This document discusses programmable logic controllers (PLCs). It provides a brief history of PLCs, describing their origins in the 1960s as replacements for relay logic systems. The document outlines the typical components of a PLC system, including the processor, memory unit, power supply, input/output modules, and programming device. It also describes ladder logic, a common programming language for PLCs using a logic diagram format. Finally, some applications of PLCs in industries like manufacturing and materials handling are listed.

1. PROGRAMMING LOGIC
CONTROLLERS (PLC)
By :-
Manish Gharte
(Symbiosis Insti. Tech.)

2. PROGRAMMABLE LOGIC CONTROLLER
 A digital electronic device that uses a
programmable memory to store instructions and to
implement specific functions such as logic,
sequence, timing, On/Off control, counting and
arithmetic to control machines and process. “
 PLCs are equipped with special input/output
interfaces .

3. HISTORY OF PLC
 Introduced in the late 1960’s.
 Developed to offer the same functionality as the existing
relay logic systems.
 Programmable
 They had no hard drive, they had battery backup.
 Could Start in Seconds.
 Used ladder logic for programming.

4. PLC SYSTEM

5. COMPONENTS OF PLC
1.Processor.
2.Memory unit .
3.Power supply.
4.I/O Modules.
5.Programming device.

6.  Processor :-
Processor is the central processing unit
(CPU) of the programmable controller, it executes the
various logic and sequencing functions by operating on
the PLC inputs to determine the appropriate output
signals.
Memory unit:-
Memory unit is connected to the CPU,
which contains the programs of logic, sequencing and
I/O operations .

7. Power supply :-
A power supply of 120v alternating
current(ac) is typically used to drive the PLC (some
units operate on 240 v ac).
 The input/output module :-
Inputs to the controller are signals
from limit-switches , push-buttons, sensors , and other
on/off devices.
Outputs from the controller are
on/off signals to operate motors, valves, and other
devices required to actuate the process.

8. A SIMPLE OVERVIEW OF CONTROL SYSTEM
C
O
N
T
R
O
L
S
Y
S
T
E
M
P
L
A
N
T
INFORMATION
COMMANDS
SENSORS
ACTUATORS
8

9.  PROGRAMMING OF PLC BY USING
LADDER LOGIC :-
 Ladder logic is the oldest programming language for
PLC.
 It is well suited to express combination logic.

10. PROGRAMMING
Normally Open
(NO)
Normally Closed
(NC)
Power flows through these contacts when they are closed. The
normally open (NO) is true when the input or output status bit
controlling the contact is 1. The normally closed (NC) is true
when the input or output status bit controlling the contact is 0.

11.  Ladder logic for Basic Gate :-
 AND OPERATION
Rung
A B C
Each rung or network on a ladder program represents
a logic operation. In the rung above, both inputs A and B
must be true (1) in order for the output C to be true (1).

12. 12
 NOT OPERATION
Rung
A C
In the rung above, it can be seen that if input A is be true (1),
then the output C is true (0) or when A is (0), output C is 1.

13. PROGRAMMING EXAMPLE:
 Ladder Logic Program for Start/Stop of Motor :
A1
Y
Y
A2
C

14.  Selecting a PLC Criteria :-
• Number of logical inputs and outputs.
• Memory
• Number of special I/O modules
• Scan Time
• Communications
• Software

15.  Leading Brands Of PLC:-
 Allen Bradley
 Texas Instruments
 Siemens
 Honeywell
 Toshiba
 General Electric

16. 16
 PLC Size :-
1. SMALL - it covers units with up to 128 I/O’s and
memories up to 2 Kbytes.
2. MEDIUM - have up to 2048 I/O’s and memories up
to 32 Kbytes.
3. LARGE - They have up to 8192 I/O’s and
memories up to 750 Kbytes.

17.  ADVANTAGES OF PLCS:
 Reliability.
 Flexibility in programming and reprogramming.
 Cost effective for controlling complex systems.
 Small physical size, shorter project time.
 High speed of operation.
 Ability to communicate with computer systems in
the plant.
 Ease of maintenance /troubleshooting.
 Reduced space.
 Energy saving.

18.  APPLICATIONS:
 Wherever automation is desired the PLCs are best
suited to meet the task.
 Few examples of industries where PLCs are used :
1) Robots manufacturing and control
2) Car park control
3) Train control station system
4) Food processing
5) Materials handling
6)Machine tools
7)Conveyer system etc.