closed in design.
Relay logic A representation of the program or other logic in a form
normally used for relays.
Remote input/output (I/O) system Any input/output system that
permits communication between the processor and input/output hard-
ware over a coaxial or twin axial cable. Remote input/output systems
permit the placement of input/output hardware at any distance from
the processor.
Resolution The smallest distinguishable increment into which a
quantity is divided.
Response time The amount of time required for a device to react to
a change in its input signal or to a request.
Retentive instruction Any programmable logic controller instruc-
tion that does not need to be continuously controlled for operation.
Loss of power to the instruction does not halt execution or operation
of the instruction.
Retentive timer An electromechanical relay that accumulates time
whenever the device receives power and maintains the current time
should power be removed from the device. Loss of power to the device
after reaching its preset value does not affect the state of the contacts.
Retentive timer instruction A programmable logic controller in-
struction that emulates the timing operation of the electromechanical
retentive timer.
Retentive timer reset instruction A programmable logic controller
instruction that emulates the reset operation of the electromechanical
retentive timer.
Ring topology A network topology that that forms a data path in
a ring.
Routine A series of instructions that perform a specific function
or task.
RS-232 An Electronic Industries Association (EIA) standard for data
transfer and communication for serial binary communication circuits.
Run The single, continuous execution of a program by a program-
mable logic controller.
Rung A group of programmable logic controller instructions that
controls an output or storage bit, or performs other control functions
such as file moves, arithmetic, and/or sequencer instructions. A rung is
represented as one section of a ladder logic diagram.
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Glossary 383
a station and the signals transmitted by the stations to be passed to the
trunk.
Task It holds the information necessary to schedule the program’s
execution and sets the execution priority for one or more programs.
Terminal address The alphanumeric address assigned to a particu-
lar input or output point. It is also related directly to a specific image
table bit address.
Thermocouple A temperature-measuring device that utilizes two
dissimilar metals for temperature measurement. As the junction of
the two dissimilar metals is heated, a proportional voltage difference,
which can be measured, is generated.
Thumbwheel switch A rotating switch used to input numeric infor-
mation into a controller.
Time base A unit of time generated by a microprocessor’s clock cir-
cuit and used by PLC timer instructions. Typical time bases are 0.01,
0.1, and 1.0 second.
Timed contact A normally open and/or normally closed contact that
is actuated at the end of a timer’s time-delay period.
Timer In relay-panel hardware, an electromechanical device that
can be wired and preset to control the operating interval of other de-
vices. In a programmable logic controller, a timer is internal to the
processor; that is, it is controlled by a user-programmed instruction.
Toggle switch A panel-mounted switch with an extended lever; nor-
mally used for on/off switching.
Token The logical right to initiate communications in a communica-
tion network.
Token passing A technique in which tokens are circulated among
nodes in a communication network.
Topology The structure of a communications network; examples are
bus, ring, and star.
Transducer A device used to convert physical parameters such as
temperature, pressure, and weight into electric signals.
Transformer An electric device that converts a circuit’s electrical
energy into a circuit or circuits with different voltages and current
ratings.
Transistor A three-terminal active semiconductor device composed
of silicon or germanium that is capable of switching or amplifying an
electric current.
Transistor-transistor logic (TTL) A semiconductor logic family
in which the basic logic element is a multiple-emitter transistor. This
family of devices is characterized by high speed and medium power
dissipation.
Transitional contact A contact that, depending on how it is pro-
grammed, will be on for one program scan every 0 to 1 transition, or
every 1 to 0 transition, of the referenced coil.
Transmission line A system of one or more electric conductors used
to transmit electric signals or power from one place to another.
Triac A solid-state component capable of switching alternating
current.
True As related to programmable logic controller instructions, an
on, enabled, or 1 state.
Truth table A table listing that shows the state of a given output as a
function of all possible input combinations.
TTL-input module Enables devices that produce TTL-level signals
to communicate with a PLC’s processor.
Single-scan function A supervisory instruction that causes the
control program to be executed for one scan, including input/output
update. This troubleshooting function allows step-by-step inspection
of occurrences while the machine is stopped.
Sink mode output A mode of operation of solid-state devices in
which the device controls the current from the load. For example,
when the output is energized, it connects the load to the negative po-
larity of the supply.
SINT A data type that stores an 8-bit (1-byte) signed integer value.
Snubber A circuit generally used to suppress inductive loads; it
consists of a resistor in series with a capacitor (RC snubber) and/or a
MOV placed across the alternating current load.
Software Programs that control the processing of data in a system,
as contrasted to the physical equipment itself (hardware).
Solid-state switch Any electronic device incorporating a transistor,
silicon-controlled rectifier, or triac semiconductor switch to control the
on/off flow of electric power.
Source mode output A mode of operation of solid-state output de-
vices in which the device controls the current to the load. For example,
when the output is energized, it connects the load to the positive polar-
ity of the supply.
Star topology A network architecture in which all network nodes
are connected to a central device that routes the nodes’ messages.
State The logic 0 or 1 condition in programmable logic controller
memory or at a circuit input or output.
Station Any programmable logic controller, computer, or data ter-
minal connected to, and communicating by means of, a data highway.
Status indicators LEDs that indicate the on-off status of an input or
output point and are visible on the outside of the PLC.
Stepper-motor module Provides pulse trains to a stepper-motor
translator that enables control of a stepper motor.
STI An acronym for selectable time interrupt, a subroutine that ex-
ecutes on a time basis rather than an event basis.
Storage bit A bit in a data table word that can be set or reset but that
is not associated with a physical input or output terminal point.
Structure Text (ST) High-level, text-based language with com-
mands that support a highly structured program development and the
ability to evaluate complex mathematical expressions.
Subroutines Program files that are scanned only when called on by
logic and can be used to break the program into smaller segments.
Subtract A programmable logic controller instruction that performs
the mathematical subtraction of one number from another.
Suppression device A unit that attenuates the magnitude of electri-
cal noise.
Surge A transient wave of current or power.
Synchronous shift register A shift register in which only one
change of state occurs per control pulse.
Synchronous transmission A type of serial transmission that main-
tains a constant time interval between successive events.
T
Tag A text-based name for an area of the controller’s memory where
data are stored.
Tap A device that provides mechanical and electrical connections
to a trunk cable. A tap allows the signals on the trunk to be passed to
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384 Glossary
W
Watchdog timer Monitors logic circuits controlling the processor. If
the watchdog timer, which is reset every scan, ever times out, the pro-
cessor is assumed to be faulty and is disconnected from the process.
Word A grouping or a number of bits in a sequence treated as a unit.
Word length The total number of bits that make up a word. Most
programmable logic controllers use either 8, 16, or 32 bits to form a
word.
Work cell A group of machines that work together to manufacture
a product; normally includes one or more robots. The machines are
programmed to work together in appropriate sequences. Work cells are
often controlled by one or more PLCs.
Write Refers to the process of loading information into memory;
can also refer to block transfer, that is, a transfer of data from the pro-
cessor data table to an intelligent input/output module.
Z
Zone The portion of a PLC ladder program that can be enabled or
disabled by a control function.
TTL-output module Enables a PLC to operate devices requiring
TTL-level signals to operate.
Twisted pair cable A pair of wires that can transmit data; the wires
are twisted to provide protection against crosstalk.
U
Unlatch instruction One-half of a programmable logic control-
ler instruction pair that emulates the unlatching action of a latching
relay. The unlatch instruction de-energizes a specified output point or
internal coil until re-energized by a latch instruction. The output point
or internal coil remains de-energized regardless of whether or not the
unlatch instruction is energized.
Up-counter An event that starts from 0 and increments up to the
preset value.
V
Variable A factor that can be altered, measured, or controlled.
Variable data Numerical information that can be changed during
application operation. It includes timer and counter accumulated val-
ues, thumbwheel settings, and arithmetic results.
Volatile memory A memory structure that loses its information
whenever power is removed. Volatile memories require a battery
backup to ensure memory retention during power outages.
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385
Index
A
AC discrete I/O modules, 23–26
ACC; see Accumulated value
ACC (Accumulator, CLX controller timers), 338
ACC word
counters, 154
timers, 130
Access control, 306–307
Accumulated time, 128
Accumulated (ACC) value
CLX controller counters, 338
counters, 151
SLC 500 counters, 154
SLC 500 timers, 130
Accumulated value (ACC) word
counters, 154
timers, 130
Accumulator (ACC, CLX controller timers), 338
Actuators, 109
in closed-loop systems, 296
in process control systems, 294
ADD (add) instruction, 227–229
CLX controllers, 353–354
SLC 500 controllers, 227–229
Addition
binary, 51, 52
math instructions, 227
Add-On instructions, 361
Addresses
conflicts, 283–284
data file elements and bits, 73
Addressing, 19
CLX controllers, 321–322, 331–332
instructions, 84
rack/slot-based, 19–20
tag-based, 19–21, 321–322
Alarm summary display screens, 295
Alias tags, 322, 323
Allen-Bradley CompactLogix system, 317
Allen-Bradley controllers, 18; see also specific
controllers
configuring communications, 286–287
horizontal scan in, 79
memory structures, 72
programmable automation controllers, 317
RSLinx software for, 286–287
RSLogix software for, 286–287
subroutine instructions, 182
Allen-Bradley ControlLogix (CLX) controllers, 317
bit-level programming, 329–335
adding ladder logic to main routine, 332–334
creating ladder logic, 330
internal relay instructions, 334
latch and unlatch instructions, 334–335
one-shot instruction, 335
program scan, 329–330
tag-based addressing, 331–332
comparison instructions, 355–358
configuration, 318–319
function block programming, 361–370
function block diagram, 361–364
ladder logic programming vs., 365–370
setup procedure for, 365
math instructions, 353–356
memory layout, 318
move instructions, 357–359
programming counters, 347–351
count-down counters, 350–351
count-up counters, 348–350
progr

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