1. 1
Faculty of Engineering & Information Technology
Al-AZHAR UNIVERSITY
Programmable Logic Controllers
( ITCE 5319 )
Introduction to PLCs
(Chapter 1)
Instructor: Eng. Mohammad Aqel
Second Semester, 2007/2008
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What is a PLC?
• Is a microprocessor base controller that uses a
programmable memory to store instructions and to
implement functions such as logic, timing, counting in
order to control machines .
• Input devices, e.g. sensors such as switches, and output
devices in the system being controlled, e.g. motor, valves,
etc., are connected to the PLC.
• The operator enters a sequence of instructions e.g. ladder
logic diagram. The program then is downloaded into the
memory of the PLC. The controller monitors the inputs’
status and outputs’ status according to this program and
carries out the control rules for which it has been
programmed.
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• In a traditional industrial control system, all
control devices are wired directly to each other .
In a PLC system, however, the PLC replaces the
wiring between the devices.
• Instead of being wired directly to each other, all
equipment is wired to the PLC. Then, the control
program inside the PLC provides the "wiring"
connection between the devices.
• The control program is the computer program
stored in the PLC’s memory that tells the PLC
what’s supposed to be going on in the system.
• The use of a PLC to provide the wiring
connections between system devices is called
softwiring.
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WHY USE PLCS?
• Softwiring makes changes in the control system easy and
cheap.
• If you want a device in a PLC system to behave differently
or to control a different process element, all you have to do
is change the control program. In a traditional system,
making this type of change would involve physically
changing the wiring between the devices.
• Flexibility: software based system
• Compact: replace hundreds of relays, timers and counters
• Reliable: less mechanical parts
• reduced costs
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BUT WHAT EXACTLY IS A PLC?
A PLC basically consists of two elements:
• the central processing unit (CPU)
• the input/output (I/O) system
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The Central Processing Unit
• is the part of a programmable controller
that retrieves, decodes, stores, and
processes information.
• It also executes the control program stored
in the PLC’s memory.
• In essence, the CPU is the "brains" of a
programmable controller.
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The CPU has three parts:
• the processor
• the memory system
• the power supply
• The processor is the section of the CPU that codes,
decodes, and computes data.
• The memory system is the section of the CPU that
stores both the control program and data from the
equipment connected to the PLC.
• The power supply is the section that provides the PLC
with the voltage and current it needs to operate.
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The Input/Output System
• The input/output unit provides the interface
between the system and the outside world,
allowing for connections to be made through
input/output channels to input devices and
output devices
• Every input and output point has a unique I/O
address which can be recognized by the CPU.
• If the CPU can be thought of as the brains of a
PLC, then the I/O system can be thought of as
the arms and legs.
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A LITTLE MORE ABOUT INPUTS
AND OUTPUTS
All of the field devices connected to a PLC can be
classified in one of two categories:
• inputs • outputs
• Inputs are devices that supply a signal/data to a PLC.
Typical examples of inputs are push buttons, switches,
and measurement de-vices.
• Outputs are devices that await a signal/data from the
PLC to perform their control functions. Lights, horns,
motors, and valves
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• There are two basic types of input and output
devices:
• discrete • analog
• Discrete devices are inputs and outputs that have
only two states: on and off.
A 1 means that the device is on and a 0 means that
the device is off.
• Analog devices are inputs and outputs that can
have an infinite number of states. These devices
can not only be on and off, but they can also be
barely on, almost totally on, not quite off, etc.
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AND A LITTLE MORE ABOUT
THE CONTROL PROGRAM
• The control program is a software program in the PLC’s
memory.
• The control program is made up of things called instructions.
• Instructions are little computer codes that make the inputs and
outputs as what you want the result, such as (add and
subtract data, time and count events, compare information,
etc.).
• remember, changing the system is a snap (fast). If you want
the system to act differently, just change the instructions in the
control program.
• all PLCs use two basic types of instructions:
• contacts • coils
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Contacts & Coils
• Contacts are instructions that refer to the input
conditions to the control program—that is, to the
information supplied by the input field devices .
• The contact waits for the input to do something
in particular (e.g., turn on, turn off).
• Coils are instructions that refer to the outputs of
the control program to what each particular
output device is supposed to do in the system.
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Ladder Logic
• Ladder logic is the main programming method
used for PLCs.
• ladder logic has been developed to mimic (like)
relay logic.
• By selecting ladder logic as the main
programming method, the amount of retraining
needed for engineers and tradespeople was
greatly reduced.
• A relay is a simple device that uses a magnetic
field to control a switch, as pictured in next slide.
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normally
open
normally
closed
input coil
OR
OR
•The contact that closes when
the coil is energized is called
normally open.
•The normally closed contacts
touch when the input coil is not
energized.
•Relays are normally drawn in
schematic form using a circle to
represent the input coil. The
output contacts are shown with
two parallel lines.
•Normally open contacts are
shown as two lines, Normally
closed contacts are shown with
two lines with a diagonal line
through them.
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Example of a Relay
115V
AC
wallplug
relaylogic
input A
(normally closed)
input B
(normally open)
output C
(normally open)
ladder logic
A B C
•The first relay on the left is
used as normally closed, and
will allow current to flow until a
voltage is applied to the input A.
•The second relay is normally
open and will not allow current
to flow until a voltage is applied
to the input B.
•If current is flowing through
the first two relays then current
will flow through the coil in the
third relay, and close the switch
for output C.
•Ladder logic form, This can be
read logically as C will be on if A
is off and B is on.
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Relay Logic In PLC
ladder
power
supply
+24V
com.
inputs
outputs
push buttons
logic
PLC
AC power
115V
a c
neut.
A B C
light
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A Seal-in Circuit
• Many relays also have multiple outputs and this allows
an output relay to also be an input simultaneously.
• The input B will only be on when the output B is on. If B
is off, and A is energized, then B will turn on.
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Reading Ladder Logic
HOT NEUTRAL
INPUTS OUTPUTS
A B X
C D
E F
G
H
Y
Note:Power needs to flow through some combination of the inputs
(A,B,C,D,E,F,G,H) to turn on outputs (X,Y).
• In the figure there are two rungs, and on each rung there are
combinations of inputs (two vertical lines) and outputs (circles).
• If the inputs are opened or closed in the right combination the power
can flow from the hot rail, through the inputs, to power the outputs,
and finally to the neutral rail.
•In the top rung the contacts are normally open and normally closed.
Which means if input A is on and input B is off, then power will flow
through the output and activate it. Any other combination of input
values will result in the output X being off.
• The second rung of Figure is
more complex, there are
actually multiple combinations
of inputs that will result in the
output Y turning on.
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Mnemonics
• This is other method for
programming PLCs.
• An example of mnemonics is
shown in Figure. In this example
the instructions are read one line at
a time from top to bottom.
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Sequential Function Chart
Start
End
power up
power down
flash
Executionfollows
multiple paths
• Sequential Function Charts (SFCs)
have been developed for the
programming of more advanced
systems.
•Power up , power down , flash : is
functions .
•Each function is a small ladder logic
program.
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PLC Connections
• When a process is controlled by a PLC it uses inputs from sensors
to make decisions and update outputs to drive actuators, as shown
in Figure below.
• The control loop is a continuous cycle
of the PLC reading inputs, solving the
ladder logic, and then changing the
outputs.
•When power is turned on initially the
PLC does a quick check to ensure that
the hardware is working properly.
•If there is a problem the PLC will halt
and indicate there is an error. For
example, if the PLC backup battery is
low and power was lost, the memory
will be corrupt
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CASE STUDY
• Problem: Try to develop a ladder logic diagram
that will allow three switches in a room to control
a single light.
?
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Solution1
• Solution: There are two possible approaches to this
problem. The first assumes that any one of the switches
on will turn on the light, but all three switches must be off
for the light to be off.
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Solution2
• The second solution assumes that each switch can turn the light on or
off, regardless of the states of the other switches.
• This method is more complex and involves thinking through all of the
possible combinations of switch positions.
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Summary
• Normally open and closed contacts.
• Relays and their relationship to ladder
logic.
• PLC outputs can be inputs, as shown by
the seal in circuit.
• Programming can be done with ladder
logic, mnemonics, SFCs.
• There are multiple ways to write a PLC
program.
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ASSIGNMENT PROBLEM
• Develop a simple ladder logic program
that will turn on an output X if inputs A and
B, or input C is on.
