6.- Chapter 2 PLC Hardware Components.ppt

Programmable
Logic
Controllers
1.1

Programmable
logic controllers
are now the
most widely used
industrial process
control technology.
A PLC is basically a digital computer
designed for use in machine control. It
has been designed to operate in the
industrial environment and is equipped
with special input/output interfaces
and a control programming language.

Initially the PLC was used to replace relay logic,
but its ever-increasing range of functions includes
timing, counting, calculating, comparing, and the
processing of analog signals.
The structure of a PLC is based on the
same principles as those employed in
computer architecture.

Programmable Logic Controller Advantages
Elimination of much of the hardwiring
associated with conventional relay
control circuits.
Relay
control
panel
PLC
control
panel

Increased Reliability - Since all the logic
is contained in the PLC’s memory, there
is no chance of making a logic wiring
error.

More Flexibility -
It is easier to
create and
change a
program in a PLC
than to wire and
rewire a circuit.

Lower Cost -
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 communicate
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. PLC
Communications
Module

Faster Response Time - 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.
Sensor
Sensor

Easier to Troubleshoot - To find and fix
problems, users can display the control
program on a monitor and watch it in real
time as it executes.
PLC
Monitor
Process

Parts of a PLC
1.2

The major parts of a PLC system 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.
A system with a closed architecture is
one whose design is proprietary,
making it more difficult to connect to
other systems.

Fixed I/O is typical of small PLCs that come in
one package with no separate, removable units.
The processor and I/O are packaged together.

Modular I/O is divided
by compartments
into which separate
modules can be
plugged. This feature
greatly increases your
options and the unit’s
flexibility.

The power supply provides DC power to all
modules that plug into the rack. For large
PLC systems, this power supply does not
normally supply power to the field devices.
Power Supply

The processor (CPU)
consists of a
microprocessor for
implementing the logic
and controlling the
communications among
the modules.
The processor requires memory for storing the results
of the logical operations performed by the
microprocessor as well as the operating system and the
PLC program.

The PLC program is executed as part of a
repetitive process referred to as a scan.
Starts with the
CPU reading the
status of all inputs.
Repeated
Continuously
Concludes
with the
updating of all
outputs.

The I/O (Input/Output) system forms the
interface by which field devices are connected to
the controller.
Input devices
such as
pushbuttons,
limit switches,
and sensors
are hardwired
to the input
module
terminals.

The I/O (Input/Output) system forms the
interface by which field devices are connected to
the controller.
Output devices
such as motor
starters, solenoid
valves, and
indicator lights
are hardwired to
the output module
terminals.

A programming device is used to enter the desired
program into the memory of the processor.
Hand-held
programming
device.
The program is entered using
the relay ladder logic
programming language.

A personal computer (PC) is the most commonly
used programming device.
The computer
monitor is able
to display more
logic on the
screen thus
simplifying
interpretation of
the program.

A program is a series of instructions that directs
the PLC to execute actions.
Relay ladder logic, the standard programming
language, is based on electromagnetic relay control.

Principles of
Operation
1.3

A mixer motor is to
be used to
automatically stir the
liquid in a vat when the
temperature and
pressure reach preset
values.
Manual operation of
the motor is provided
by means of a separate
pushbutton station.
Mixer Process Control Problem

Hardwired relay method for control of motor
The motor starter coil (M) is energized when both the
pressure and temperature switches are closed or when
the manual pushbutton is pressed.

PLC method for control of motor
The same input field devices
(pressure switch,
temperature switch, and
pushbutton) are used.
Field devices are
hardwired to an
input module.

PLC method for control of motor
The same output field
device (motor starter
coil) is used.
Starter coil is
hardwired to an
output module.

Enter the PLC ladder logic program into the
memory of the CPU.
The symbols represent instructions and the numbers
represent the instruction’s addresses.
Each input and output device is given an address,
which lets the PLC know where it is connected.

• For the program to operate, the controller is placed
in the RUN mode.
•During each scan the controller examines the status of input
devices, executes the user program, and changes outputs
accordingly.
•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
across the rung that includes the coil.

LogixPro simulation of the PLC program
Input
Module
Output
Module
Ladder Logic Program

Typical wiring required to implement the process
control scheme using a fixed PLC controller.

Modifying the
Operation
1.4

One of the important features of a PLC is the
ease with which the program can be changed.
Original hardwired process control circuit modified.
Change requires that the manual pushbutton control be
permitted to operate at any pressure, but not unless the specified
temperature setting has been reached.

If a relay system were used, it would require some rewiring
of the circuit
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.

LogicPro simulation of the modified PLC
program

PLCs versus
Computers
1.5

The architecture of a PLC is basically the
same as that of a personal computer.
Unlike PCs, the
PLC is designed
to operate in the
industrial
environment with
wide ranges of
ambient
temperature and
humidity.

A properly designed PLC installation is not as
affected 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.

Computers are complex
computing machines capable
of executing several programs
or tasks simultaneously
and in any order.
PLCs 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.

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 programming
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 conditions

Programmable
Automation
Controllers (PACs)
combine PLC
ruggedness with PC
functionality.
Using PACs, you can build advanced systems
incorporating software capabilities such as advanced
control, communication, data logging, and signal
processing with rugged hardware performing logic,
motion, process control, and vision.

PLC Size and
Application
1.6

The criteria used in categorizing PLCs include
functionality, number of inputs and outputs,
cost, and physical size

The I/O count is the most important
size 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.

There are three major types of PLC
applications: single ended, multitask, and
control management.
A single ended application involves one PLC controlling
one process

A multitask PLC application involves one PLC
controlling several processes.
Adequate I/O
capacity is a
significant factor
in this type of
installation.

A control management PLC application
involves one PLC controlling several others.
This kind of application requires a large PLC
processor designed to communicate with other PLCs
and computers
The control management PLC supervises several
PLCs by downloading programs that tell the other PLCs
what has to be done.

Memory is the part of a PLC
controller that stores data,
instructions, 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).
 Depending on memory type, 1 K can mean 1024
bits, 1024 bytes, or 1024 words.

The amount of memory
required depends on:
• 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 different types of instructions supported.

The I/O Section
2.1

The input/output (I/O) section of a PLC is the
section to which all field devices are connected
and provides the interface between them and
the CPU.
Input/output
arrangements
are built into a
fixed PLC.
Inputs
Outputs
Modular types use
external I/O
modules that plug
into the PLC.
I/O Modules

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 controller.
Output interface modules convert controller signals
into external signals used to control the machine or
process.

The hardware assembly that houses I/O modules,
processor modules, and power supplies is referred to as
the chassis.
A logical rack is an addressable unit consisting of 128
input points and 128 output points.

The ability to locate the I/O modules near the field
devices minimizes the amount of wiring required.
A rack is referred to as
a remote rack when it
is located away from
the processor module.
The remote rack is linked to the local rack
through a pair of
communications modules.

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.
 Rack/slot-based addressing schemes are used with
Allen-Bradley PLC-5 and SLC 500 controllers,
Tag-based addressing is used with Allen-Bradley
ControlLogix controllers.

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
O0:1/1- Output, file 0, rack 0, group 1, bit 1
PLC-5 rack/slot-based addressing format.

SLC 500 rack/slot-based addressing format.
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.
The bit part of
the address is
usually not used
but can be
addressed by the
programmer if
necessary.

Allen-Bradley ControlLogix controllers use a
tag-based addressing format.
Instead of a fixed numeric format, a tag
(alphanumeric name) is used to address data.
The field devices are assigned tag names when the
PLC ladder logic program is developed.

PC-based control runs on personal or industrial
hardened computers. Also known as soft PLCs,
they simulate the functions of a PLC on a PC,
This implementation uses
an input/output card in
conjunction with the PC
as an interface for the
field devices.

Combination I/O
modules can
have both input
and output
connections in
the same
physical module.

An I/O module is
made up of a printed
circuit board and a
terminal assembly.
The printed circuit
board contains
electronic circuitry.
The terminal
assembly is used
for making field-
wiring connections.
Status
Lights

Most PLC modules have plug-in wiring terminal
strips that plug into the actual module.
If there is a problem
with a module, the
entire strip is
removed, a new
module is inserted,
and the terminal
block is plugged into
the new module.

I/O modules can be 8, 16, 32, or 64 point cards.
The points refers to the number of inputs or outputs
available.
A high-density 64 point card provides a greater
space saving but with less rated current output per
output.

Discrete I/O
Modules
2.2

The discrete I/O
interface module
connects
field input
devices of the
ON/OFF nature.
The classification of
discrete I/O covers
bit oriented
inputs and outputs.

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.

The modules themselves receive their voltage
and current for proper operation from the
backplane of the rack enclosure into which they
are inserted.
Backplane power is provided by the PLC module
power supply and is used to power the electronics that
reside on the I/O module circuit board.
Currents required
by the loads are
normally provided
by user-supplied
power.

AC discrete input module block diagram.
The circuit is made up of a power section and a logic
section.
An optical isolator is used to provide electrical isolation
between the power and logic circuitry.
The input LED turns on or off, indicating the status of
the input device.

AC discrete input module schematic diagram.
When the pushbutton is closed AC 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.
When light from the LED strikes the phototransistor,
it switches it into conduction.

AC discrete output module block diagram.
The module is composed of the power section and
the logic section, coupled by an isolation circuit.
The power output interface can be thought of as
an electronic switch that turns the output load
device on and off.

AC discrete output module schematic diagram.
When the processor calls for the output load to be
energized, a voltage is applied across the LED of the
opto-isolator.
This in turn triggers the triac AC semiconductor
switch into conduction allowing current to flow to the
output load.

Individual AC outputs are usually limited by
the size of the triac to 1 A or 2 A.
For controlling larger
loads an interposing
relay is connected to
the output module.
The contacts of the
relay can then be used
to control a larger load
or motor starter,

Discrete output modules are used to turn field
output devices either on or off.
Output modules can be purchased with transistor,
triac, or relay output

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.

If the module is a current-sinking module,
then the input or output device must be a
current-sourcing device.

Analog I/O
Modules
2.3

Analog input
and output
devices 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.
The analog input interface module accepts an
analog signal and converts it to a digital signal.
The analog output module accepts a digital signal and
converts it to an analog signal that operates the output.

The two basic types of analog input modules are
voltage sensing and current sensing.
A varying DC voltage in the low millivolt range,
proportional to the temperatures being monitored,
is produced by the thermocouples.

The main element of the analog input module is
an analog-to-digital (A/D) converter
 Unipolar input modules can accept an input signal
that varies in the positive direction only.
 Bipolar input modules can accept signals that swing
between a negative and positive value.
 Resolution refers to the smallest change in input
signal value that can be sensed

Unlike voltage input signals, current signals are
not as sensitive to noise and typically are not
distance limited.
The current sensing loop power may be supplied by
the sensor or the analog output module.
Shielded twisted pair cable is normally recommended
for connecting any type analog input signal.

The main element of the analog
output module is an
digital-to-analog (D/A) converter
The analog output module receives from the
processor digital data, which are converted into
a proportional voltage or current to control an
analog field device.
The analog output signal is varied
under the control of the PLC
program and can be used for
control of an analog control valve.

Analog I/O control system
The PLC controls the
amount of fluid placed
in a holding tank by
adjusting the
percentage of the valve
opening.
The valve is initially opened 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.

Special I/O
Modules
2.4

High-speed counter modules are used to count
pulses from sensors, encoders, and switches
that operate at very high speeds.
A typical count rate available is 0 to 100 kHz, which
means the module would be able to count 100,000 pulses
per second.
They have the
electronics needed to
count independently of
the processor.

The thumbwheel module allows the use of
thumbwheel switches for feeding
information to the PLC to be used in the
control program.

The TTL module allows the
transmitting and receiving of
TTL (Transistor-Transistor-
Logic) signals.
This module allows devices that
produce TTL-level signals to
communicate with the PLC’s
processor

An encoder-counter
module allows the
user to read the
signal from an
encoder on a
real-time basis
and stores this
information so it can
be read later by the
processor.

The BASIC or ASCII module runs user
written BASIC and C programs.
Typical applications include
interfaces to bar code readers,
robots, printers, and displays.

The stepper-motor module provides pulse trains
to a stepper-motor translator, which enables
control of a stepper motor.
The commands for the module are determined by the
control program in the PLC.

The BCD-output module enables a PLC to
operate devices that require BCD-coded signals
such as seven-segment displays

The proportional-integral-derivative (PID) module
is used in process control applications that
incorporate PID algorithms.
This arrangement prevents the
CPU from being burdened with
complex calculations.

Motion and position control modules are used in
applications involving accurate high-speed
machining and packaging operations.
Intelligent position
and motion
control modules
permit PLCs to
control stepper
and servo motors.

Serial communications modules
are used to establish point-to-
point connections with other
intelligent devices for the
exchange of data.
Communication
modules provide for
connection to PLC
networks.

I/O
Specifications
2.5

Discrete I/O Module Specifications
Nominal Input Voltage - 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 Threshold Voltages - Specifies the minimum
ON-state voltage at which logic 1 is recognized and
the maximum OFF-state voltage at which logic 0 is
recognized.
Nominal Current Per Input - Specifies the minimum
input current that the discrete input devices must be
capable of driving to operate the input circuit.

Ambient Temperature Rating - Specifies what the
maximum temperature of the air surrounding the I/O
modules should be for best operating conditions.
Input ON/OFF Delay (response time) - 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).
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.

Output Current - Specifies the maximum current that a
single output and the module as a whole can safely carry
under load (at rated voltage).
Inrush Current - 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 - Specifies whether the
particular output 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.
Points Per Module - This specification defines the
number of field inputs or outputs that can be
connected to a single module.
Electrical Isolation - The specification for
electrical isolation, typically 1500 or 2500
volts AC, rates the module’s capacity for
sustaining an excessive voltage at its input
or output terminals.

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.
Backplane
Power
Supply

Analog I/O Module Specifications
Channels Per Module - Whereas individual circuits on
discrete I/O modules are specified as points per module,
circuits on analog I/O modules are specified as channels
per module.
Input Current/Voltage Range(s) - These are the voltage
or current signal ranges that an analog input module is
designed to accept.
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.

Input Protection - Analog input circuits are usually
protected against accidentally connecting a voltage that
exceeds the specified input voltage range.
Resolution - The resolution specifies how accurately
an analog value can be represented digitally. This will
determine the smallest measurable unit of current or
voltage change that can be detected.
Input Impedance and Capacitance - For analog I/Os,
these values must be matched to the external
device connected to the module. Typical ratings are
in the Megohm and Picofarad range.

Common-Mode Rejection - Noise is generally caused by
electromagnetic interference, radio frequency
interference, and ground loops.
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.

2.6
The Central
Processing
Unit (CPU)

The central processing unit (CPU) is built into
fixed PLCs while modular 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.

A processor module can be divided into two
sections: the CPU section and the memory section
The CPU
executes the
program.
The memory
stores the
program along
with other
retrievable
data.

The PLC power supply provides the necessary
power to the processor and I/O modules plugged
into the backplane of the rack.
The power supply
converts the AC input
voltage into the usable
DC voltage required by
the CPU, memory, and
I/O electronic circuitry.

The CPU contains the similar type of
microprocessor found in a personal computer.
A PLC microprocessor is designed to facilitate
industrial control rather than provide general
purpose computing.

The CPU of a PLC system may contain more
than one processor.
Fault-tolerant PLC systems support dual processors for
critical processes. These systems allow the user to
configure the system with redundant (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 information to the operator.

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.
When not in use, store
modules in a static-shield
bag.

2.7
Memory Design

Memory is the element that stores information,
programs, and data in a PLC.
The complexity of the program determines the amount
of memory required.

Memory location refers to an address in the
CPU’s memory where a binary word can be
stored. Each binary piece of data is a bit
and eight bits make up one byte.
The program is stored in the memory as 1s and 0s, which
are typically assembled in the form of 16-bit words.

Sections of memory used to store the status of
inputs are called input status files or tables.

Input Table Simulation

Sections of memory used to store the status of
outputs are called output status files or tables.

Output Table Simulation

2.8
Memory Types

Memory types can be placed into two general
categories: volatile and nonvolatile.
Volatile memory will lose its stored information if all
operating power is lost or removed.
Nonvolatile memory has the ability to retain stored
information when power is removed accidentally or
intentionally.
PLCs 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.

Nonvolatile Read Only Memory (ROM) stores
programs, and data that cannot be changed
after the memory chip has been manufactured.
ROM is used by the PLC
for the operating system
and controls the system
software that the user uses
to program the PLC.

Random Access Memory (RAM) is designed so
that information can be written into or read
from the memory.
PLCs use RAM as a temporary storage area of data
that may need to be quickly changed.
RAM is volatile so
battery backup is
required for it to avoid
losing data in the event
of a power loss

Erasable Programmable Read-Only Memory
(EPROM) provides some level of security
against unauthorized 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. UV EPROM memory can
only be erased with an
ultraviolet light.

Electrically erasable programmable read-only
memory (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.

Flash EEPROMs are similar to EEPROMs in
that they can only be used for backup storage.
Flash memory is
extremely fast at
saving and
retrieving files.
Flash memory is also sometimes built into the
processor module, where it automatically
backs up parts of RAM.

2.9
Programming
Terminal Devices

A programming terminal device is needed to enter,
modify, and troubleshoot the PLC program.
The hand-held proprietary
programming terminal has
a connecting cable so that it
can be plugged into a PLC’s
programming port.
Hand-held programmers are compact and inexpensive
but have limited display capabilities.

The most popular method of PLC programming
is to use a personal computer in conjunction with
the manufacturer’s programming software.

2.10
Recording and
Retrieving Data

Printers are used to provide hard-copy printouts
of the processor’s memory in ladder program
format.
A printout
can show
programs of
any length
and analyze
the complete
program.

The program in the PLC is entered directly
from the keyboard or downloaded from the
computer hard drive or thumb drive.

Some CPUs support the
use of a memory
cartridge that provides
portable EEPROM
storage for the user
program.
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, selector switches, pilot
lights, thumbwheels, and other operator control
panel devices

Human machine
interfaces allow you to
view the operation in real
time.
You can configure display screens to:
• Replace hardwired pushbuttons and pilot lights
with realistic-looking icons.
• Allow the operator to change timer and counter
presets.
• Show alarms, complete with time of occurrence.

6.- Chapter 2 PLC Hardware Components.ppt

Processor Memory
Organization
5.1

The memory structure for a PLC processor
consists of several areas, some of these having
specific roles.
With tag-based memory structures all data are assigned
a variable name called a tag. A program can be
developed using only tag names but you must assign
input and output terminals to input and output tags
before the program can be executed
With rack-based memory structures
addresses are derived using the rack
number, the I/O module slot number
and the screw terminal number where
the I/O device is wired.

The memory space can be divided into two
broad categories: program files and Data files.
Program and
Data file
organization for
the SLC 500
controller.

Program files are the areas of processor memory
where ladder logic programming is stored.
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.

The Data file portion of the processor’s memory
stores input and output status, processor status,
the status of various bits, and numerical data.
These files are
organized by
the type of data
they contain.

I/O address format for the SLC family of PLCs.

Simulated I/O addressing format for the SLC
family of PLCs.

Memory
organization
for a PLC-5
controller.

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.
The addresses in the output data file and the input
data file are potential locations for I/O modules
mounted in the chassis.

The status data file
contains information about
the processor status
The bit data file stores
bit status and frequently
serves for storage when
using internal outputs.

The timer file stores
the timer status and
timer data.
The counter file
stores the counter
status and counter
data.

The integer file
stores integer
data values.
The floating point file
can store data that
requires a decimal
point.

The input image table
is allocated to storing
the on/off status of
connected discrete
inputs.
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.

Simulated operation
of the input image
table on/off status of
connected discrete
inputs.

The output image table is
allocated to storing the
on/off status of connected
discrete outputs.
If the program calls for
an output to be ON, its
corresponding bit in the
table is set to 1.
If the program calls for
an output to be OFF, its
corresponding bit in the
table is set to 0.

Simulated operation
of the output image
table on/off status of
connected discrete
outputs.

Typically, micro PLCs have a fixed number of
inputs and outputs.
This controller has 20 discrete inputs with addresses I/0 through
I/19 and 12 discrete outputs with addresses O/1 through O/11.

During each program scan cycle, the processor
reads all the inputs, takes these values, and
energizes or de-energizes the outputs according to
the user program.

The time it takes to complete a scan cycle is a
measure of how fast the controller can react to
changes in inputs.
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.

The scan time is a function of:
• The speed of the processor module
• The length of the ladder program
• The type of instructions executed
• The actual ladder true/false conditions
The PLC computes the scan time each
time the END instruction is executed.
Typical scan time data include the
maximum scan time and the last scan time.

Overview of the data flow during the scan process.

Scan process applied to a single rung program.

Scan process
applied to a
multiple rung
program.

Vertical versus horizontal scan patterns.

PLC programming language refers to the method
by which the user communicates information to
the PLC.
Standard PLC programming languages

Ladder diagram language is the most commonly
used PLC language and is designed to mimic
hardwired relay logic.
Hardwired relay
control circuit
Equivalent ladder
diagram program

Instruction list programming language consists
of a series of instructions that refer to the basic
AND, OR, and NOT logic gate functions.
Hardwired relay
control circuit
Equivalent instruction
list program

Functional block diagram programming uses
instructions that are programmed as blocks wired
together to accomplish certain functions.

Ladder diagram and functional block diagram
programming used to produce the same logical
output.
Ladder diagram
Equivalent
function block
diagram.

Sequential
function chart
programming
language is
similar to a
flowchart of
your process.
The program is split into
steps with multiple
operations happening in
parallel branches.

Structured text is a high level language primarily
used to implement more complex procedures.
Ladder diagram
Equivalent
structured text
program.

The ladder diagram language is a symbolic set
of instructions used to create the controller
program.
Representations of contacts and coils are the basic
symbols of the logic ladder diagram instruction set.

The Examine If Closed (XIC) instruction looks
and operates 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.

The memory bit is set to 1 or 0 depending on the
status of the input.
A 1 corresponds to a true status or on condition.
If the instruction memory bit is a 1 (true) this
instruction will allow rung continuity through itself,
like a closed relay contact.

A 0 corresponds to a false status or off condition.
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.

Simulated Examine If Closed (XIC) instruction
operation.

The Examine If Open (XIO) instruction looks
and operates like a normally closed relay
contact.
This instruction asks the PLC’s processor to examine
if the contact is open.
It does this by examining the bit at the memory
location specified by the address for a 0 or 1.

As with any other input the memory bit is set to 1 or 0
depending on the status of the input. A 1 corresponds to
a true status or on condition.
The instruction is interpreted as false when the bit is
1 and will not allow rung continuity through itself.

A 0 corresponds to a off condition.
The instruction is interpreted as true when the bit is
0 and will not allow rung continuity through itself.

Simulated Examine If Open (XIO) instruction
operation.

The Output Energize (OTE) instruction looks and
operates like a relay coil.
This instruction signals the PLC to energize
(switch on) or de-energize (switch off ) the output.
The instruction is associated with a memory bit that
energizes the output when set to 1 and de-energizes
the output when reset to 0.

A true logic
path is
established by
the input
instructions in
the rung.
OTE instruction
is set to 1 to
energize the
output.

Simulated Output Energize (OTE) instruction
operation.

Action of the field device and PLC bit.
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.

Simulated operation of the field input device and
the PLC bit.

The main function of the ladder logic diagram
program is to control outputs based on input
conditions.
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.

For an output to be activated or energized, at
least one left-to-right true logical path must exist.
A complete closed
path is referred to
as having logical
continuity.
When logical continuity exists in at least one path, the
rung condition and Output Energize instruction are
said to be true.

Simulated operation of logic continuity.

To complete the entry of a relay-type instruction,
you must assign an address to each instruction.
Address indicates
what input is
connected to what
input device
Address indicates
what output is
connected to what
output device

Simulated operation of instruction addressing.

The assignment of an I/O address can be
included in the I/O connection diagram.
Inputs and outputs are typically represented by
squares and diamonds, respectively.

Branch instructions are used to create parallel
paths of input condition instructions.
The rung will be
true if either
instruction
A or B is true.

Simulated branch instructions.

Parallel branches can be used to allow more than
one combination of input conditions.
Either A and not B, or C provides logical
continuity and energizes output D.

Simulated program, either A and not B, or C
provides logical continuity and energizes
output D.

Output branching allows a true logic path to
control multiple outputs
Either A or B provides a
true logical path to all
three output instructions:
C, D, and E.
Additional input
instructions can be
programmed in the
output branches.

Simulated program, either A or B provides a
true logical path to all three output instructions:
C, D, and E.

Input and output branches can be nested to avoid
redundant instructions and to speed up processor
scan time.
A nested branch starts or ends within another branch.

In some PLC models, the
programming of a nested
branch cannot be done
directly.
It is possible, however,
to program a logically
equivalent branching
condition.

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.

The PLC will not allow
for programming of
vertical contacts.
Reprogrammed to eliminate
vertical contact.

The processor examines the ladder logic rung for logic
continuity from left to right.
If programmed as shown,
contact combination
FDBC would be ignored.
Reprogrammed circuit.

Internal Relay
Instructions
5.7

An internal output does not directly control an
output field device.
The advantage of using internal outputs is that there
are many situations in which an output instruction is
required in a program but no physical connection to a
field device is needed.

SLC 500 controllers use bit file B3 for internal bit
addressing.

Internal relay used for a program that requires
more series contacts than the rung allows.
This PLC allows for only 7 series contacts when 12
are actually required for the programmed logic.

Simulated internal relay program.

Programming Examine
If Closed and Examine
If Open Instructions
5.8

Examine If Closed (XIC) instruction
Both the NO and the NC pushbuttons are represented
in the program by the Examine If Closed instruction.
The normal state of the field input device (NO
or NC) does not matter to the controller.
What matters is that contacts need to closed to energize
the output.

Simulated Examine If Closed (XIC) instruction

Examine If Open (XIO) instruction
The pushbutton is represented in the user program 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.

Simulated Examine If Open (XIO) instruction

The logic states (0 or 1) indicate whether an instruction
is true or false and is the basis of controller operation.

The time aspect relates to the repeated scans of the
program, wherein the input table is updated with the
most current status bits.

Entering The Ladder
Diagram
5.9

Allen-Bradley’s RSLogix software
packages are windows
programming packages used to
develop ladder logic programs.
Software, in various versions,
can be used to program the
PLC-5, SLC 500,
ControlLogix, and MicroLogic
family of processors.
A personal computer is most often used and is adapted
to the particular PLC model through the use of the
relevant programmable controller software.

Instruction toolbar with bit instructions selected.
To place an instruction on a rung, click its icon on
the toolbar and simply drag the instruction straight
off the toolbar onto the rung of the ladder.

The programming software needs to know what
processor is being used in conjunction with the
user program.
The Select Processor Type screen contains a list of the
different processors that the RSLogix software can
program.

The I/O Configuration screen lets you double
click or drag-and-drop a module from an all-
inclusive list to assign it to a slot in your
configuration.

Data File screens contain data that are used in
conjunction with ladder program instructions
and include input and output files as well as
timer, counter, integer, and bit files.
The bit file B3 is used for internal relays.

Relay ladder logic is a graphical programming
language designed to closely represent the
appearance of a wired relay system.
The logic is apparent
from the
highlighting which
identifies the logic
state of contacts in
real time and which
rungs have logic
continuity.

A processor has basically two modes of operation:
the program mode and some variation of the run
mode.
A three-position keyswitch may be used to select
different processor modes of operation.

The program mode is used to enter
a new program, edit or update an
existing program, upload files and
download files.
The run mode is used to
execute the user program.
The test mode is used to operate
or monitor the user program
without energizing any outputs.
The remote position allows the PLC to be remotely
changed between program and run mode by a
personal computer connected to the PLC processor.

6.- Chapter 2 PLC Hardware Components.ppt

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