Page 309 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Input section – Input section consists of push-buttons, switches and sensors which are connected to specific ad-
dresses in the program. They transfer address information to the processing section.
Processing section – The microprocessor receives the input signal from input section and execute the information
in the software program and sends the processed signals to output section.

Output section – Takes the signal from processing section and modify the signal from the processor to operate
output devices connected to specific output addresses.
Modules of PLCs

It is built with several components that are plugged into a common rack or bus with extendable I/O capabilities.
It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which
are from same manufacturers or from other manufacturers
These modular PLCs come in different sizes with variable power supply computing capabilities, I/O connectivity etc.

PLC is essentially a microcomputer consisting of hardware and software. The major components are
1 Power Supply module
2 Input module
3 Central processing unit
4 Output modules

5 Software
1 Power supply module:
Usually input output modules require 24V DC and processor require 5V DC. Usually power supply is integral
part of PLC. Power supply units convert 120/230 V AC line voltage to standard supply 24 VDC or 5V DC using
standard rectifier circuits
2 Input module
Input devices include push buttons, sensors, potentiometers, pressure switches. The function of the input module
is to covert high voltages from input devices to low level logic voltages that the CPU uses internally for process-
ing. Input module can process both analog input and digital input. Digital inputs are more preferred in Industry.
Analog Input module :-Analog input module is used to convert analog signal form analog devices like temperature
sensors, pressure sensors etc. to digital signals using ADC (Analogto digital convertor).Analog signal is varying
voltage in the range of 0-12 V or current in the range of 5-20 mA. These values of current or voltage is converted
into integer value (say16 bit word)
3 Central processing unit

The central processing unit controls and processes all operations within the PLC and hence called brain of the
PLC. The CPU can perform various arithmetic and data manipulation function with the local and remotely located
Input/output sections. Further, the processor can perform many communication functions it needs to interface with
a personal computer, remote Input/Output, other PLCs and peripheral devices Functions of CPU are :
A It receives Input from various sensing devices and switches

B It executes the user program
C It makes various decisions to control the operation of the equipment or process
D It can perform various arithmetic and data manipulation functions
E It delivers corresponding output signals to various load control devices such as relay coils and solenoids
4 Output module
Output devices include contactor coils, solenoid coils, lamps, etc. Output module amplifies the low-Level logic signals
generated by the CPU and pass these modified signals to the final control elements to operate the output devices.
5 Software
PLC consists of two parts: Operating systems and user program. The PLC operating system provides effective

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CITS : E & H - Electronics Mechanic - Lesson 180 - 187