A PLC is a digitally operated electronic device that uses programmable memory to implement specific functions. It has several advantages including flexibility, easy error correction, cost effectiveness, and reliable operation. The PLC processor and memory are located in the CPU. The CPU contains fixed and alterable memory. The control unit manages input/output scanning and logic processing based on the user-written program. PLCs are commonly used in industrial manufacturing applications for process control.

1. Compiled by
- Shruti Bhatanagar Dasgupta

2. 
A PLC is a digitally operational electronic
equipment which uses programmable
memory of internal storage of instructions to
implement a specific function.

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Flexibility: can run many types of machines at the same
time with their own distinct programs.
Easy implementation of changed and correction of errors.
Large no. of contact coils available for programming.
Its cost effective.
It has pilot running feature enabling the pre-run and
evaluation of a program in the office or lab.
Its speed of operation can be determined through its
scan time.
The programming method is through ladder logic using
Boolean logic.
It is more reliable and easy to maintain as compared to
solid-state components
Ease of documentation of PLC Circuit.
Fail safe operation.

4. INPUTS
Multiple
Wires
C
R
C
O
E
OUTPUTS
D
E
TO
OUTPUTS
R
O
D
TO INPUTS
Connection
s to I/P
Contacts
Connectio
ns to O/P
Coils, Alar
ms,etc
CPU
PROGRAMME
R MONITOR
PRINTE
R
RECORDE
R

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Data processing and Process Control Computer
Keyboard
Light-Pen
Mouse
Data
Processing
Computer
System
Heat Sensor
Plotter
Motor
Motion Sensor
Sound Sensor
Video
Display
Terminal
Printer
Process Control
Computer
System
Solenoid
Heater

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The PLC Processor and Memory are always in
the same unit called the CPU.
The CPU fixed memory contains the program
set by the manufacturer
System program is set into special IC Chip
called Read Only Memory (ROM), which cant
be altered or erased.
The memory space that can be erased or
programmed by the user is stored in the CPR
Random Access Memory (RAM).

7. 
Operational Section of PLC CPU
Fixed Memory
- Logic
- Edit
- Monitor
- ROM
Power Supply
Alterable
Memory
- Diagram
- Numerics
- I/O Status
- RAM
Processor
- Logic
- Clock, et
c.
I/P
Peripherals
Battery Backup
O/P
Peripherals

8. ROM with fixed
operating system
Control
Unit
ALU
Control Section
Registe
rs
Analysi
s
Peripherals
- Keyboard
s
- Tape
- Print
- Disk
I/P
INPUT
SCAN
BLOCK
Logic Scan
Program
(Where User
Writes for
Process
Cntrl)
OUTPUT
SCAN
BLOCK
O/P
Interfaces
with other
CPUs etc.

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The processor is programmable, and since it can perform
many logic and arithmetic functions its also known as the
Microprocessor.
Its classified as per bit size and clock speed
S.No.
Bit Size
(bit)
Clock Speed
(M Hz)
1
8085
8
1
2
8086
16
4.77
3
80186
16
8
4
80286
16
12.5
5
80386
32
33
6
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Microprocessor
80486
32
50
The bit size determines the no. of bits of data the
microprocessor can manipulate and clock speed
determines the speed of execution of the instruction.

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The ROM with fixed operating system program
interfaces to the control section. It manages the
operation of the PLC.
The control Section, contains a Control unit with
storage registers. The Control Section determines
which operating sections are to be functional, in what
order and for how long.
The I/P Scan block, scans the I/Ps and places the I/p
statuses in the Ram Memory, when required to do so.
After analysis the Logic Scan updates the O/P Scan
Block to the Appropriate state and changes the O/P
respectively.
The O/P Scan Block then scans the O/P and updates
their statuses in the RAM.

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Used in typical industrial processes in manufacturing where the
cost of developing and maintaining the automation system is
high relative to the total cost of the automation, and where
changes to the system would be expected during its operational
life.
Used in Automotive applications.
Used in very complex process control, such as used in the
chemical industry, may require algorithms and performance
beyond the capability of even high-performance PLCs.
Also used in very high-speed or precision controls may also
require customized solutions; for example, aircraft flight
controls.
Programmable controllers are widely used in motion
control, positioning control and torque control.
Some manufacturers produce motion control units to be
integrated with PLC.
PLC with feedback is also used in temperature control units also.