What is PLC (Programmable Logic Controller) and why we need PLC?

Updated: Feb 3

In this article, we will learn Programmable Logic Controller or in short, you can say it a PLC which is widely used in process industries. We will also discuss the needs of PLC in the process industries.

We will also learn advantages of PLC over relay, what is a need of PLC, what isPLC, Types of PLC programming language, PLC applications and standards. In short everything about PLC.

PLC or you can say it Programmable logic controller is a small yet powerful computer used to control field devices and thus controlling the processes.

To control a field devices means that in PLC it contains a set point which is compared with process variable (PV) and produced an error in order to keep processes at a set point.

Here, I am going to start an article on PLC and on another automation system like DCS, HMI, SCADA and also will cover a topic industrial network, too.

First, we will start with a very basic and important topic on the PLC.

In this article, we will learn what is PLC and why we use PLC?

Let start with a history which provides you with a knowledge that how and when PLC was introduced.

It was in 1968 when Dick Morley known as a father of the PLC introduced PLC which has replaced a complex relay logic system. Name of that PLC was Modicon (Modular Digital Controller).

The Modicon brand was sold to Gould Electronics and later by famously know the brand of today Schneider Electric.

Earlier PLC was used only for Digital devices but today it changes a lot and can use with Analog devices too. Nowadays you can hardly find differences between PLC and DCS.

Before going through PLC let's distinguish the difference between PLC and relay.


  • Relays are versatile and reliable used to control and any functions but due to continuous operation of contacts, wear and tear happens and also it heats a lot whereas in the PLC all the above mention things won't happen.

  • When any problem occurs it harder to work with relay and its time-consuming process whereas in the PLC very easy to identify a problem.

  • Require more number of wiring which make troubleshooting even harder whereas in the PLC require less wiring reduces complexity and make troubleshooting easier.

  • Relay logic system contains timer if the process requires timing which needs a timer to put in the slot and need a lot of wiring whereas in the PLC you just need software and you can easily program it.

WHAT IS PLC (Programmable Logic Controller)?

PLC or programmable logic controller is a small but powerful computer used to control industrial devices. Simply it read the input from the field devices then execute the logic by seeing in the program and write the output.

So basically, PLC performs a certain task based on the program which can be in the form of the ladder. The program can be written by the user.

There are different programming languages are available to write a program. If you want to write a program for complex logic then you can choose another programming language other than ladder logic.


The International Electrotechnical Commission (IEC) has formed a unique standard for programming language as there are lots of difference between PLC manufacturer and IEC standard.

IEC1131 is the standard for which larger manufacturers like Siemens and Allen Bradley help IEC and these two manufacturers have made other manufacturers follow them.

IEC1131 has few sections which are listed below.

  • General section

  • programming manual

  • Fuzzy logic

  • communications

  • user manuals

  • Programming language types

  • Hardware and test requirement


For small size projects:

  • ABB

  • Schneider

  • LG

  • Siemens

  • Zilog

For Medium size projects:

  • Omron

  • Modicon

  • Mor

  • Toshiba

For larger size projects:

  • Siemens

  • Allen Bradley


  • There is no need for changing wiring for changes in the procedure you can do all by within software environment

  • PLC hardware contains an indication of the fault and inside the software, you can find the actual place of the fault that is a very useful feature.

  • It requires very less installation place.

  • Wiring is very simple and no requirement of any confusion.

  • If you have to do programming for the same thing you just need to write logic over and over.

  • PLC doesn't contain any moving parts.

  • Scanning time of the PLC is very fast.

  • PLCs are small but very powerful PCs.

All the above reasons are enough to make us know why we need PLC.


  • Ladder Logic (most commonly use)

  • Functional Block Diagram (FBD)

  • Structural Text (ST)

  • Instruction List (IL)

  • Sequential Flow Chart (SFC)

  • Continuous flow chart (CFC)

In my next article, I will cover the main component which makes up the PLC.

If you have any queries related to this article or wants to know more about it please do comment. Please like and share this article among your friends.

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