Introduction to Programmable
Logic Controller(PLC)
Akram Hossain, Professor,
Purdue University Calumet
Hammond, IN 46323
Definition of PLC
 Programmable Logic Controllers are solid state
devices that can be programmed to performed
sequential and discrete state operation on external
equipment
 They are designed to perform the logic functions
previously accomplished by electromechanical relays,
drum switches, mechanical and electronic timers and
counters, standalone digital PID controllers etc.
Major Manufacturer of PLC
Allen-Bradley (A-B)
General Electric
Gould-Madicon
Texas Instruments
Square-D


Reliance Electric
Siemens
Areas of PLC Applications
Annunciators
Auto Insertion
Bagging
Baking
Blending
Boring
Brewing
Calendaring
Casting
Chemical Drilling
Color Mixing
Compressors
Conveyors
Injection Molding
Assembly
Motor Winding
Oil Fields
Painting
Palletizers
Pipelines
Polishing
Reactors
Robots
Rolling
Security Systems
Stretch Wrap
Cranes
Crushing
Cutting
Digesters
Drilling
Electronic Testing
Elevators
Engine Test Stands
Extrusion
Forging
Generators
Gluing
Grinding
Heat Treating
Slitting
Sorting
Stackers
Stitching
Stack Precipitators
Threading
Tire Building
Traffic Control
Textile Machine
Turbines
Turning
Weaving
Web Handling
Welding
History of PLC
The Hydramatic Division of the General Motors Corporation specified the design criteria for the first
programmable controller in 1968. Their primary goal was to eliminate the high costs associated with
inflexible, relay-controlled systems. The specifications required a solid-state system with computer
flexibility able to
(1) survive in an industrial environment,
(2) Be easily programmed and maintained by plant engineers and technicians, and
(3) Be reusable. Such a control system would reduce machine downtime and
provide expandability for the future. Some of the initial specifications
included the following:
•
The new control system had to be price competitive with the use of relay systems.
•
The system had to be capable of sustaining an industrial environment.
•
The input and output interfaces had to be easily replaceable.
•
The controller had to be designed in modular form, so that subassemblies could be removed
easily for replacement or repair.
•
The control system needed the capability to pass data collection to a central system.
•
The system had to be reusable.
•
The method used to program the controller had to be simple, so that it could be easily understood
by plant personnel.
PLC and Computer
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A PLC and a computer both are electronic processor unit. The architecture of a PLC’s CPU is basically
same as that of a general purpose computer; however, some important characteristics set them
apart.
Unlike computer, PLCs are specifically designed to survive the harsh conditions of the industrial
environment. A well-designed PLC can be placed in an area with substantial amounts of electrical
noise, electromagnetic interference, mechanical vibration, and noncondensing humidity.
Distinction of PLCs is that their hardware and software are designed for easy use by plant
electricians and technicians. The hardware interfaces for connecting field devices are actually part
of the PLC itself and are easily connected.
The modular and self-diagnosing interface circuits are able to pin point malfunctions and moreover,
are easily removed and replaced.
Software programming uses conventional relay ladder symbols, or other easily learned languages,
which are familiar to plant personnel.
A computer can execute a complex programming task and also multitasking. An standard PLC is
designed to executes a single program in an orderly fashion. As PLCs are rapidly changing, modern
PLCs have multitasking capabilities.
Why PLCs ?
Soft Manufacturing Process
Flexible Manufacturing Process
Retrofit Existing Process
Less Maintenance
Easy to Debug
Why PLCs are so Popular?
 Programmable logic controller have made it possible to precisely control large
process machines and driven equipment with less physical wiring and wiring
time than it requires with standard electro-mechanical relays, pneumatic system,
timers, drum switches, and so on.
 The programmability allows for fast and easy changes in the relay ladder logic to
meet the changing needs of the process or driven equipment without the need
for expensive and time consuming rewiring process.
 Modem PLCs are "electrician friendly", PLC can be programmed and used by
plant engineers and maintenance electricians without much electronic and
computer programming background. They can programmed by using the
existing ladder diagrams.
Advantages of PLC
A.
B.
C.
Flexibility
 Universal Controller - can replace various independent/ standalone
controller.
Implementing Changes and Correcting Errors
 Do not have to rewiring relay panel.
 Change program using keyboard.
Large Quantity of Contact
 Large number of' Soft Contact' available.
D. Lower Cost
 Advancement in technology and open architecture of PLC will
reduce the market price.
E.
Pilot Running (Simulation Capability)
 A program can be simulated or run without actual input connection.
Advantages of PLC
F.
G.
Visual Observation.

Can observe the opening and closing of contact switch on CRT .

Operator message can be programmed for each possible malfunction.
Speed of Operation

Depends on scan time -millisecond.

Asynchronous operation.
H.
Ladder or Boolean Programming Method.

Easy for 'Electrician ,
I.
Reliability

In general -very reliable
J.
Simplicity of Ordering Control Sys. Components

One package with Relay, Timers, Control Block, etc.
Advantages of PLC
K.
Documentation

L.
Printout of ladder logic can be printed easily
Security

M.
Software lock on a program (Password)
Ease of Changes by Programming

Ability to program and reprogram, loading and down loading
Disadvantages of PLC
A.
New Technology

Change from ladder and relay to PLC concept
B.
Fixed program Application

Not cost effective for single- function application
C.
Environment Consideration

Not adapted for very high temperature, high humidity level, high vibration, etc.
D.
Fail-safe operation

Does not start automatically when power failure ( can be programmed into )

Not "Fail-safe" -Fail-shorted rather than OPEN
E.
Fixed-circuit operation

Fixed control system -less costly
What is a Ladder Diagram?
A complete control scheme normally drawn as a series of
contacts and coils arranged between two vertical control
supply lines so that the horizontal lines of contacts appear
similar to rungs of a ladder. The control contacts (input
devices) are to left and coils (output devices) on the right.
Ladder diagrams are an industrial standard for representing
relay-logic control system
PLC Series 5 Input/Output Chassis
Allen-Bradley PLC Series 5
Processor Module
PLC 5/25 Processor Mode of Operation
A-B Series 5 Power Supply
120 VAC Input Module Connection
120 VAC Output Module Connection
Two Slot Addressing
PLC Scan and Update Sequence
How PLC Works?
Input
Map
00
0
Output
Map
00
I:001
01
1
I:001
O:001
01
00
03
04
0
02
1
03
0
04
00
1
01
0
02
0
03
0
04
O:001
O:001
01
03
O:001/01
03
01
120 VAC
Input
Module
01
01
0
Ladder Logic in
Processor Memory
120 VAC
Output
Module
PLC Input & Output Mapping
M
START SW
STOP SW
RELAY COIL
M
SELF HOLDING FOR "M"
L1
CONTROL
POWER
I:
INPUT
START
STOP
N
ON LIGHT
00
00
01
01
02
02
03
03
MOTOR
RELAY
COIL
O:
OUTPUT
I:001
0
RACK
0
RACK
I:001
01
SLOT
01
SLOT
/
/
03
ADDRESS
03
ADDRESS
O:001
00
L1
04
04
MOTOR
ON LIGHT N
01
01
O:001/01
O:001
01
O:001
03
PLC Program and Data Files
Program Files
-Reserved Files (File # 0)
-SFC file (File # 1 )
-Ladder file (File #2- 999) -File #2: Main Ladder File
-Subroutine file (File #3 -999)
-Selectable Interrupt file (File #3 -999)
-Fault Routine File (File #3 -999)
Data Files
-Files which store data of the I/0 module.
-Can be integer data, floating point (real) data, timer
data, counter data, discrete input data, discrete output data, etc.
Input/Output Addressing
Memory Organization for Data Files
Memory Map of File Types
A-B PLC Series-5 Data Organization
Floating Point Files
Timer and Counter Data Files
Control Data File
PLC Logical Addressing
General Instruction Format
General Instruction Format
Status File
TIME ON DELAY (TON)