Overview of PLC

Basic of plc programmable logic control 



Logic Controllers

Third Edition

Frank D. Petruzella



Chapter 1


Controllers (PLCs)
An Overview


Programmable Logic Controller

A programmable logic controller (PLC) is a
specialized computer used to control
machines and process.

It uses a programmable memory to store instructions
and execute specific functions that include
On/Off control, timing, counting, sequencing,
arithmetic, and data handling.


Advantages of a PLC Control System

Eliminates much of the hard wiring that
was associated with conventional relay
control circuits.

The program takes
the place of much of
the external wiring
that would be required
for control of a process.


Advantages of a PLC Control System

Increased Reliability:
Once a program has been written and tested
it can be downloaded to other PLCs.

Since all the
logic is contained
in the PLC’s
memory, there
is no chance
of making a
logic wiring


Advantages of a PLC Control System

More Flexibility:
Original equipment manufacturers (OEMs) can
provide system updates for a process by simply
sending out a new program.

It is easier to
create and change
a program in a PLC
than to wire and
rewire a circuit.
End-users can modify
the program in the


Advantages of a PLC Control System

Lower Costs:
Originally PLCs were designed to replace relay
control logic. The cost savings using PLCs have
been so significant that relay control is becoming
obsolete, except for power applications.

Generally, if an application
requires more than about
6 control relays, it will
usually be less expensive
to install a PLC.


Advantages of a PLC Control System

Communications Capability:
A PLC can communicate with other controllers
or computer equipment.

They can be networked
to perform such functions
as: supervisory control, data
gathering, monitoring
devices and process
parameters, and downloading
and uploading of programs.


Advantages of a PLC Control System

Faster Response Time:
PLCs operate in real-time which means that an
event taking place in the field will result in an
operation or output taking place.

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.


Advantages of a PLC Control System

Easier To Troubleshoot:
PLCs have resident diagnostic and override
functions that allows users to easily trace and
correct software and hardware problems.

The control program
can be watched in
real-time as it
executes to find
and fix problems


PLC Architecture

Refers to the PLC hardware or software.


PLC Architecture

• An open architecture design allows the system to be
connected easily to devices and programs made by
other manufacturers.

• A closed architecture or proprietary system, is one
whose design makes it more difficult to connect devices
and programs made by other manufacturers.

NOTE: When working with PLC systems that are
proprietary in nature you must be sure that any generic
hardware or software you use is compatible with your
particular PLC.


I/O Configurations

Fixed I/O

• Is typical of small PLCs

• Comes in one package, with
no separate removable units.

• The processor and I/O are
packaged together.

• Lower in cost – but
lacks flexibility.


I/O Configurations

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. You can choose
from all the modules available
and mix them in any way you


I/O Configurations

Modular I/O

When a module slides into the rack,
it makes an electrical connection
with a series of contacts – called the
backplane. The backplane is located
at the rear of the rack.


PLC System


Power Supply

Supplies DC power to other
modules that plug into the rack.

In large PLC systems, this power supply does not
normally supply power to the field devices.

In small and micro PLC systems, the power supply
is also used to power field devices.


Processor (CPU)

Is the “brain” of the PLC.

Consists of a microprocessor
for implementing the logic, and
controlling the communications
among the modules.

Designed so the desired circuit can be
entered in relay ladder logic form. Processor


The processor accepts input data from
various sensing devices, executes the
stored user program, and sends
appropriate output commands
to control devices.


I/O Section

Consists of input modules
and output modules.


I/O Section

Input Module

Forms the interface
by which input field
devices are connected
to the controller.

The terms “field” and “real world”are used to distinguish
actual external devices that exist and must be physically
wired into the system.


I/O Section

Output Module

Forms the interface
by which output field
devices are connected
to the controller.

PLCs employ an
optical isolator which
uses light to
electrically isolate the
internal components
from the input and
output terminals.


Programming Device

PC with appropriate

The computer monitor is used to display the logic on
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Programming Device

Hand-held unit
with display

Hand-held programming devices are sometimes used to
They are compact, inexpensive, and easy to use, but

pHrogram small PLCs.
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programs to multiple machines.


1. A programmable logic controller (PLC) is a
specialized ___________ used to control
machines and process.

a. relay

b. computer

c. disk

d. monitor


2. A PLC system eliminates much of the hard
wiring that was associated with conventional
relay control circuits. (True/False)

3. PLC control systems are more reliable and
easier to troubleshoot than relay based control

4. A PLC operates in real-time. (True/False)

5. A proprietary PLC system allows the system to
be connected easily to devices made by other
manufacturers. (True/False)


6. The fixed I/O module configuration:

a. is typical of small PLCs units

b. has no separate removable units

c. is limited as to the availability of modules

d. all of these


7. In which module of the PLC is the
decision making performed?

a. processor module

b. input module

c. output module

d. power supply module


8. The input interface module:

(a) conditions the signal received from a
field device.

(b) allows the programmer to input the program.

(c) allows the CPU to input messages to a CRT

(d) provides inputs to motor controllers and
similar field devices.


9. The output interface module connects to:

(a) sensing devices such as switches or

(b) load devices such as lamps or solenoids.

(c) the programming device that controls the
machine process.

(d) the tape or disc drive circuits


10. The programming device must be
connected to the the controller:

(a) at all times.

(b) when entering a program.

(c) when monitoring a program.

(d) both b and c.


PLC Mixer Process Control Problem

Mixer motor to automatically
stir the liquid in the vat
when the temperature and
pressure reach preset values.

Alternate manual
pushbutton control
of the motor to be

The temperature and
pressure sensor switches
close their respective contacts when conditions reach
their preset values.


Process Control Relay Ladder Diagram

Motor starter coil is energized when both the pressure
and temperature switches are closed or when the manual
pushbutton is pressed.


PLC Input Module Connections

The same input field
devices are used.

These devices are wired
to the input module
according to the
manufacturer’s labeling


PLC Output Module Connections

Same output field
device is used and

wired to the output

Triac switches motor
ON and OFF in accordance
with the control signal from
the processor.


PLC Ladder Logic Program

The numbers represent addresses

I/O address format will differ, depending on
The symbols represent instructions

the PLC manufacturer. You give each input and
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Entering And Running The PLC Program

To enter the program into the PLC, place
the processor in the PROGRAM mode and
enter the instructions one-by-one using
the programming device.

To operate the program, the controller
is placed in the RUN mode, or
operating cycle.


PLC Operating Cycle

During each operating cycle, the controller examines the
status of input devices, executes the user program, and
changes outputs accordingly.

The completion of one cycle
of this sequence is called
a scan. The scan time, the
time required for one full
cycle, provides a measure
of the speed of response
of the PLC.


PLC Operating Cycle

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Modifying A PLC Program

Relay ladder diagram for
modified process.

The change requires that the manual pushbutton
If a relay system were used, it would require some
control should be permitted to operate at any
rewiring of the system, as shown, to achieve the
pressure but not unless the specified temperature
desired change.
setting has been reached.


Modifying A PLC Program

PLC ladder logic diagram for

modified process.

If a PLC is used, no rewiring is necessary!

The inputs and outputs are still the same.

All that is required is to change the PLC program


PLCs Versus Personal Computers

Same basic architecture

-operates in the industrial -capable of executing several
environment programs simultaneously, in
-is programmed in relay any order
ladder logic -some manufacturers have
-has no keyboard, CD drive, software and interface cards
monitor, or disk drive available so that a PC can do
-has communications ports, the work of a PLC
and terminals for input and
output devices


PC Based Control Systems

– lower initial cost
– less proprietary hardware

and software required
– straightforward data exchange

with other systems
– speedy information processing
– easy customization


PLC Size Classification

– number of inputs and outputs (I/O count)
– cost
– physical size

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– handles up to 16 I/O points


PLC Instruction Set

The instruction set for a particular PLC type lists the
different types of instructions supported.

An instruction is a command that will cause a PLC
to perform a certain predetermined operation.


Typical PLC Instructions

XIC (Examine ON) . . . . . . . Examine a bit for an ON condition
XIO (Examine OFF) . . . . . . Examine a bit for an OFF condition
OTE (Output Energize) . . . . Turn ON a bit (non retentive)
OTL (Output Latch) . . . . . Latch a bit (retentive)
OTU (Output Unlatch) . . . Unlatch a bit (retentive)
TOF (Timer Off-Delay) . . . . Turn an output ON or OFF after its

rung has been OFF a preset time

TON (Timer On-Delay) . . . Turn an output ON or OFF after its
rung has been ON for a preset time

CTD (Count Down) . . . . . . Use a software counter to count down
from a specified value

CTU (Count Up) . . . . . . . . Use a software counter to count up
to a specified value


11. The control plan stored in the PLC is called:

(a) a program

(b) a Boolean ladder


(d) a microprocessor


12. This symbol in a ladder logic diagram:

(a) can be thought of as a normally closed contact.

(b) can be thought of as a normally open contact.

(c) represents a capacitor.

(d) is always at logic 1.


13. This symbol in a ladder logic
diagram represents a(n):

(a) set of normally open contacts.

(b) imaginary coil that, when energized can close
an output interface contact.

(c) contact that will seal itself in once it closes

(d) field load or sensing device


14. Any change to a PLC program normally
involves some change in wiring as well.

15. To operate the program, the controller
is placed in the PROGRAM mode. (True/False)

16. An instruction is a command that will cause a
PLC to perform a certain predetermined operation.


17. During each operating cycle, the controller
examines the status of output devices, executes the
user program, and changes inputs accordingly.

18. A PLC has no keyboard, CD drive, monitor,
or disk drive. (True/False)

19. PC based control systems require the use of
interface cards. (True/False)


20. Which of the following is not a criteria
for classification of PLCs?

(a) number of inputs and outputs

(b) cost

(c) physical size

(d) manufacturer

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