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PLC : An Overview

Written By: 

Rose Mary


Initially industries used relays to control the manufacturing processes. The relay control panels had to be regularly replaced, consumed lot of power and it was difficult to figure out the problems associated with it. To sort these issues, Programmable logic controller (PLC) was introduced. Graphical Representation Of PLC
Fig. 1: Graphical Representation Of PLC
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 cable. These loaded programs are stored in non – volatile memory of the PLC. During the transition of relay control panels to PLC, the hard wired relay logic was exchanged for the program fed by the user. A visual programming language known as the Ladder Logic was created to program the PLC.
PLC Hardware
The hardware components of a PLC system are CPU, Memory, Input/Output, Power supply unit, and programming device. Below is a diagram of the system overview of PLC.
An Overview of Hardware Components Of A PLC System
Fig. 2: An Overview Of Hardware Components Of A PLC System


  • CPU – Keeps checking the PLC controller to avoid errors. They perform functions including logic operations, arithmetic operations, computer interface and many more. 
  • Memory – Fixed data is used by the CPU. System (ROM) stores the data permanently for the operating system. RAM stores the information of the status of input and output devices, and the values of timers, counters and other internal devices. 
  • I/O section – Input keeps a track on field devices which includes sensors, switches. 
  • O/P Section - Output has a control over the other devices which includes motors, pumps, lights and solenoids. The I/O ports are based on Reduced Instruction Set Computer (RISC). 
  • Power supply – Certain PLCs have an isolated power supply. But, most of the PLCs work at 220VAC or 24VDC.
  • Programming device – This device is used to feed the program into the memory of the processor. The program is first fed to the programming device and later it is transmitted to the PLC’s memory. 

System Buses – Buses are the paths through which the digital signal flows internally of the PLC. The four system buses are:

·         Data bus is used by the CPU to transfer data among different elements.
·         Control bus transfers signals related to the action that are controlled internally.
·         Address bus sends the location’s addresses to access the data.
·         System bus helps the I/O port and I/O unit to communicate with each other.

Working & Application

Working of PLC (Programmable Logic Controller)
The Programmable logic controller functions in four steps.
Typical Block Diagram Of Programmable Logic Controller Functions
Fig. 3: Typical Block Diagram Of Programmable Logic Controller Functions
  • Input scan: The state of the input is scanned which is connected externally. The inputs include switches, pushbuttons, and proximity sensors, limit switches, pressure switches. Ideally, they are transformers and not relays. 
  • Program scan: The loaded program is executed to carry out the function appropriately. 
  • Output scan: The input sources have a control over the output ports to energize or de-energize them. The outputs include solenoids, valves, motors, actuator, and pumps. Depending on the model of PLC, these relays can be transistors, triacs or relays. 
  • Housekeeping
PLC Applications
The simple suitable application is a conveyor system. The requirements of the conveyor systems are as follows:
  • A programmable logic controller is used to start and stop the motors of the conveyor belt.
  • The conveyor system has three segmented conveyor belts. Each segment is run by a motor.
  • To detect the position of a plate, a proximity switch is positioned at the segment’s end.
  • The first conveyor segment is turned ON always.
  • The proximity switch in the first segment detects the plate to turn ON the second conveyor segment.
  • The third conveyor segment is turned ON when the proximity switch detects the plate at the second conveyor.
  • As the plate comes out of the detection range, the second conveyor is stopped after 20 secs.
  • When the proximity switch fails to detect the plate, the third conveyor is stopped after 20 secs.


Programmable Logic Controllers were discovered by the automotive industry to substitute the re-wiring of the machine’s control panel.
Prior to the invention of PLC, automobiles were manufactured using plenty of relays, cam timers, and closed loop controllers. The electricians had to re-wire every part of the machine daily which was time consuming and highly expensive on the financial front.
Later in the year 1968, a request for an electronic device for the hard-wired relay systems was made by GM hydramatic. Bedford Associates won the proposal and started a new company to develop, fabricate, sell, and service this new launched product. The first PLC launched was designated 084 as it was the eighty fourth projects of Bedford Associates. Dick Morley worked on this project and is being considered as the Father of PLC. In the year 1977, the brand invented by Modicon was sold to Gould Electronics. The Gould Electronics later sold it to German Company AEG which was later taken over by French Schneider Electric.
The first 084 model of PLC was revealed in North Andover, Massachusetts at the Modicon headquarters. 
The automotive industry is one of the largest users of PLC.

Advantages & Disadvantages

  • PLCs can be programmed easily which can be understood clearly well.
  • They are fabricated to survive vibrations, noise, humidity, and temperature. 
  • The controller has the input and output for interfacing.
  • It is a tedious job when replacing or bringing any changes to it. 
  • Skilful work force is required to find its errors.
  • Lot of effort is put to connect the wires.
  • The hold up time is usually indefinite when any problem arises.





nice short but sweet

thank were  thes niseeeeee 

plc rocks,..just one click,then everything's undercontrol....

I dont think So >>>>

can anyone gie weight of the plc


wow keep Up I was wondering what is a PLC from a time thankz

Pls give information about How to programming of PLC through Controller using seriel Prot....

PLC can be programmed via usb port


This is not a PLC image
Image shown above is a HMI not a PLC. 

My dought is cleard

very informative

i am kama l kishor

The picture shown over there is an HMI not PLC.

Easy to understand.


Controlled many fouction in one time

Useful content.

For more detail about PLC CLICK ON

We need more data for interview

great work 



Nice pdf sir ,so much thinks ,vv simple language is easily understood
First of all thanks to explain in proper manner. i am so great full to is very nice explanation.

very simple & easy to understand


Its a very imformative notes about PLC. We can get a overview of PLC controller. Nice job friend.and keep it continue. Post more imfo about PLC and scada as well. Well done .

Thank you for providing such a good information

i wish it have more in-depth explanation tho

i wish it have more in-depth explanation tho

This is good work
I think its I/P SECTION instead of I/O SECTION
Very nice and knowledgeable thanks

This is a good knowlage to me i wish to have the material to study on it.


Very useful

oh nice prossing its really nice proguram