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Programmable Controllers
075065757X-ch000-prelims.fm Page i Saturday, June 28, 2003 4:46 PM

In memory of Arthur Parr, 1913–1992.
Man is still the most extraordinary computer of all.
John F. Kennedy
21 May 1963
075065757X-ch000-prelims.fm Page ii Saturday, June 28, 2003 4:46 PM

Programmable Controllers
An engineer’s guide
Third edition
E.A. Parr, MSc, CEng, MIEE, MInstMC
AMSTERDAM BOSTON HEIDELBERG LONDON NEWYORK OXFORD
PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Newnes
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Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
200 Wheeler Road, Burlington, MA 01803
A division of Reed Educational and Professional Publishing Ltd
A member of the Reed Elsevier plc group
First published 1993
Second edition 1999
Third edition 2003
Copyright © E.A. Parr 1993, 1999, 2003. All rights reserved.
The right of E.A. Parr to be identified as the author of this work
has been asserted in accordance with the Copyright, Designs and
Patents Act 1988
No part of this publication
may be reproduced in any material form (including
photocopying or storing in any medium by electronic
means and whether or not transiently or incidentally
to some other use of this publication) without the
written permission of the copyright holder except
in accordance with the provisions of the Copyright,
Designs and Patents Act 1988 or under the terms of a
licence issued by the Copyright Licensing Agency Ltd,
90 Tottenham Court Road, London, England W1T 4LP.
Applications for the copyright holder’s written permission
to reproduce any part of this publication should be addressed
to the publishers
British Library Cataloguing in Publication Data
A catalogue record for this book is available
from the British Library
ISBN 0 7506 5757 X
Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India
www.integra-india.com
Printed and bound in Great Britain
For information on all Newnes publications visit our website at:
newnespress.com
075065757X-ch000-prelims.fm Page iv Saturday, June 28, 2003 4:46 PM

Contents
Preface xi
1 Computers and industrial control 1
1.1 Introduction 1
1.2 Types of control strategies 1
1.2.1 Monitoring subsystems 2
1.2.2 Sequencing subsystems 2
1.2.3 Closed loop control subsystems 4
1.2.4 Control devices 5
1.3 Enter the computer 6
1.3.1 Computer architectures 7
1.3.2 Machine code and assembly language programming 11
1.3.3 High level languages 11
1.3.4 Application programs 14
1.3.5 Requirements for industrial control 14
1.3.6 The programmable controller 18
1.4 Input/output connections 21
1.4.1 Input cards 21
1.4.2 Output connections 22
1.4.3 Input/output identification 28
1.5 Remote I/O 29
1.6 The advantages of PLC control 31
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vi Contents
2 Programming techniques 33
2.1 Introduction 33
2.2 The program scan 36
2.3 Identification of input/output and bit addresses 40
2.3.1 Racks, cards and signals 40
2.3.2 Allen Bradley PLC-5 41
2.3.3 Siemens SIMATIC S5 42
2.3.4 CEGELEC GEM-80 42
2.3.5 ABB Master 45
2.3.6 Mitsubishi F2 47
2.3.7 Internal bit storage 48
2.4 Programming methods 48
2.4.1 Introduction 48
2.4.2 Ladder diagrams 49
2.4.3 Logic symbols 52
2.4.4 Statement list 55
2.5 Bit storage 58
2.6 Timers 63
2.7 Counters 67
2.8 Numerical applications 72
2.8.1 Numeric representations 72
2.8.2 Data movement 75
2.8.3 Data comparison 77
2.8.4 Arithmetical operations 78
2.9 Combinational and event-driven logic 81
2.9.1 Combinational logic 81
2.9.2 Event-driven logic 86
2.10 Micro PLCs 95
2.11 IEC 1131-3, towards a common standard 99
2.12 Programming software 105
2.13 Programming software tools 109
3 Programming style 115
3.1 Introduction 115
3.2 Software engineering 116
3.3 Top-down design 118
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Contents vii
3.4 Program structure in various PLCs 119
3.5 Housekeeping and good software practice 128
3.6 Speeding up the PLC scan time 135
4 Analog signals, closed loop control
and intelligent modules 140
4.2 Common analog signals 140
4.2.1 Temperature 140
4.2.2 Pressure 142
4.2.3 Flow 144
4.2.4 Speed 146
4.2.5 Weighing systems 146
4.2.6 Level 147
4.2.7 Position 148
4.2.8 Output signals 149
4.3 Signals and standards 149
4.4 Analog interfacing 151
4.4.1 Resolution 151
4.4.2 Multiplexed inputs 152
4.4.3 Conversion times 153
4.4.4 Channel selection and conversion to engineering units 156
4.4.5 Analog input cards 158
4.4.6 Filtering 160
4.5 Analog output signals 160
4.6 Analog-related program functions 163
4.7 Closed loop control 164
4.7.1 Introduction to control theory 164
4.7.2 Stability and loop tuning 167
4.7.3 Closed loop control and PLCs 168
4.8 Specialist control processors 172
4.9 Bar codes 173
4.10 High-speed counters 178
4.11 Intelligent modules 178
4.12 Installation notes 179
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viii Contents
5 Distributed systems 182
5.1 Parallel and serial communications 182
5.2 Serial standards 185
5.2.2 Synchronization 185
5.2.3 Character codes 186
5.2.4 Transmission rates 186
5.2.5 Modulation of digital signals 189
5.2.6 Standards and protocols 191
5.2.7 Error control 196
5.2.8 Point to point communication 202
5.3 Area networks 205
5.3.2 Transmission lines 205
5.3.3 Network topologies 207
5.3.4 Network sharing 209
5.3.5 A communication hierarchy 210
5.4 The ISO/OSI model 212
5.5 Proprietary systems 214
5.5.2 Allen Bradley Data Highway 215
5.5.3 Gem-80 Starnet, ESP and CORONET 217
5.5.4 Siemens SINEC 218
5.5.5 Ethernet 218
5.5.6 Towards standardization 219
5.5.7 Profibus 223
5.6 Safety and practical considerations 224
5.7 Fibre optics 227
6 The man–machine interface 232
6.2 Simple digital control and indicators 234
6.3 Numerical outputs and inputs 236
6.3.1 Numerical outputs 236
6.3.2 Multiplexed outputs 237
6.3.3 Leading zero suppression 240
6.3.4 Numerical inputs 240
6.4 Alarm annunciation 242
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Contents ix
6.5 Analog indication 247
6.6 Computer graphics 250
6.6.2 The Allen Bradley Panelview 254
6.6.3 Pixel graphics; the CEGELEC Imagem 256
6.6.4 The Siemens Simatic HMI family 265
6.6.5 Practical considerations 267
6.6.6 Data entry 270
6.7 Message displays 271
6.8 SCADA packages 271
7 Industrial control with conventional computers 276
7.2 Bus-based machines 277
7.2.2 IEEE-488 parallel interface bus 278
7.2.3 Backplane bus systems 281
7.2.4 IBM PC clones 282
7.3 Programming for real time control 285
7.4 Soft PLCs 292
8 Practical aspects 293
8.2 Safety 293
8.2.2 Risk assessment 294
8.2.3 PLCs, computers and safety 296
8.2.4 Emergency stops 308
8.2.5 Guarding 312
8.2.6 Safety legislation 314
8.2.7 IEC 61508 315
8.3 Design criteria 320
8.4 Constructional notes 322
8.4.1 Power supplies 322
8.4.2 Equipment protection 325
8.5 Maintenance and fault finding 331
8.5.2 Statistical representation of reliability 332
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x Contents
8.5.3 Maintenance philosophies 335
8.5.4 Designing for faults 337
8.5.5 Documentation 339
8.5.6 Training 344
8.5.7 Fault-finding aids, EDDI and FIMs 348
8.6 Electromagnetic compatibility (EMC)
and CE marking 354
8.7 Other programmable devices 359
9 Sample ladder logic 362
9.2 One Shot 364
9.3 Toggle action 365
9.4 Alarm annunciator 368
9.5 First order filter 370
9.6 Level control 373
9.7 Linearization 380
9.8 Flow totalization 385
9.9 Scaling 391
9.10 Gray code conversion 394
9.11 BCD to Binary conversion 398
9.12 Binary to BCD conversion 400
9.13 A hydraulic system 403
Appendix Number systems 416
Index 421
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Preface
All industrial processes need some form of control system if they are to
run safely and economically. In recent years a specialist control computer,
called a programmable controller, has evolved and revolutionized control
engineering by combining computing power and immense flexibility at
a reasonable price.
This book is concerned with the application and use of programmable
controllers. It is not an instructional book in programming, and is certainly
not a comparative guide to the various makes of machine on the market.
To some extent, choosing a programmable controller is rather like
choosing a word processor. You ask people for their views, try a few
simple examples in a shop, and buy the cheapest that you think meets
your requirements. Only after several months do you really know the
system. From then on, all other word processors seem awkward.
Programmable controllers are similar. Unless there are good reasons
for a particular choice (ready experience in the engineering or maintenance
staff, equipment being supplied by an outside contractor and similar
considerations), there are good and bad points with all (the really bad
machines left the market years ago).
At the Sheerness Steel Company where I work, the plant control is
based on about sixty programmable controllers consisting of Allen
Bradley PLC 2s and 5s, GEC (now CEGELEC) GEM-80s, ASEA (now
ABB) Masters and Siemens SIMATIC S5s, with small machines primar-
ily from Mitsubishi. These controllers are somewhat like the trees at
Galleons Lap in Winnie the Pooh; there never seems to be the same
number on two successive days, even if you tie a piece of string around
each one!
As with most plants, the background to this distribution of controllers
is largely historical chance (the original Mitsubishi came on a small
turn-key plant from an outside contractor, for example), but the ready
access to these machines is the reason for their prominence in this book.
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xii Preface
Even within this range of PLC families, the coverage in this book is
not complete. The PLCs have been chosen to cover the application points
I wish to make, not as a complete survey of a manufacturer’s range.
In ‘previous lives’ I have worked with PLCs from AEG, GE, Landys
and Gyr, Modicon, Telemecanique, Texas Instruments and many other
companies. To these manufacturers I offer my sincere apologies for not
giving them more coverage, but to do so would have made a tedious book
and masked the application points I have tried to make. I could happily
use any of these machines, and there is not a major difference in style or
philosophy between them (the manufacturers would no doubt disagree!).
The guideline is therefore choose a machine that suits you, and do not
change manufacturers for purely economic reasons. Knowledge, consistency
of spares and a good relationship with a manufacturer are very valuable.
A book like this requires much assistance, and I would like to thank
Peter Bark and Dave Wilson of ABB, Adrian Bishop, Bob Hunt, Julian
Fielding, John Hanscombe, Hugh Pickard, Jennie Holmes and Hennie
Jacobs of Allen Bradley, Peter Backenist, David Slingsby and Stuart
Webb of GEC/CEGELEC, Peter Houldsworth, Paul Judge, Allan
Norbury, Dickon Purvis, Paul Brett and Allan Roworth of Siemens,
and Craig Rousell who all assisted with information on their machines,
commented constructively on my thoughts and provided material and
photographs.
My fellow engineers at Sheerness Steel also deserve some praise for
tolerating my PLC systems (and who will no doubt compare my written
aims with our actual achievements!).
A book takes some time to write, and my family deserve considerable
thanks for their patience.
Andrew Parr
Minster on Sea
eaparr2002@yahoo.co.uk
Note for second edition
This revision incorporates additional material covering recent develop-
ments, and reflects the increasing importance of health and safety
legislation.
Notes for third edition
This edition includes a new chapter giving example ladder rungs for
common industrial problems. Screen shots of Windows based program-
ming software have been included to show how programs are entered.
Health and Safety issues, particularly the introduction of IEC 61508,
have been updated.
075065757X-ch000-prelims.fm Page xii Saturday, June 28, 2003 4:46 PM

1 Computers and industrial
control
1.1 Introduction
Very few industrial plants can be left to run themselves, and most need
some form of control system to ensure safe and economical operation.
Figure 1.1 is thus a representation of a typical installation, consisting of
a plant connected to a control system. This acts to translate the commands
of the human operator into the required actions, and to display the plant
status back to the operator.
At the simplest level, the plant could be an electric motor driving
a cooling fan. Here the control system would be an electrical starter
with protection against motor overload and cable faults. The operator
controls would be start/stop pushbuttons and the plant status displays
simply running/stopped and fault lamps.
At the other extreme, the plant could be a vast petrochemical
installation. Here the control system would be complex and a mixture
of technologies. The link to the human operators will be equally varied,
with commands being given and information displayed via many
devices.
In most cases the operator will be part of the control system. If an
alarm light comes on saying ‘Low oil level’ the operator will be expected
to add more oil.
1.2 Types of control strategies
It is very easy to be confused and overwhelmed by the size and
complexity of large industrial processes. Most, if not all, can be
simplified by considering them to be composed of many small sub-
processes. These sub-processes can generally be considered to fall into
three distinct areas.
075065757X-ch001.fm Page 1 Wednesday, July 9, 2003 3:31 PM

2 Programmable Controllers
1.2.1 Monitoring subsystems
These display the process state to the operator and draw attention to
abnormal or fault conditions which need attention. The plant condition
is measured by suitable sensors.
Digital sensors measure conditions with distinct states. Typical
examples are running/stopped, forward/off/reverse, fault/healthy,
idle/low/medium/high, high level/normal/low level. Analog sensors
measure conditions which have a continuous range such as temperature,
pressure, flow or liquid level.
The results of these measurements are displayed to the operator via
indicators (for digital signals) or by meters and bargraphs for analog
signals.
The signals can also be checked for alarm conditions. An overtravel
limit switch or an automatic trip of an overloaded motor are typical
digital alarm conditions. A high temperature or a low liquid level could
be typical analog alarm conditions. The operator could be informed of
these via warning lamps and an audible alarm.
A monitoring system often keeps records of the consumption of
energy and materials for accountancy purposes, and produces an event/
alarm log for historical maintenance analysis. A pump, for example,
may require maintenance after 5000 hours of operation.
1.2.2 Sequencing subsystems
Many processes follow a predefined sequence. To start the gas burner
of Figure 1.2, for example, the sequence could be:
Figure 1.1 A simple view of a control system

Computers and industrial control 3
(a) Start button pressed; if sensors are showing sensible states (no air
flow and no flame) then sequence starts.
(b) Energize air fan starter. If starter operates (checked by contact on
starter) and air flow is established (checked by flow switch) then
(c) Wait two minutes (for air to clear out any unburnt gas) and then
(d) Open gas pilot valve and operate igniter. Wait two seconds and
then stop igniter and
(e) If flame present (checked by flame failure sensor) open main gas
valve.
(f) Sequence complete. Burner running. Stays on until stop button
pressed, or air flow stops, or flame failure.
The above sequence works solely on digital signals, but sequences can
also use analog signals. In the batch process of Figure 1.3 analog sensors
are used to measure weight and temperature to give the sequence:
1 Open valve V1 until 250kg of product A have been added.
2 Start mixer blade.
3 Open valve V2 until 310kg of product B have been added.
4 Wait 120s (for complete mixing).
5 Heat to 80°C and maintain at 80°C for 10min.
6 Heater off. Allow to cool to 30°C.
7 Stop mixer blade.
8 Open drain valve V3 until weight less than 50kg.
Figure 1.2 Gas-fired burner, a sequence control system

1.2.3 Closed loop control subsystems
In many analog systems, a variable such as temperature, flow or
pressure is required to be kept automatically at some preset value or
made to follow some other signal. In step 5 of the batch sequence above,
for example, the temperature is required to be kept constant to 80°C
within quite narrow margins for 10 minutes.
Such systems can be represented by the block diagram of Figure 1.4.
Here a particular characteristic of the plant (e.g. temperature) denoted
by PV (for process variable) is required to be kept at a preset value SP (for
setpoint). PV is measured by a suitable sensor and compared with the
SP to give an error signal
error=SP−PV (1.1)
If, for example, we are dealing with a temperature controller with
a setpoint of 80°C and an actual temperature of 78°C, the error is 2°C.
This error signal is applied to a control algorithm. There are many
possible control algorithms, and this topic is discussed in detail in
Chapter 4, but a simple example for a heating control could be ‘If the
error is negative turn the heat off, if the error is positive turn the heat on.’
The output from the control algorithm is passed to an actuator which
affects the plant. For a temperature control, the actuator could be
a heater, and for a flow control the actuator could be a flow control valve.
Figure 1.3 A batch process

The control algorithm will adjust the actuator until there is zero error, i.e.
the process variable and the setpoint have the same value.
In Figure 1.4, the value of PV is fed back to be compared with the
setpoint, leading to the term ‘feedback control’. It will also be noticed
that the block diagram forms a loop, so the term ‘closed loop control’ is
also used.
Because the correction process is continuous, the value of the
controlled PV can be made to track a changing SP. The air/gas ratio
for a burner can thus be maintained despite changes in the burner
firing rate.
1.2.4 Control devices
The three types of control strategy outlined above can be achieved in
many ways. Monitoring/alarm systems can often be achieved by
connecting plant sensors to displays, indicators and alarm annunciators.
Sometimes the alarm system will require some form of logic. For
example, you only give a low hydraulic pressure alarm if the pumps
are running, so a time delay is needed after the pump starts to allow the
pressure to build up. After this time, a low pressure causes the pump to
stop (in case the low pressure has been caused by a leak).
Sequencing systems can be built from relays combined with timers,
uniselectors and similar electromechanical devices. Digital logic (usually
based on TTL or CMOS integrated circuits) can be used for larger
systems (although changes to printed circuit boards are more difficult
to implement than changes to relay wiring). Many machine tool
applications are built around logic blocks: rail-mounted units containing
logic gates, storage elements, timers and counters which are linked by
terminals on the front of the blocks to give the required operation. As
with a relay system, commissioning changes are relatively easy to
implement.
Closed loop control can be achieved by controllers built around DC
amplifiers such as the ubiquitous 741. The ‘three-term controller’
Figure 1.4 A closed loop control system

(described further in Chapter 4) is a commercially available device that
performs the function of Figure 1.4. In the chemical (and particularly
the petrochemical) industries, the presence of potentially explosive
atmospheres has led to the use of pneumatic controllers, with the signals
in Figure 1.4 being represented by pneumatic pressures.
1.3 Enter the computer
A computer is a device that performs predetermined operations on
input data to produce new output data, and as such can be represented
by Figure 1.5(a). For a computer used for payroll calculations the input
data would be employees’ names, salary grades and hours worked.
These data would be operated on according to instructions written to
include current tax and pension rules to produce output data in the
form of wage slips (or, today, more likely direct transfers to bank
accounts).
Early computer systems were based on commercial functions: payroll,
accountancy, banking and similar activities. The operations tended to
be batch processes, a daily update of stores stock, for example.
The block diagram of Figure 1.5(a) has a close relationship with the
control block of Figure 1.1, which could be redrawn, with a computer pro-
viding the control block, as in Figure 1.5(b). Note that the operator’s
actions (e.g. start process 3) are not instructions, they are part of the
input data. The instructions will define what action is to be taken as the
input data (from both the plant and the operator) change. The output
data are control actions to the plant and status displays to the operator.
Early computers were large, expensive and slow. Speed is not that
important for batch-based commercial data processing (commercial
Figure 1.5 The computer in industrial control: (a) a simple overview of
a computer; (b) the computer as part of a control system

programmers will probably disagree!) but is of the highest priority in
industrial control, which has to be performed in ‘real time’. Many emer-
gency and alarm conditions require action to be taken in fractions of a
second.
Commercial (with the word ‘commercial’ used to mean ‘designed for
use in commerce’) computers were also based on receiving data from
punched cards and keyboards and sending output data to printers. An
industrial process requires possibly hundreds of devices to be read in
real time and signals sent to devices such as valves, motors, meters and
so on.
There was also an environmental problem. Commercial computers are
designed to exist in an almost surgical atmosphere; dust-free and an
ambient temperature that can only be allowed to vary by a few degrees.
Such conditions can be almost impossible to achieve close to a manufac-
turing process.
The first industrial computer application was probably a monitoring
system installed in an oil refinery in Port Arthur, USA in 1959. The reli-
ability and mean time between failure of computers at this time meant
that little actual control was performed by the computer, and its role
approximated to the earlier Section 1.2.1.
1.3.1 Computer architectures
It is not essential to have intimate knowledge of how a computer works
before it can be used effectively, but an appreciation of the parts of a com-
puter is useful for appreciating how a computer can be used for industrial
control.
Figure 1.5(a) can be expanded to give the more detailed layout of
Figure 1.6. This block diagram (which represents the whole computing
range from the smallest home computer to the largest commercial
mainframe) has six portions:
1 An input unit where data from the outside world are brought into the
computer for processing.
2 A store, or memory, which will be used to store the instructions the
computer will follow and data for the computer to operate on. These
data can be information input from outside or intermediate results
calculated by the machine itself. The store is organized into a
number of boxes, each of which can hold one number and is identi-
fied by an address as shown in Figure 1.7. Computers work inter-
nally in binary (see the Appendix for a description of binary,
hexadecimal (hex) and other number systems) and the store does
not distinguish between the meanings that could be attached to the
data stored in it. For example, in an 8-bit computer (which works

with numbers 8 bits long in its store) the number 01100001 can be
interpreted as:
(a) The decimal number 97.
(b) The hex number 61 (see Appendix).
(c) The letter ‘a’ (see Chapter 6).
(d) The state of eight digital signals such as limit switches.
(e) An instruction to the computer. If the machine was the old Z80
microprocessor, hex 61 moves a number between two internal
stores.
A typical desktop computer will use 16-bit numbers (called a 16-bit
word) and have over a million store locations. The industrial computers
we will be mainly discussing have far smaller storage, 32000 to 64000
Figure 1.6 The component parts of a computer
Figure 1.7 A simple view of a computer’s store

store locations being typical for larger control machines, but even
smaller machines with just 1000 store locations are common.
3 Data from the store can be accessed very quickly, but commercial
computers often need vast amounts of storage to hold details such as
bank accounts or names and addresses. This type of data is not
required particularly quickly and is held in external storage. This is
usually magnetic disks or tapes and is called secondary or backing
storage. Such stores are not widely used on the types of computer
we will be discussing.
4 An output unit where data from the computer are sent to the outside
world.
5 An arithmetic and logical unit (called an ALU) which performs
operations on the data held in the store according to the instructions
the machine is following.
6 A control unit which links together the operations of the other five
units. Often the ALU and the control unit are known, together, as
the central processor unit or CPU. A microprocessor is a CPU in a
single integrated circuit.
The instructions the computer follows are held in the store and, with
a few exceptions which we will consider shortly, are simply followed
in sequential order as in Figure 1.8(a).
The control unit contains a counter called an instruction register (or IR)
which says at which address in the store the next instruction is to be
Figure 1.8 Program flow in a computer: (a) simple sequential flow;
(b) conditional jump; (c) subroutine call

found. Sometimes the name program counter (and the abbreviation PC)
is used.
When each instruction is obeyed, the control unit reads the store
location whose address is held in the IR. The number held in this store
location tells the control unit what instruction is to be performed.
Instructions nearly always require operations to be performed on
data in the store (e.g. add two numbers) so the control unit will bring
data from the store to the ALU and perform the required function.
When the instruction has been executed, the control unit will
increment the IR so it holds the address of the next instruction.
There are surprisingly few types of instruction. The ones available on
most microprocessors are variations on:
1 Move data from one place to another (e.g. input data to a store
location, or move data from a store location to the ALU).
2 ALU operations on two data items, one in the ALU and one in
a specified store location. Operations available are usually add,
subtract, and logical operations such as AND, OR.
3 Jumps. In Figure 1.8(a) we implied that the computer followed a simple
sequential list of instructions. This is usually true, but there are occa-
sions where simple tests are needed. These usually have the form
IF (some condition) THEN
Perform some instructions
ELSE
Perform some other instructions
To test a temperature, for example, we could write
IF Temperature is less than 75 °C THEN
Turn healthy light on
Turn fault light off
ELSE
Turn healthy light off
Turn fault light on
Such operations use conditional jumps. These place a new address
into the IR dependent on the last result in the accumulator.
Conditional jumps can be specified to occur for outcomes such as
result positive, result negative or result zero, and allow a program to
follow two alternative routes as shown in Figure 1.8(b).
4 Subroutines. Many operations are required time and time again within
the same program. In an industrial control system using flows
measured by orifice plates, a square root function will be required
many times (flow is proportional to the square root of the pressure drop
across the orifice plate). Rather than write the same instruction sev-
eral times (which is wasteful of effort and storage space) a subroutine

instruction allows different parts of the program to temporarily
transfer operations to a specified subroutine, returning to the instruc-
tion after the subroutine call as shown in Figure 1.8(c).
1.3.2 Machine code and assembly language programming
The series of instructions that we need (called a ‘program’) has to be
written and loaded into the computer. At the most basic level, called
machine code programming, the instructions are written into the machine
in the raw numerical form used by the machine. This is difficult to do,
prone to error, and almost impossible to modify afterwards.
The sequence of numbers
16 00 58 21 00 00 06 08 29 17 D2 0E 40 19 05 C2 08 40 C9
genuinely are the instructions for a multiplication subroutine starting at
address 4000 for a Z80 microprocessor, but even an experienced Z80
programmer would need reference books (and a fair amount of time) to
work out what is going on with just these 19 numbers.
Assembly language programming uses mnemonics instead of the raw
code, allowing the programmer to write instructions that can be rela-
tively easily followed. For example, with
LOAD Temperature
SUB 75
JUMP POSITIVE to Fault_Handler
it is fairly easy to work out what is happening.
A (separate) computer program called an assembler converts the
programmer’s mnemonic-based program (called the source) into an
equivalent machine code program (called the object) which can then
be run.
Writing programs in assembly language is still labour-intensive,
however, as there is one assembly language instruction for each
machine code instruction.
1.3.3 High level languages
Assembly language programming is still relatively difficult to write, so
ways of writing computer programs in a style more akin to English
were developed. This is achieved with so-called ‘high level languages’
of which the best known are probably Pascal, FORTRAN and the
ubiquitous BASIC (and there are many, many languages: RPG,
FORTH, LISP, CORAL and C to name but a few, each with its own
attractions).

In a high level language, the programmer writes instructions in some-
thing near to English. The Pascal program below, for example, gives a
printout of a requested multiplication table.
program multtable (input, output);
var number, count : integer
begin
readln (‘Which table do you want’, number);
for count= 1 to 10 do
writeln (count, ‘times’, number, ‘is’, count*number);
end. (of program)
Even though the reader may not know Pascal, the operation of the
program is clear (if asked to change the table from a ten times table to
a twenty times table, for example, it is obvious which line would need
to be changed).
A high level language source program can be made to run in two
distinct ways. A compiler is a program which converts the entire high
level source program to a machine code object program offline. The
resultant object program can then be run independently of the source
program or the compiler.
With an interpreter, the source program and the interpreter both
exist in the machine when the program is being run. The interpreter
scans each line of source code, converting them to equivalent machine
code instructions as they are obeyed. There is no object program with
an interpreter.
A compiled program runs much faster than an interpreted program
(typically five to ten times as fast because of the extra work that the inter-
preter has to do) and the compiled object program will be much smaller
than the equivalent source code program for an interpreter. Compilers
are, however, much less easy to use, a typical sequence being:
1 A text editor is loaded into the computer.
2 The source program is typed in or loaded from disk (for modification).
3 The resultant source file is saved to disk.
4 The compiler is loaded from disk and run.
5 The source file is loaded from disk.
6 Compilation starts (this can take several minutes). If any errors are
found go back to step 1.
7 An object program is produced which can be saved to disk and/or
run. If any runtime errors are found, go back to step 1.
An interpreted language is much easier to use, and for many applications
the loss of speed is not significant. BASIC is usually an interpreted
language; Pascal, C and Fortran are usually compiled. Figure 1.9 sum-
marizes the operation of compiled and interpreted high level languages.

Figure 1.9 Compiled and interpreted high level languages:
(a) compiled program (e.g. Pascal, C); (b) interpreter
(e.g. most BASICs)

1.3.4 Application programs
Increasingly, as computers become more widespread, many programs
have been written which allow the user to define the tasks to be
performed without worrying unduly about how the computer achieves
them. These are known as application programs and are typified
by spreadsheets such as Lotus 123 and Excel and databases such as
Approach and Access. In these the user is defining complex mathematical
or database operations without ‘programming’ the computer in a conven-
tional sense.
1.3.5 Requirements for industrial control
Industrial control has rather different requirements than other applica-
tions. It is worth examining these in some detail.
A conventional computer, shown schematically in Figure 1.10(a),
takes data usually from a keyboard and outputs data to a VDU screen
or printer. The data being manipulated will generally be characters or
numbers (e.g. item names and quantities held in a stores stock list).
The control computer of Figure 1.10(b) is very different. Its inputs
come from a vast number of devices. Although some of these are
numeric (flows, temperature, pressures and similar analog signals) most
will be single-bit, on/off, digital signals.
Figure 1.10 The difference between commercial and industrial
computers: (a) commercial computer; (b) industrial control computer

There will also be a similarly large amount of digital and analog
output signals. A very small control system may have connections to
about 20 input and output signals; figures of over 200 connections are
quite common on medium-sized systems. The keyboard, VDU and
printer may exist, but they are not necessary, and their functions will
probably be different to those on a normal desktop or mainframe
computer.
Although it is possible to connect this quantity of signals to a conven-
tional machine, it requires non-standard connections and external boxes.
Similarly, although programming for a large amount of input and
output signals can be done in Pascal, BASIC or C, the languages are
being used for a purpose for which they were not really designed, and
the result can be very ungainly.
In Figure 1.11(a), for example, we have a simple motor starter. This
could be connected as a computer-driven circuit as in Figure 1.11(b).
The two inputs are identified by addresses 1 and 2, with the output (the
relay starter) being given the address 10.
If we assume that a program function bitread (N) exists which gives
thestate(on/off)ofaddressN,andaprocedurebitwrite(M,var) whichsends
the state of program variable var to address M, we could give the actions of
Figure 1.11 by
repeat
start: = bitread(1);
stop: = bitread(2);
Computer
Digital input
card
Digital output
card
(b)
Stop
1
2
Start
L
Start
(supply)
L
Stop
C1
C1
C1
(a)
N
N
(neutral)
10
Figure 1.11 Comparison of hardwire and computer-based schemes:
(a) hardwire motor starter circuit; (b) computer-based motor starter

run: = ((start) or (run)) & stop;
bitwrite (10,run);
until hellfreezesover
where start, stop and run are 1-bit variables. The program is not very
clear, however, and we have just three connections.
An industrial control program rarely stays the same for the whole of
its life. There are always modifications to cover changes in the oper-
ations of the plant. These changes will be made by plant maintenance
staff, and must be made with minimal (preferably no) interruptions to
the plant production. Adding a second stop button and a second start
button to Figure 1.11 would not be a simple task.
In general, computer control is done in real time, i.e. the computer
has to respond to random events as they occur. An operator expects a
motor to start (and more important to stop!) within a fraction of a second
of the button being pressed. Although commercial computing needs
fast computers, it is unlikely that the difference between one and
two second computation time for a spreadsheet would be noticed by the
user. Such a difference would be unacceptable for industrial control.
Time itself is often part of the control strategy (e.g. start air fan, wait
10s for air purge, open pilot gas valve, wait 0.5s, start ignition spark,
wait 2.5s, if flame present open main gas valve). Such sequences are
difficult to write with conventional languages.
Most control faults are caused by external items (limit switches, solen-
oids and similar devices) and not by failures within the central control
itself. The permission to start a plant, for example, could rely on signals
involving cooling water flows, lubrication pressure, or temperatures
within allowable ranges. For quick fault finding the maintenance staff
must be able to monitor the action of the computer program whilst it is
running. If, as is quite common, there are ten interlock signals which
allow a motor to start, the maintenance staff will need to be able to
check these quickly in the event of a fault. With a conventional computer,
this could only be achieved with yet more complex programming.
The power supply in an industrial site is shared with many antisocial
loads; large motors stopping and starting, thyristor drives which put
spikes and harmonic frequencies onto the mains supply. To a human
these are perceived as light flicker; in a computer they can result in storage
corruption or even machine failure.
An industrial computer must therefore be able to live with a ‘dirty’ mains
supply, and should also be capable of responding sensibly following a total
supply interruption. Some outputs must go back to the state they were in
before the loss of supply; others will need to turn off or on until an operator
takesavailablecorrectiveaction.Thedesignermusthavethefacilitytodefine
what happens when the system powers up from cold.

The final considerations are environmental. A large mainframe com-
puter generally sits in an air-conditioned room at a steady 20°C with
carefully controlled humidity. A desktop PC will normally live in a fairly
constant environment because human beings do not work well at
extremes. An industrial computer, however, will probably have to operate
away from people in a normal electrical substation with temperatures as
low as −10°C after a winter shutdown, and possibly over 40°C in the
height of summer. Even worse, these temperature variations lead to a
constant expansion and contraction of components which can lead to
early failure if the design has not taken this factor into account.
To these temperature changes must be added dust and dirt. Very few
industrial processes are clean, and the dust gets everywhere (even with
IP55 cubicles, because an IP55 cubicle is only IP55 when the doors are
shut and locked; IP ratings are discussed in Section 8.4.2). The dust will
work itself into connectors, and if these are not of the highest quality,
intermittent faults will occur which can be very difficult to find.
In most computer applications, a programming error or a machine
fault can at worst be expensive and embarrassing. When a computer
controlling a plant fails, or a programmer misunderstands the plant’s
operation, the result could be injuries or fatalities. Under the UK Health
and Safety at Work Act, prosecution of the design engineers could
result. It behoves everyone to take extreme care with the design.
Our requirements for an industrial control computer are very
demanding, and it is worth summarizing them:
1 They should be designed to survive in an industrial environment
with all that this implies for temperature, dirt and poor-quality mains
supply.
2 They should be capable of dealing with bit-form digital input/output
signals at the usual voltages encountered in industry (24V DC
to 240V AC) plus analog input/output signals. The expansion of the
I/O should be simple and straightforward.
3 The programming language should be understandable by main-
tenance staff (such as electricians) who have no computer training.
Programming changes should be easy to perform in a constantly
changing plant.
4 It must be possible to monitor the plant operation whilst it is running
to assist fault finding. It should be appreciated that most faults will
be in external equipment such as plant-mounted limit switches,
actuators and sensors, and it should be possible to observe the action
of these from the control computer.
5 The system should operate sufficiently fast for realtime control. In
practice, ‘sufficiently fast’ means a response time of around 0.1s, but
this can vary depending on the application and the controller used.

6 The user should be protected from computer jargon.
7 Safety must be a prime consideration.
1.3.6 The programmable controller
In the late 1960s the American motor car manufacturer General Motors
was interested in the application of computers to replace the relay
sequencing used in the control of its automated car plants. In 1969 it
produced a specification for an industrial computer similar to that
outlined at the end of Section 1.3.5.
Two independent companies, Bedford Associates (later called
Modicon) and Allen Bradley, responded to General Motor’s specification.
Each produced a computer system similar to Figure 1.12 which bore
little resemblance to the commercial minicomputers of the day.
The computer itself, called the central processor, was designed to live
in an industrial environment, and was connected to the outside world
via racks into which input or output cards could be plugged. In these
early machines there were essentially four different types of cards:
1 DC digital input card
2 DC digital output card
3 AC digital input card
4 AC digital output card
Each card would accept 16 inputs or drive 16 outputs. A rack of eight
cards could thus be connected to 128 devices. It is very important to
appreciate that the card allocations were the user’s choice, allowing great
flexibility. In Figure 1.12(b) the user has installed one DC input card,
one DC output card, three AC input cards, and two AC output cards,
leaving one spare position for future expansion. This rack can thus be
connected to
• 16 DC input signals
• 16 DC output signals
• 48 AC input signals
• 16 AC output signals
Not all of these, of course, need to be used.
The most radical idea, however, was a programming language based
on a relay schematic diagram, with inputs (from limit switches, push-
buttons, etc.) represented by relay contacts, and outputs (to solenoids,
motor starters, lamps, etc.) represented by relay coils. Figure 1.13 shows
a simple hydraulic cylinder which can be extended or retracted by
pushbuttons. Its stroke is set by limit switches which open at the end of
travel, and the solenoids can only be operated if the hydraulic pump
is running. This would be controlled by the computer program of

Figure 1.13(b) which is identical to the relay circuit needed to control the
cylinder. These programs look like the rungs on a ladder, and were
consequently called ‘ladder diagrams’.
The program was entered via a programming terminal with keys
showing relay symbols (normally open/normally closed contacts, coils,
timers, counters, parallel branches, etc.) with which a maintenance
electrician would be familiar. Figure 1.14 shows the programmer
Figure 1.12 The component parts of a PLC system: (a) an early PLC
system; (b) a typical rack of cards

keyboard for an early PLC. The meaning of the majority of the keys
should be obvious. The program, shown exactly on the screen as in
Figure 1.13(b), would highlight energized contacts and coils, allowing
the programming terminal to be used for simple fault finding.
The processor memory was protected by batteries to prevent
corruption or loss of program during a power fail. Programs could be
stored on cassette tapes which allowed different operating procedures
(and hence programs) to be used for different products.
The name given to these machines was ‘programmable controllers’ or
PCs. The name ‘programmable logic controller’ or PLC was also used,
but this is, strictly, a registered trademark of the Allen Bradley Company.
Unfortunately in more recent times the letters PC have come to be used
Figure 1.13 A simple PLC application. (a) A simple hydraulic cylinder
controlled by a PLC. (b) The ‘ladder diagram’ program used to control the
cylinder. This is based on American relay symbols. –][– means that signal
is present, and –]/[– means that signal is not present

for personal computer, and confusingly the worlds of programmable
controllers and personal computers overlap where portable and lap-top
computers are now used as programming terminals. To avoid confusion,
we shall use PLC for a programmable controller and PC for a personal
computer. Section 2.12 gives examples of programming software on
modern PCs.
1.4 Input/output connections
1.4.1 Input cards
Internally a computer usually operates at 5V DC. The external devices
(solenoids, motor starters, limit switches, etc.) operate at voltages up to
110V AC. The mixing of these two voltages will cause severe and
possibly irreparable damage to the PLC electronics. Less obvious
problems can occur from electrical ‘noise’ introduced into the PLC from
voltage spikes on signal lines, or from load currents flowing in AC
neutral or DC return lines. Differences in earth potential between the
PLC cubicle and outside plant can also cause problems.
The question of noise is discussed at length in Chapter 8, but there
are obviously very good reasons for separating the plant supplies from the
PLC supplies with some form of electrical barrier as in Figure 1.15. This
ensures that the PLC cannot be adversely affected by anything happening
on the plant. Even a cable fault putting 415V AC onto a DC input would
only damage the input card; the PLC itself (and the other cards in the
system) would not suffer.
This is achieved by optical isolators, a light-emitting diode and photo-
electric transistor linked together as in Figure 1.16(a). When current is
passed through the diode D1 it emits light, causing the transistor TR1 to
Figure 1.14 The programming terminal keypad for an early Allen
Bradley PLC (reproduced by permission of Allen Bradley)

switch on. Because there are no electrical connections between the diode
and the transistor, very good electrical isolation (typically 1–4kV) is
achieved.
A DC input can be provided as in Figure 1.16(b). When the push-
button is pressed, current will flow through D1, causing TR1 to turn on,
passing the signal to the PLC internal logic. Diode D2 is a light-emitting
diode used as a fault-finding aid to show when the input signal is present.
Such indicators are present on almost all PLC input and output cards.
The resistor R sets the voltage range of the input. DC input cards are
usually available for three voltage ranges: 5V (TTL), 12–24V, 24–50V.
A possible AC input circuit is shown in Figure 1.16(c). The bridge
rectifier is used to convert the AC to full wave rectified DC. Resistor R2
and capacitor C1 act as a filter (of about 50ms time constant) to give
a clean signal to the PLC logic. As before, a neon LP1 acts as an input
signal indicator for fault finding, and resistor R1 sets the voltage range.
Figure 1.17(a) shows a typical input card from the Allen Bradley
range. The isolation barrier and monitoring LEDs can be clearly seen.
This card handles eight inputs and could be connected to the outside
world as in Figure 1.17(b).
1.4.2 Output connections
Output cards again require some form of isolation barrier to limit
damage from the inevitable plant faults and also to stop electrical ‘noise’
corrupting the processor’s operations. Interference can be more of
a problem on outputs because higher currents are being controlled by
Figure 1.15 Protection of the PLC from outside faults. The PLC supply
L1/N1 is separate from the plant supply L2/N2

Figure 1.16 Optical isolation of inputs: (a) an optical isolator;
(b) DC input card; (c) AC input card

Figure 1.17 A PLC input card: (a) Allen Bradley eight-way input card;
(b) wiring of input card

the cards and the loads themselves are often inductive (e.g. solenoid and
relay coils).
There are two basic types of output card. In Figure 1.18(a), eight
outputs are fed from a common supply, which originates local to the
PLC cubicle (but separate from the supply to the PLC itself). This
arrangement is the simplest and the cheapest to install. Each output has
its own individual fuse protection on the card and a common circuit
breaker. It is important to design the system so that a fault, say, on load
3 blows the fuse FS3 but does not trip the supply to the whole card,
shutting down every output. This topic, called ‘discrimination’, is
discussed further in Chapter 8.
A PLC frequently has to drive outputs which have their own individual
supplies. A typical example is a motor control centre (MCC) where each
starter has a separate internal 110-V supply derived from the 415-V bars.
The card arrangement of Figure 1.18(a) could not be used here without
separate interposing relays (driven by the PLC with contacts into the
MCC circuit).
An isolated output card, shown in Figure 1.18(b), has individual out-
puts and protection and acts purely as a switch. This can be connected
directly with any outside circuit. The disadvantage is that the card is
more complicated (two connections per output) and safety becomes
more involved. An eight-way isolated output card, for example, could
have voltage on its terminals from eight different locations.
Contacts have been shown on the outputs in Figure 1.18. Relay
outputs can be used (and do give the required isolation) but are not
particularly common. A relay is an electromagnetic device with moving
parts and hence a finite limited life. A purely electronic device will have
greater reliability. Less obviously, though, a relay-driven inductive load
can generate troublesome interference and lead to early contact failure.
A transistor output circuit is shown in Figure 1.19(a). Optical isolation
is again used to give the necessary separation between the plant and the
PLC system. Diode D1 acts as a spike suppression diode to reduce the
voltage spike encountered with inductive loads. Figure 1.19(b) shows
the effect. The output state can be observed on LED1. Figure 1.19(a) is a
current sourcing output. If NPN transistors are used, a current sinking
card can be made as in Figure 1.19(c).
AC output cards invariably use triacs, a typical circuit being shown in
Figure 1.20(a). Triacs have the advantage that they turn off at zero
current in the load, as shown in Figure 1.20(b), which eliminates the
interference as an inductive load is turned off. If possible, all AC loads
should be driven from triacs rather than relays.
Figure 1.21 is a photograph of the construction of AC and DC output
cards; the isolation barrier, the state indication LEDs and the protection
fuses can be clearly seen.

Figure 1.18 Types of output card: (a) output card with common supply;
(b) output card with separate supplies

An output card will have a limit to the current it can supply, usually
set by the printed circuit board tracks rather than the output devices. An
individual output current will be set for each output (typically 2 A) and
a total overall output (typically 6 A). Usually the total allowed for the
card current is lower than the sum of the allowed individual outputs. It is
Figure 1.19 DC output circuits: (a) DC output circuit, current sourcing;
(b) effect of spike suppression diode; (c) current sinking output

therefore good practice to reduce the total card current by assigning
outputs which cannot occur together (e.g. forward/reverse, fast/slow) to
the same card.
1.4.3 Input/output identification
The PLC program must have some way of identifying inputs and out-
puts. In general, a signal is identified by its physical location in some
form of mounting frame or rack, by the card position in this rack, and
by which connection on the card the signal is wired to.
In Figure 1.22, a lamp is connected to output 5 on card 6 in rack 2. In
Allen Bradley notation, this is signal
Figure 1.20 AC output circuit: (a) AC output stage – sourcing/sinking is
irrelevant on AC outputs; (b) effect of triac output

O:26/05
The pushbutton is connected to input 2 on card 5 in rack 3, and (again
in Allen Bradley notation) is
I:35/02
Most PLC manufacturers use a similar scheme. The topic is discussed
further in Chapter 2.
1.5 Remote I/O
So far we have assumed that a PLC consists of a processor unit and
a collection of I/O cards mounted in local racks. Early PLCs did tend to
be arranged like this, but in a large and scattered plant with this arrange-
ment, all signals have to be brought back to some central point in
expensive multicore cables. It will also make commissioning and fault
finding rather difficult, as signals can only be monitored effectively at
a point possibly some distance from the device being tested.
In all bar the smallest and cheapest systems, PLC manufacturers
therefore provide the ability to mount I/O racks remote from the
processor, and link these racks with simple (and cheap) screened single
Figure 1.21 Output cards

pair or fibre optic cable. Racks can then be mounted up to several
kilometres away from the processor.
There are many benefits from this. It obviously reduces cable costs as
racks can be laid out local to the plant devices and only short multicore
cable runs are needed. The long runs will only need the communication
cables (which are cheap and only have a few cores to terminate at each
end) and hardwire safety signals (which should not be passed over
remote I/O cable, or even through a PLC for that matter, a topic
discussed further in Chapter 8).
Less obviously, remote I/O allows complete units to be built, wired to
a built-in rack, and tested offsite prior to delivery and installation. The
pulpit in Figure 3.2 contains three remote racks, and connects to the
controlling PLC mounted in a substation about 500m away, via
a remote I/O cable, plus a few power supplies and hardwire safety
signals. This allowed the pulpit to be built and tested before it arrived
on site. Similar ideas can be applied to any plant with I/O that needs to
be connected to a PLC.
Figure 1.22 Identification of plant signals

If remote I/O is used, provision should be made for a program terminal
to be connected local to each rack. It negates most of the benefits if the
designer can only monitor the operation from a central control room
several hundred metres from the plant. Fortunately, manufacturers
have recognized this and most allow programming terminals to be
connected to the processor via similar screened twin cable.
We will discuss serial communication further in Chapter 5.
1.6 The advantages of PLC control
Any control system goes through four stages from conception to
a working plant. A PLC system brings advantages at each stage.
The first stage is design; the required plant is studied and the control
strategies decided. With conventional systems design must be complete
before construction can start. With a PLC system all that is needed is
a possibly vague idea of the size of the machine and the I/O requirements
(how many inputs and outputs). The input and output cards are cheap
at this stage, so a healthy spare capacity can be built in to allow for the
inevitable omissions and future developments.
Next comes construction. With conventional schemes, every job is
a ‘one-off’ with inevitable delays and costs. A PLC system is simply
bolted together from standard parts. During this time the writing of the
PLC program is started (or at least the detailed program specification
is written).
The next stage is installation, a tedious and expensive business as
sensors, actuators, limit switches and operator controls are cabled. A
distributed PLC system (discussed in Chapter 5) using serial links and
pre-built and tested desks can simplify installation and bring huge cost
benefits. The majority of the PLC program is written at this stage.
Finally comes commissioning, and this is where the real advantages
are found. No plant ever works first time. Human nature being what it
is, there will be some oversights. Changes to conventional systems are
time consuming and expensive. Provided the designer of the PLC
system has built in spare memory capacity, spare I/O and a few spare
cores in multicore cables, most changes can be made quickly and relatively
cheaply. An added bonus is that all changes are recorded in the PLC’s
program and commissioning modifications do not go unrecorded, as is
often the case in conventional systems.
There is an additional fifth stage, maintenance, which starts once the
plant is working and is handed over to production. All plants have
faults, and most tend to spend the majority of their time in some form of
failure mode. A PLC system provides a very powerful tool for assisting
with fault diagnosis. This topic is discussed further in Chapter 8.

A plant is also subject to many changes during its life to speed pro-
duction, to ease breakdowns or because of changes in its requirements.
A PLC system can be changed so easily that modifications are simple
and the PLC program will automatically document the changes that
have been made.

2 Programming techniques
2.1 Introduction
Chapter 1 described the evolution of the programmable controller leading
to a system similar to that of Figure 1.12. This consists of a CPU linked
to one or more I/O racks. These racks contain cards which are connected
to the plant signals.
There are many variations on the details of Figure 1.12. Modern
central processors tend to be small, live in one of the racks, and not be
readily identifiable. In the smallest systems every part has been encapsu-
lated in one unit. All, however, behave as in Figure 1.12.
In this chapter we shall consider how a PLC can be programmed.
Each manufacturer, of course, has its own standards and it would be
rather restrictive to deal with only one machine. This chapter is therefore
written around five manufacturers’ ranges:
1 The Allen Bradley PLC-5 series (Figure 2.1(a)). Allen Bradley, now
owned by Rockwell, were one of the original PLC originators (and
actually have the USA copyright on the name PLC). They have
been responsible for much of the development of the ideas used in
PLCs and have succeeded in maintaining a fair degree of upward
compatibility from their earliest machine without restricting the fea-
tures of the latest.
2 The Siemens Simatic S5 range (Figure 2.1(b)) which has become
widely used in Europe in the early part of the 1990s.
3 The British GEM-80 (Figure 2.1(c)), originally designed by GEC
through a long association with industrial computers dating back to
English Electric. This part of GEC is now known as CEGELEC and
is part of a French group in which Alsthom is a major shareholder.
4 The ASEA Master System (Figure 2.1(d)), now manufactured by the
ABB company formed by the merger of ASEA and Brown Boveri.
The Master System has features more akin to a conventional computer
075065757X-ch002.fm Page 33 Friday, July 25, 2003 2:49 PM

(a)
(b)
Figure 2.1 The four medium-sized PLCs discussed: (a) the Allen Bradley
PLC-5; (b) the Siemens S5-1154;

Programming techniques 35
(c)
(d)
Figure 2.1 (continued) (c) the CEGELEC GEM-80; (d) the ABB Master.
Photographs courtesy of the manufacturers

system and its programming language has some interesting and
powerful features.
5 Many PLC systems are now very small; the author recently found it
cost-effective to build a system with a PLC rather than the 12 four-
pole relays that could have been conventionally used. There are
many cheap small machines, and as an example of this bottom end
of the market we shall consider the Japanese Mitsubishi F2-40,
shown later in Figure 2.12.
Significant differences will be found in this selection (a PLC-5, for
example, has three different types of timer, the Siemens 115-U has five
timers, and a GEM-80 just one, which, because of its different approach,
can be used in various ways). Between them most of the standards
adopted by other manufacturers will be covered.
2.2 The program scan
A PLC program can be considered to behave as a permanent running
loop similar to that in Figure 2.2(a). The user’s instructions are obeyed
sequentially, and when the last instruction has been obeyed the operation
starts again at the first instruction. A PLC does not, therefore, communicate
continuously with the outside world, but acts, rather, by taking ‘snapshots’.
The action of Figure 2.2(a) is called a program scan, and the period of
the loop is called the program scan time. This depends on the size of the
PLC program and the speed of the processor, but is typically 2–5ms per
K of program. Average scan times are usually around 10–50ms.
Figure 2.2 PLC program scan and memory organization: (a) PLC
operation; (b) program sequence; (c) PLC memory organization

Programming techniques 37
Figure 2.2(a) can be expanded to Figure 2.2(b). The PLC does not
read inputs as needed (as implied by Figure 2.2(a)) as this would be
wasteful of time. At the start of the scan it reads the state of all the con-
nected inputs and stores their state in the PLC memory. When the PLC
program accesses an input, it reads the input state as it was at the start of
the current program scan.
As the PLC program is obeyed through the scan, it again does not
change outputs instantly. An area of the PLCs memory corresponding
to the outputs is changed by the program, then all the outputs are
updated simultaneously at the end of the scan. The action is thus: read
inputs, scan program, update outputs.
The PLC memory can be considered to consist of four areas as
shown in Figure 2.2(c). The inputs are read into an input mimic area at
the start of the scan, and the outputs updated from the output mimic
area at the end of the scan. There will be an area of memory reserved
for internal signals which are used by the program but are not connected
directly to the outside world (timers, counters, storage bits, e.g. fault
signals, and so on). These three areas are often referred to as the data
table (Allen Bradley) or the database (ASEA/ABB).
This data area is smaller than may be at first thought. A medium-size
PLC system will have around 1000 inputs and outputs. Stored as indi-
vidual bits this corresponds to just over 60 storage locations in a PLC
with a 16-bit word. An analog value read from the plant or written to
the plant will take one word. Timers and counters take two words (one
for the value, and one for the preset) and 16 internal storage bits take
just one word. The majority of the store, therefore, is taken up by the
fourth area, the program itself.
The program scan obviously limits the speed of signals to which
a PLC can respond. In Figure 2.3(a) a PLC is being used to count a series
of fast pulses, with the pulse rate slower than the scan rate. The PLC
counts correctly. In Figure 2.3(b) the pulse rate is faster than the scan
rate and the PLC starts to miscount and miss pulses. In the extreme case
of Figure 2.3(c) whole blocks of pulses are totally ignored.
In general, any input signal that a PLC reads must be present for
longer than the scan time; shorter pulses may be read if they happen to
be present at the right time but this cannot be guaranteed. If pulse trains
are being observed, the pulse frequency must be slower than 1/(2×scan
period). A PLC with a scan period of 40ms can, in theory, just about
follow a pulse train of 1/(2×0.04)=12.5Hz. In practice other factors
such as filters on the input cards have a significant effect and it is always
advisable to be conservative in speed estimates.
Less obviously, the PLC scan can cause a random ‘skew’ between
inputs and outputs. In Figure 2.4 an input is to cause an ‘immediate’
output. In the best case of Figure 2.4(a), the input occurs just at the start

of the scan, resulting in the energization of the output one scan period
later. In Figure 2.4(b) the input has arrived just after the inputs are read,
and one whole scan is lost before the PLC ‘sees’ the input, and the rest
of the second scan passes before the output is energized. The response
can thus vary between one and two scan periods.
In the majority of applications this skew of a few tens of milliseconds
is not important (it cannot be seen, for example, in the response of a plant
Figure 2.3 The effect of program scan on fast pulses
Figure 2.4 The effect of program scan on response time: (a) best case;
(b) worst case

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Tres Rios, Monte de los, 685
Treviño, Mex., 685
Trimbelle r., 73
Trinchera cr., 494, 497, 507
Trinchera mt., 445
Trinidad r., 707, 708
Trinidad, Tex., 708
Trinity r., 696, 703, 706, 707, 708, 710,
779, 781, 782, 784, 786
Trinity, Tex., 708
Trinity, The, 707
Trionyx ferox, 539
Triplet lakes, near Morrison l. and
Whipple l., named by Brower
Trompledo, Schlc., 1855, for
Trempealeau
trout, 297
Trout cr., Col., 469, 471
Trout cr., Itasca co., Minn., 143
Trout Creek pass, 469, 847
Trout Creek Pass hills, 469
Trout cr., Winona co., Minn., 53
Trout cr., Wis., 41
Trout r., 143
Truchas, Truches mt. or pk., 606, 736
Trudeau, Gov. Z., 358
Trujillo, 659
Trumbull, Jonathan, 711
Truro, Col., 465
Truxillo, 659, 839
Trying to Walk is E. trans. of name of
Nicollet's guide Gaygwedosay, 1836,
who lately died at supposed age of
115 years; portrait published, 1895, by
Brower
Tsea, 745

Tshiquite, 737
Tsia, 745
Tsuga canadensis, 320
Tubac, Ariz., 771, 773, was an Indian
mission about 1699
Tubson, 773
Tucayan, 743, 744
Tucson, Ariz., 639, 734, 773. The orig.
Piman rancheria, pop. 331 in 1760-67,
became site of a Spanish presidio
about 1772, and actual settlement by
Spaniards was in 1776. The contention
of great antiquity of Tucson as a white
settlement is thus a popular myth
Tuerto cr., 616
Tulenos, 770
Tulip, see La Tulip
Tully, 12
Tully isl., 12
Tumbling rock, 56
Tuque cr., 364
Turk, see Coronado
Turkey cr., br. of Ark. r., in Col., 452, 454,
455, 456, 457, 459, 460
Turkey cr., br. of Ark. r. in Okla., 550
Turkey cr., br. of Huerfano r., 491
Turkey cr., br. of Kansas r., 519
Turkey cr., br. of Little Ark. r., in Kas.,
518, 522
Turkey cr., br. of Neosho r., 399
Turkey cr., br. of Osage r., 377, 378
Turkey cr., br. of Smoky Hill r., 403
Turkey isl., 381
Turkey r., 32, 34, 293, 294, 339, 355
Turkey's foot, 22, 292
Turkish ladies, 790

Turnbull, Peter, opened a road in Itasca
basin, 1882
Turnbull pt., 167, named by Brower for
Peter Turnbull, first white resident at L.
Itasca
Turner, Capt. E. D., xxvii, 725
Turner's, Turner's isl., 5
Turning Point, 297
Turn isl., 356
Turn, The, 356
Turtle brook is Turtle r. near Turtle l.,
Minn., D. Thompson, 1798
Turtle isl., 8
Turtle l., 161, 332
Turtle mt., 255, 278
Turtle Portage r., 324
Turtle r., 157, 158, 159, 167, 168, 263,
328
Turtle River lakes, 157, 158
turtles, 539
Tusayan, 742
Tuscumbia, Mo., 373
Tuttasuggy, 371, 386, 389, 391, 540,
550, 552, 591
Tuzan, 744
Twin Creek pass, 465
Twin isls., 18
Twin lakes, Col., 471, 472
Twin lakes, Minn., 166, so named by
Brower
Twin Lakes sta., Col., 471
Twin r., 103
Two Branch, Ill., 4
Two Branch isl., 4
Two Butte cr., 442, 443
Two Creek pass, 465

Two Lakes l., 317
Two Mountains l., 351
Two Rivers Baptist Association, 10
Two Rivers brook, 103
Two Rivers, Minn., 103, 105, 110, see
Little
Two Rivers, Mo., 10, 11, see North and
South
Two Rivers tp., Minn., 103
Tympanuchus americanus, 98
Tyrrell, J. B., 168, 278
Tyson's cr., 17
Tzia, 745
Tziguma, old pueblo near Cienega, N. M.,
pop. under 1000, abandoned after
revolt of 1680
U
Ugarte, Capt., 698
Ukaqpa, 559
Ulloa, A. d', 214
Ulloa, Count, 357
Ulloa, Francisco de, was first in Gulf of
California in 1539
Ultimate Reservoir Bowl, see Greater and
Lesser do.
Ultima Thule, 457
Ulua, see Ulloa
Ulysses, Kas., 439
Umas, 736, see Yuma Inds.
Unadilla, N. Y., 405
Una Vida, 630
Uncle Sam, 711
Undine region, 68
Union Avenue, Pueblo, Col., 453, 454

Union gulch, Col., 471
Unionists, 560
Union Jack down, c
Union Pacific R. R., 404, 471
United Empire Loyalists, lxxvi
United States, 231, 232, 236, 237, 238,
240, 241, 247, 248, 249, 250, 262,
263, 265, 266, 270, 279, 280, 727,
808, 810, 811, 812, 813, 814, 816,
817, 818, 821, 822, 823, 825, 837,
838, 840, 842, 843, 844, 846, 850,
851, 853, 854
United States and Mexican Boundary,
644, 645, 646, 647, 691, 692
United States and New Spain Boundary,
656
United States Geological Survey, 369,
370, 491
United States mt., 606
United States Northern Boundary, 279
upékan is Chip. name of the portage-
strap in Wilson's Dict., 1874; see p.
136
Upham sta., N. M., 636
Upper Canada, xxiii, lxxvi, lxxxviii, cii
Upper cañon of S. Platte r., 466
Upper chain, 15
Upper chain of Rock r., 25
Upper Cottonwoods, 688
Upper crossing of Ark. r., 439
Upper Ford, Tex., 645
Upper Fox r., 295, 300, 301
Upper Gravel r., 376, 377
Upper Iaway r., 48
Upper Iowa r., 42, 44, 45, 48, 206, 305,
307, 308, 339, 342

Upper l., near Red Wing, Minn., 70
Upper Nicollet l., 165
Upper or Eleven Mile cañon, 466
Upper Pajarito, N. M., 626
Upper Pimas, 735
Upper Red Cedar l., xlviii, 153, 157, 158,
159, 323, 326, 351, 356
Upper Red r. of Pike, 535
Upper Rio Grande, 474
Upper St. Croix l., 72, 309, 310
Upper St. Croix r., 309
Upper Zumbro outlet, 61
Uraba was a name of Taos
Ures, 773
Usawa, Usaw-way l., 162, 331
Usaya, 744
Utah, 630, 731, 732, 733, 734, 735, 736
Utah Inds., 508, 535, 537, 591, 618, 744,
746, 849, 850
Utah l., 738
Ute cr., 494
Ute cr. = Brush Hollow cr., 462
Ute Inds., xlvi, 448, 453, 492, 596, 743,
816
Ute pass, 456, 464
Utica, N. Y., xlvi
V
Vacamora, 771
Vache Blanche, 347
Valasco, F., 817, 819, 820
Valencia co., N. M., 628, 629, 742
Valencia, N. M., 618, 628
Valladolid, Mex., 720, 721, 723
Valladolid, Spain, 720

Vallance, J., 553
Valley City, Ia., 25
Vallois, Don P., 661, 662
Vallois, Señora M., 659
Valverde ford is near the ruins of
Valverde, and about 5 m. N. of Fort
Craig, N. M.
Valverde, N. M., 633, 634
Van Bibber, Mr., 367
Van Buren, Ark., 559
Van Buren, M., 358
Vandals, 632
Van Dalsem, Capt. H. H., cviii
Vandermaelen l., 160
Vaqueria, 684
vaquero, 684
vara, 669
Varennes, P. G. de, 254
Vargas, 737
Vasquez, A. F. B., or "Baroney," lxiv, 359,
360, 361, 362, 364, 365, 368, 371,
386, 387, 390, 393, 401, 403, 414,
416, 420, 421, 422, 429, 432, 435,
449, 459, 470, 472, 474, 477, 478,
480, 481, 482, 490, 506, 509, 510,
545, 579, 580, 612, 834, 845, 853,
855
Vaugondy, 559, 695, 734
Veau, Mr. Jacques, 194, 195
Vegas, Col., 459
Velasco, see Valasco, F.
Velasco, viceroy, 755
Vellita, N. M., 629
Venadito, 685
Venus' spr., 651
Vequeria cr., 683

Vequeria, Mex., 683, 684
Vera Cruz, administration or State of,
673, 718, 720, 721, 722, 723, 724
Vera Cruz, city of, 721, 722, 791
Veragua, Veraqua, 726
Verdegris r., 400
Verde r., Ariz., 727, 730, 734
Verde r., Mex., 721
Verdigris, Kas., 400
Verdigris, Verdigrise r., 399, 400, 515,
532, 555, 556, 557, 560, 584
Verendrye, Le Sieur de, 254, 255, 256
Veritas, Caput, 331
Vermijo r., 558
Vermilion cr., br. of Osage r., 377, 378
Vermilion r., br. of Ark. r., 395, 400, 514,
515, 555, 557
Vermilion r., br. of Miss. r., Cass co.,
Minn., 147
Vermilion r., br. of Miss. r., Dakota co.,
Minn., 72, 73
Vermilion r. of Beltrami = Deer r., Minn.,
147
Vermilion sea, old name of the Gulf of
California, for Red sea
Vermilion sl., 73
Vermillion isl., 298, 356
Vermillion r. of Pike, br. of Osage r., 379
Vermont, 570
Vermonter, 242
Vernon co., Mo., 370, 385
Vernon co., Wis., 49
Verte, Isle, 297
Verte r., 77, 78
Verum Caput, 165, 331
Verumontanum, 165

Verwyst, 101
Veta pass, 492, 494
Viana, Capt. or Don F., 412, 709, 710,
839
Viceroyalty of New Spain, 719
Vicksburg, Miss., 708
Victoria City, Mex., 724
Victoria, Wis., 49
Victor, Kas., 422
Victory, Wis., 45
Vide-poche, 215
Vieau, Jacques, 194
Vieux Desert l., 128
Vigil, D., 607
Village Creek, Ia., 43
Village de Charette, 568, 572
Villamil, Don B., 659, 661, 662
Villa Rica de la Vera Cruz, 722
Villiers, N. de, 214
Villineuve, a person, 413
Vimont's Relations, 31
Vine cr., 559
vineyards, 681
Vingt-une isl., 361
Viola, Wis., 41
Virginia, a boat, 84
Virginia, a State, xxvii, xxviii, liii, lxxxviii,
656, 691, 715, 826, 833
Virgin r., 732
Visscher, Capt. N. J., xxvi, xxvii
Vitior, 613, 614. This baffling name is
clearly a misprint, Mr. F. W. Hodge
believes, for Sienega (Cienega), place
on a cr. of same name, br. of Santa Fé
r., 2 m. S. E. of Cieneguilla, which
appears on most maps of to-day.

Cienega and Cieneguilla were both
towns of Santa Fé co. in 1844, but La
Bajada may be later. Cienega had pop.
500, and Cieneguilla, pop. 300, in
1853-54, according to Whipple, P. R.
R. Rep. III., Pt. 3, p. 12
Vocabulary, etc., 355
volcano, 723
Volcano sta., N. M., 597
Volney, Count, 154
Voltaire, 154, 801
Vulgate, 182
W
Wabasha, 43, 171, 206, 260, 342, 347,
348
Wabasha I., II., III., 44
Wabasha co., Minn., 56, 57, 64
Wabasha, Minn., 57, 59, 60, 61
Wabasha st., St. Paul, 74
Wabashaw, 44, 88
Wabash r., 68, 438
Wabash Ry., 8, 15
Wabash, St. Louis, and Pacific R. R., 360
Wabesapinica, Wabezipinikan,
Wabisapencun, Wabisapincun,
Wabisipinekan r., 26
Wabezi r., 122
Wabiscihouwa, 44
Wabisipinekan r., 293
Wabizio-sibi, 122
Wablo cr., 383
Waboji, 338
Wacanto, Wacantoe, 343, 347
Wachpecoutes, 263

Waconda, Mo., 12
Waconda, Wacondaw pra., 12
Wacoota, Wacouta, 69
Wacouta, Goodhue co., Minn., 63
Waddapawmenesotor, 81
Wadena co., Minn., 128
Wade, Pvt., 332
Wadub r., 101
Wagoner's cr., 15
Wahkantahpay, 88
Wahkanto, 349
Wahkootay is Wacouta, 88
Wahkpakotoan, 344, 345, 349
Wahkpatoan, 345, 349
Wahpatoota, 349
Wahpaykootans, 88
Wahpeton Sioux, 85
Wahpetonwans, 118
Wajhustachay, 61
Wakan-tibi, 200
Wakarusa pt., 520
Wakarusa r., 408, 520
Wakoan, 706
Wakomiti is the Ojibway name of the
stream misnamed Hennepin r., and
should stand: see Annals of Iowa, Apr.,
1895, p. 26
Wakon-teebe, 198
Wakouta, 62, 69
Wakouta, Goodhue co., Minn., 63
Wakpatanka is Sioux name of the Miss.
r., meaning Great river
Wakpatons, 313
Wakuta, Wakute, see Wacoota, 69
Walapais, 736
Walbach, Gen. J. De B., xxvii, xxviii

Walker, Capt. Joel P., of Cal., 446
Walker, Lt. J. P., 656, 658, 660, 664, 665,
666, 761, 767, 817, 819, 820, 821, 830
Walking Buffalo, a chf., 69, 88
Wallace co., Kas., 404
Wallace, Joseph, 531, 532, 560, 714
Wallace, N. M., see Santo Domingo, N.
M.
Wall, a Mr., xxi
Walnut cr., br. of Ark. r., 424, 425, 426,
429, 517, 518, 522, 545, 546, 547
Walnut cr., or White Water r., 549
Walnut Hills, lii, 657
Walpi, 744, is Pike's Gualpi
Walworth, Capt. John, lxxxvii, lxxxviii
Wamaneopenutah, 347
Wamdetanka, 85
Wamendetanka, 348
Wamendi-hi, 118
Waminisabah, 347
Wanomon r., 147
Wanotan, 349
Wanyecha cr., 94
Wapahasha, Wapasha, Wapashaw, 43,
44, 61, 86, 348
Wapello, Louisa co., Ia., named for a chf.
who had his village on Iowa r. near
present city
Wapsipinecon, Wapsipinicon r., 26
Wapuchuseamma, 745
Waqpatonwan, Waqpetonwan, 313, 343
Waqpekute, 344
Waraju r., 66
War Department, xix, xx, xxii, l, lviii, lix,
lx, lxi, lxiii, lxv, lxxii, xcvi, cxiii, 236,

239, 446, 549, 554, 593, 645, 812,
842, 844, 851, 855
Ward, see Johnston and, 554
Ward, Prof. L. F., 38
Ward's isl., 11
War Eagle, 348
Warm spr., Mex., 652, 653
Warm spr., N. M., 597
Warner's ldg., 49
Warpekutes, 44
Warpetonwans, 313
Warpool l., 129, 317
Warren co., Mo., 363, 364, 365, 366
Warren, Gen. G. K., xlv, 56, 57, 58, 62,
91, 295, 310, 333, 457, 558
Warren, Gen. Jos., cvi, cix
Warren, Hon. W. W., 31, 127, 134, 150,
157, 170, 171, 176
Warsaw, Ill., 14
Warsaw isl., 379
Warsaw, Mo., 379
wars of Pawnees, 535
Warwater r., 128
Wasaba Tunga, 591
Wasbasha, 590
Wascheta r., 827
Wasetihoge r., 555
Washburn co., Wis., 309
Washington co., Ark., 558
Washington co., Minn., 72, 73, 74
Washington, D. C., xxxiv, xlii, li, liv, lvi,
lviii, lix, lx, lxiii, lxv, 358, 410, 510,
538, 550, 551, 582, 583, 587, 647,
656, 812, 813, 827, 833, 835, 845,
851, 855

Washington, Geo., lvi, lxvi, lxviii, lxx, 49,
289, 408, 656, 701
Washington Irving's l., see Irving l.
Washington isl., 279
Washington, Mo., 363, 364
Washington, Tex., 560, 707
Washione, 347
Washita r., 612, 704, 827
Washpecoute, 344, 345, 346, 347
Washpetong, 343, 344, 345, 346, 347
Wasonquianni, 343, 347
watab, 101, 102
Watab rap., 101, 102
Watab r., 101
Watapan Menesota, 81
Watapan Tancha, a form of the Sioux
name of the Miss. r.
watap, watapeh, 101
Watchawaha, Watchkesingar, 591
Wate-paw-mené-Sauta as Sioux name of
Minnesota r. in Schlc., 1820
water-oats, water-rice, 38
Waters, John, liii
Watertown, Ill., 25, 210
Watpà-menisothé, 81
Watson, Prof. S., 39
Wattah r., 101
wattap, 102
Waubojeeg, 127
Waucon Junction, 41
Waucouta, see Wacouta
Waukan r., 301
Waukenabo l., 135
Waukon, Ia., 41
Waukon Junction, 41
Wauppaushaw, 43, 54

Wawana, 143
Way Aga (or Ago) Enagee, 86, 231, 238
Wayne co., Mich., xxvi, cxi
Wayne co., N. C., cxi
Wayne co., O., cxi
Wayne, Gen. A., xxvi, 438, 712
Wazi Oju r., 57
Weablo cr., 383
Wea cr., 519
Weakaote, 349
Weather Diary, 216, 217, 218, 219, 220,
716, 717
Weaubleau cr., 383
Weaver mts., 730
Webber cr., 559
Webber's falls, 558, 559
Webber's Falls, Ind. Terr., 558
Webb, Tex., 690
Webster co., Mo., 376, 380
Webster co., Neb., 404, 410
Webster park, 462, 478, 479
Weed Bush is a corruption of Vide Poche
Weekly Register, see Niles'
Wejegi, 630
Weller-Conde line, 644
Weller, J. B., 644, 645
Wellman, W. D., 365
Well of Mineral Water, 680
Well of Putrid Water, 680
Wells' br., 377
Wells cr., 63, 65
wells in Mexico, 680
Wellsville sta., 475
Wenepec l., 322
Wentaron is a form of the Ind. name of
Lake Simcoe

West Arm of Lake Itasca, so named by
Brower, see Itasca l.
West Brainerd, Minn., 130
Western battery, lxxvii, lxxviii, lxxix,
lxxxviii, xc, xcvi, cii
Western Ocean, 524, 721, 722
West Fork of Miss. r., on Eastman's map
of 1855, is the main stream from Lake
Itasca
West Indies, 718
West Lake Champlain, lxxii
Westminster, Engl., 167
West Monument cr., 452
West Naiwa r., 162
West Newton chute, 57
West Oil cr., 465
West Ojibway, see Ojibway, Minn.
Weston gulch, 471
West Point, N. Y., xxvi, lxvii, lxxxvii, 405,
656, 734
Westport isl., 5
Westport, Jackson co., Mo., 408, 517,
518, 519
Westport, Lincoln co., Mo., 5
West Quincy, Mo., 9
West Savannah r., 138
West Swan r. of Nicollet, 143
West Turkey cr., Col., 455
West Turkey cr., Kas., 522
Wet Glaize cr., 375
Wetmore's Gazetteer, 3
Wet mts., 448, 451, 463, 482, 483, 484,
485, 487, 488, 848
Wet Mountain valley, 482, 483, 434, 485,
848
Wet Stone, a chf., 531

Wet Walnut cr., 425, 426
Whakoon-Thiiby, 200
Wheeler, Lt. G. M., 483
Whelply, Pvt. D., 1, 854
Whipple, A. W., 645, 735
Whipple, Capt. J., xxvi, xxvii
Whipple l., named by Rev. J. A. Gilfillan
for Bishop H. B. Whipple, 165
Whistler, Maj. J., 358
White Bear-skin r., 131, 177
White Blanket, a chf., 347
White Buffalo, a chf., 347
White Bustard or Buzzard, a chf., 89
White Dog, a chf., 121, 189
whitefish, 169, 297
White Fisher, a chf., 127, 176
Whitefish l., 133, 173, 174, 175, 319,
334
White Hair, a chf., 387, 388, 390, 551,
558, 563, 565, 576, 578, 579, 591,
and see Cheveux Blancs
Whitehouse's isl., 366
Whitely cr. is a name of the Rice or
Nagajika cr. 2 m. N. E. of Brainerd
Whitely isl. is just below Seventh or
French raps.
White Mountain Apaches, 748
White mts., Ariz., 730
White mts., N. H., 454
White mts., N. M., 631, 640, 736
White mts. of Pike, in Col., 483, 492, 493
White Nails, a chf., 349
White Oak l., 147, 148
White Oak or Stephens l., 147, 148, 150
White Oak pt., 146, 147, 148, 150

White r., br. of Miss. or of Ark. r., 514,
515, 757
White r. or Ark. r., 692, 694
White r. or Cottonwood r., 402
White r. or Neosho r., 397, 398, 473, 514,
515, 584
White r., Wis., 301
White Rock cr., Kas., 404, 405, 408, 409
White Rock, Kas., 409
White Rock, on Minnesota r., 343
White Rock, St. Paul, Minn., 75
Whiteside co., Ill., 27
White Skin, a chf., 347
White Snow mts. of Pike, 479
white spruce, 102
White Water cr., N. M., 637
White Water r., Kas., 549
White Water r., Minn., 56, 57, 305
White Wolf, a chf., 542, 551, 591
whitewood, a tree, 315
Whiting, Gen. H., xix, xxx, xxxiv, xlv,
lxviii, lxxiv, lxxix, lxxxv, lxxxix, xc, xcix,
cvi, 44, 275, 499, 500, 501
Whitman, Kas., 549
Whitney, a Mr., 302
Who-walks-pursuing-a-hawk, a chf., 85
Whymper, Edw., 461
Wiahuga, 349
Wibru, Corporal, 332
Wichaniwa r., 95, 97
Wichita, Kas., 548
Wichita res., 412
Wiggins, O. P., 457
Wild Bull r., 58
Wildcat cr., 49
wild hogs, 697

Wild Horse cr., 442
wild horses, 433, 435, 436, 738, 782,
783
wild oats or rice, 38
Wild Oats r. of L. and C. map, 1814, and
of Beltrami, is that Rice r. which is
between Aitkin and Willow r., 138
wild pigeons, 211
Wild Rice l., on Willow River route, 320
Wild Rice Sissetons, 349
wild rye, 47
wild sheep, 438
Wild Swan r., 143
Wilkinson, Gen. James, xxi, xxii, xxvii,
xxix, xlvi, li, lv, lvi, lvii, lviii, lxiii, lxiv,
lxxiv, 2, 15, 17, 37, 84, 85, 206, 221,
222, 223, 224, 225, 229, 232, 237,
239, 240, 244, 246, 255, 259, 269,
270, 271, 273, 293, 358, 361, 375,
381, 386, 388, 392, 418, 431, 481,
500, 504, 539, 561, 562, 563, 564,
565, 566, 567, 568, 569, 570, 571,
572, 573, 574, 575, 582, 585, 586,
587, 588, 589, 592, 593, 594, 662,
697, 703, 712, 810, 817, 819, 820,
821, 824, 836, 841, 842, 844, 845,
852
Wilkinson, Lieut. J. B., xxxvi, xli, li, 223,
225, 263, 359, 360, 361, 364, 365,
372, 373, 381, 382, 385, 386, 387,
389, 390, 392, 393, 403, 406, 407,
409, 414, 421, 425, 426, 427, 431,
432, 514, 515, 518, 532, 539, 540,
541, 542, 543, 544, 545, 546, 547,
565, 568, 572, 577, 578, 580, 585,
587, 589, 592, 708, 818, 819, 824,

826, 827, 833, 834, 835, 845, 846,
852, 855
Wilkinson's Report on the Arkansaw, 432,
539 to 561
Wilkinsonville, xxvi
Williamsburg, Col., 482
Williams, Gen. J. R., xxiii, xxx
Williams, Helen M., xli
Williams isl., in Osage r., 377
Williams, J. Fletcher, xxxv, 31, 44, 76, 85,
87, 88, 201
Williams, Lt.-Col. Jonathan, xxvii, xxix
Williams, Lt. J. R., preface, xxiii, xxx
Williams, Lt., unidentified, xxvi, xxviii, is
no doubt Thomas W. Williams of N. Y.,
ensign 12th Infantry Jan. 14th, 1799,
second lieutenant Mar. 2d, 1799,
honorably discharged June 15th, 1800,
second lieutenant 1st Infantry Feb.
16th, 1801, resigned July 28th, 1801
Williams' pass, 491
Williams, Peter, 18
Williamson, Lt. R. S., see Fort Ripley
Willibob l., 143
Willow cr., br. of Ark. r., Col., 443
Willow cr., br. of Cottonwood r., Kas., 401
Willow cr., br. of Osage r., Mo., 385
Willow cr. or Lost cr., Kas., 518
Willow Inds., 278
Willowmarsh l., 26
Willow portage, 142
Willow r., 137, 142, 153, 155, 320
Willows, Mex., 684
Willow spr., 518, 519, 520
Willow Springs cr., 459
Wilmot, Mr., 207, 209, 211

Wilson co., Kas., 555
Wilson cr., br. of Oil cr., 464, 465
Wilson isl., 98
Wilson, Lt. J., xxvi, xxviii
Wilson, Mr., on Pike's pk., 457
Wilson, Pvt. J., 359, 432, 548, 845, 855
Wilson's cr., br. of Huerfano r., 491
Winapicane, 819
Winboshish appears on Stieler's Hand
Atlas for Winnibigoshish
Winchell, Prof. N. H., preface, 95, 106,
333
Winchester, Va., liii
Wind, a chf., 371, 387, 389, 392, 393,
540, 550, 578, 580, 581
windshake or windshock, 109
Wind that Walks, a chf., 203, 347
Windy pt., 324
Winebagos, 346, 347
Winepie l., 168
Wing pra., 54
Winipec or Winnibigoshish l., 322
Winipeg l., 351, 353
Winipeque br. of Miss. r., or main r. above
Leech Lake br., 325
Winipeque or Winnibigoshish l., 149,
152, 322, 323
Winipie or Winnibigoshish l., 149, 152,
351
Winipie or Winnipeg l., 278, 280, 322,
327, 351, 353
Winnebago cape, 42, 43
Winnebago chain, 25
Winnebago council, 207, 208, 209
Winnebago co., 300, 301
Winnebago cr. or r., 42, 43, 46

Winnebago Inds., 31, 39, 43, 265, 266,
340, 341
Winnebago l., 24, 295, 300, 301, 340
Winnebago pra., Stearns co., Minn.,
between Watab raps. and Brockway
Winnebago rap., 300
Winnebago village, 300
Winnebeegogish l. of Schlc., 1855
Winnepegoosis l., 322
Winnepeg or Winnibigoshish l., 322
Winneshiek sl., 42
Winnibigoshish l., 138, 149, 152, 153,
158, 159, 168, 317, 322, 323, 324,
325
Winnipec or Winnibigoshish l., 322, 327
Winnipeg l., 351, 353
Winnipegoos is Winnibigoshish l., D.
Thompson, 1798
Winnipek is Winnibigoshish l., Schlc.,
1855
Winona, a maiden of myth, 66
Winona co., Minn., 52, 53, 57
Winona, Minn., 54, 55, 56, 88, 206
Winship, W. W., preface
Winsor, J., 296
Winterbotham's map, xli, 696, 697, 702,
707
wintering grounds, 99
Winter's ldg., 53
Wisconsan r., 35
Wisconsin Central R. R., 302
Wisconsin r., 3, 34, 35, 71, 78, 224, 295,
302, 303, 304, 338
Wiscoup, 156, 259, 347, 351
Wise Family, a chf., 591
Wise, Kas., 397

wishtonwish, 429, 430, 431
Wislizenus, Dr., 339, 437, 446, 518, 521,
631, 635, 649, 650, 652, 653, 654,
667, 668, 669, 670, 671, 672, 674,
675, 680, 681, 682, 683, 684, 739,
747, 759
Wissakude r., 309
Withlachoochee r., lxxxvii
Without Ears, a chf., 591
Without Nerve, a chf., 591
Wiyakonda, Mo., 12
Woco-sibi, Wokeosiby, 127
Wolf cr., br. of Ark. r. in Col., 442
Wolf cr. of Pike, 103, 184
Wolfe, Gen., lxxxiii, c
Wolf Inds., 35, 338
Wolf r., 300, 301, 356
Wollstonecraft, Maj. C., 715
Wolverine cr., 396
Woman in White mt., 723
women and children, 286
Woodbridge, N. J., lix
Woodcock, 88
Wood cr., br. of Miss. r., 2
Wood cr., br. of Mo. r., 363, 364
Wood, Mr., 201, 202, 205, 206, 207
Woodruff, J. C., 554
Woods, Lake of the, 279, 281, 351
Woods, Mr., 37, 42
Woodson co., Kas., 395, 398, 399
Wool, Gen. J. E., 669, 674, 679, 684
Woolstoncraft, Capt., 715
Wooster, Maj. Gen. D., cvi
Worcester, Mass., xxxiv
Word of God, 182
wounded, see killed and

Wright co., Minn., 96, 97, 98
Wright's cr., 381
Wright's isl., 379
Wrightstown, 299
Wuckan l., 340
Wuckiew Nutch, 208, 343, 347
Wukunsna, 347
Wyaconda, Mo., 12
Wyaconda r., 9, 291
Wyaganage, 86, 342, 347
Wyalusing, Wis., 34
Wyoming co., N. Y., cx
Wyoming, Ia., 23
Wyoming sl., 23
X
Xacco l., 673
Xalisco, 719
Xaxales, 628, 629, has been thought to
have been so called as once a
temporary Apache rancheria of huts,
jacales, or xacales; but see Jarales.
The form Xarales is also found
Xenia, Kas., 396, 397
Xicarilla for Jicarilla, in Don José Cortez,
1799
Xila is Gila
Xisuthros, 182
Xocoyotzin, 737
Xougapavi is Shongapavi
X. Y. Company, 139, 277
X. Y. Z., one, 336
Y

Yaatze, see San Marcos
Yabijoias, 735, of Pike, simply error in
copying Indiens Yabipias à longues
barbes from Humboldt's map
Yahowa r., 22, see Iowa r.
Yahowa r., 44, see Upper Iowa r.
Yakwal, 706
Yamajab is Mojave
Yamaya, 735
Yampancas r., 738
Yamparicas Inds., 738
Yanctongs, 120, 121, 197, 207, 208, 258,
264, 267, 343, 344, 345, 346, 347, see
Yanktons
Yanctons for Hietans, 563
Yanga r., 540
Yankee, 188
Yanktoan, 345, 349
Yanktoanan, 345, 349
Yanktonnais, 343
Yanktons, 120, 343, 346, 347
Yanos, 653
Yaos, 598
Yaqui r., 771, 773
Yattasses, 713
Yavapais, 735
Yavasupai Inds., 731, 736, see Havasupai
Yawayes or Yawayhaws are Iowa Inds.
Yawoha, Yawowa r., 22, see Iowa r.
Yawowa r., 44, see Upper Iowa r.
Yeager's ldg., 366
Yellow banks, 19
Yellow Head, a chf., 158
Yellow Head r., 334
Yellow r., br. of Miss. r., 38, 41, 305, 355
Yellow r., br. of St. Croix r., 309

Yellow Skin Deer, a chf., 591
Yellow Spider, a chf., 347
Yellowstone Park, 473
Yellowstone r., 168, 479, 642, 729, 733
Yeo, Sir J., lxxiv
York harbor, lxxiv
York, killed and wounded at, xci
Yorktown, lxvi
York, U. C., xxiii, lxxvi, lxxvii, lxxviii, lxxix,
lxxx, lxxxiii, lxxxiv, xxxv, lxxxvii, xcii,
xciii, xciv, xcv, xcvii, c, ci, ciii, civ, cv,
cvi, and see Fort York
Youngar r., 376
Young, Brigham, 411
Young, Col., xcv
Youngs, Capt. White, lxxxvi, cix
Yrujo, see Cassa Yrujo
Yucatan, 718, 726
Yucca arborescens, 776
Yucca canaliculata, 776
yuccas, 776
Yucca treculeana, 776
Yuma, Col., 646
Yuma, Yuman Inds., 735, 736, 744
Yungar r., 375, 376, 513, 540
Yunque, Yuque, Yunque, see Chamita
Yuraba was a name of Taos
Yutas, 816, see Utes
Z
Zacataca, Zacatecas, administration, city
and State, 719, 720, 723, 724, 725,
755, 759, 775
Zakatagana-sibi, 103
Zandia mts., 618

Zandia pueblo, 618
Zanguananos r., 732
Zapato cr., 493
Zaragoza, 674
Zaragoza, Coahuila de, 775
Zavalza, 674
Zenia, Kas., 396
Zerbin, Dr., 698, 699, 700
Zesuqua, 605
Zia, 745
Ziamma, 745
Zibola, 742
Zizania aquatica, 38, 39
Zond, see Fond
Zoto for Les Otoes, 346
Z. R., cv
Zuloaga, Don M., 659, 660, 662, 665
Zumbro r., 56, 57, 58, 59, 61, 305
Zumi, Zuñi, Zuñian, 629, 630
Zuñi, see Old Zuñi
Zuñian mts., 730
Zuñiga y Azevedo, C. de, 725
Zuñi is a Keresan name of the Zuñis
Zuñi r., 630
Zuñis, Zuñians, 737, 742, 743, 744, 745

THE END.
MAP OF THE MISSISSIPPI RIVER FROM
ITS Source TO THE MOUTH OF THE
Missouri:
Laid down from the notes of Lieutt
Z. M.
Pike, by Anthony Nau.
Reduced, and corrected by the
Astronomical Observations of Mr
.
Thompson at its source; and of Captn
. M.
Lewis, where it receives the waters of the
Missouri.
By. Nich
s
. King.
Engraved by Francis Shallus. Philadelphia.
View larger image.

THE FIRST PART OF CAPTN.
PIKE'S CHART
OF THE INTERNAL PART OF LOUISIANA
Plate 1.
See Plate 2d. & References.
Reduced and laid down on a Scale of 40 miles to
the Inch. By Anthony Nau.
View larger image.

A CHART OF THE INTERNAL PART OF
LOUISIANA,
Including all the hitherto unexplored
Countries, lying between the River La
Platte of the Missouri on the N: and the
Red River on the S: the Mississippi East
and the Mountains of Mexico West; with a
Part of New Mexico & the Province of

Texas by Z. M. PIKE Capt.n
U.S.I.
PLATE II
View larger image.
A MAP OF THE INTERNAL PROVINCES OF
NEW SPAIN.
The Outlines are from the Sketches of but
corrected and improved by Captain
ZEBULON M. PIKE who was conducted
through that COUNTRY in the Year 1807,
by Order of the Commandant General of
those Provinces.

View larger image.
A SKETCH OF THE VICE ROYALTY
EXHIBITING THE SEVERAL PROVINCES
AND ITS APROXIMATION TO THE
INTERNAL PROVINCES OF NEW SPAIN.
Harrison sct
View larger image.

Historico=Geographical Chart OF THE Upper
Mississippi River
COMPILED AND DRAWN TO ACCOMPANY Pike's
Expeditions, UNDER THE DIRECTION OF Dr Elliott
Coues, BY Dan'l W. Cronin.
1895
Copyright, 1895 by Francis P. Harper, N. Y.
The Infant Mississippi OR Cradled Hercules.
(AFTER BROWER)
View larger image.

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