PLC - Programmable Logic Controller

What is PLC?

Programmable Logic Controller (PLC) is a digital computer used for the automation of various electro-mechanical processes in industries. These controllers are specially designed to survive in harsh situations and shielded from heat, cold, dust, and moisture etc. PLC consists of a microprocessor which is programmed using the computer language.

The program is written on a computer and is downloaded to the PLC via different communication cable. These loaded programs are stored in non – volatile memory of the PLC.

Types Of PLCs :-
There are three types of programmable logic controllers
They are Unitary, Modular and Rack mounted.
All three types have different features both good and bad which make each type suitable for many different applications.
We have dedicated Blog about this topic -

Components of PLCs :-

1) Power Supply
Power Supply is to start PLC. Certain PLCs have an isolated power supply. But, most of the PLCs work at 220VAC or 24VDC.

2) 24VDC Out
24VDC out is used for doing connection of Input or Output wiring.

3) CPU

The CPU- Central Processing Unit acts as the brain of the PLC. It contains a memory unit into which the PLC program is written into.

It is basically used to process the information it receives from inputs and carries out instructions to the outputs according to the logic programmed into it.

This process is called the scan cycle and it occurs every 5ms. The scan cycle is shown in image.

When a program is written on a programming device such as a laptop it is uploaded to the CPU, once it has been uploaded it is then written into the CPU's memory. The programming device can then be disconnected without losing any data from the PLC because the program is now saved into the memory of the CPU.

4) Input and Output Devices
In PLC's data is collected from inputs which are connected to the input modules and sent to the CPU, this data is then executed within the program logic and then sent to the output devices via the output modules to perform a specific task.

In terms of PLC's there are two different types of inputs, analogue and digital. An example of an analogue input would be a sensor or transmitter and an example of a digital input would be a switch, push button etc.

=> Input will be operated with 24VDC only
=> PLC has different types of output options like -
 1) Relay Output - Supports AC/DC supply (depends upon Relays)
 2) Transistor Output - Supports only DC Voltage
 3) Triac Output - Supports only AC Voltage

5) Communication Port
Communication port is given for download/upload program to/from PLC. There are main three communication ports RS232, RS485 or Ethernet. Where Ethernet supports multiple protocol. In some PLCs USB also using for downloading/uploading process.

=> Leading Brands of PLCs in India - 
01. Allen Bradley
02. Schneider Electric
03. Siemens
04. Mitsubishi
05. ABB
06. GeFanuc
07. Delta.
08. Yakogawa
09. Honeywell
10. Omron