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What is PLC and why PLC?

As today I am starting my article on PLC. You will get to know what is PLC and why we use PLC?

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 brand of today Schneider Electric.

Earlier PLC was used only for Digital devices but today it expands 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 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 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.


PLC or programmable logic controller is a small but powerful controller used to control an industrial process. Simply it read the input from the field devices the 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.


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 projects:

  • ABB

  • Schneider

  • LG

  • Siemens

  • Zilog

For Medium size project:

  • Omron

  • Modicon

  • Mor

  • Toshiba

For larger size project:

  • 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 to make us know why we need PLC.


  • Ladder Logic (most commonly use)

  • Functional Block Diagram

  • Structural Text

  • Instruction List

  • Sequential Flow Chart

In my next article, I will cover the main component which makes up the PLC. This is just an introduction article on the PLC.

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

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