What is PLC (Programmable Logic Controller)?


Have you ever thought of “what is PLC and why it is called as Programmable Logic Controller?”

You can think for a while before you scroll down to find the answer.

In this article, I would answer you what is PLC and why PLC is called Programmable Logic Controller.

According to NEMA Standard PLC is;

“A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process.”

Let me make you understand with an example.

Say you want to control the water level of an overhead tank. You can do it with the help of PLC.

Water Level control of Overhead Tank

What do you need to do? You have to program the logic into the PLC.

You have to make the logic as per the set of conditions or the process requirement. Let us take our above example further.

To control the water level of the overhead tank these are the following conditions:

Condition #1: When you press start push-button, the pump should get ON.

Condition #2: When you press the stop push-button, the pump should get OFF.

Condition #3: When the overhead tank high level sensor is sensed, the pump should stop automatically and should not get ON even though you press start push button again (to avoid wastage of water).

Now according to the above set conditions, you have to make the program. The programming is where the magic happens. IEC 61131-3 standard mentions five different methods to program the PLC. Ladder logic is one of them. Ladder Logic is nothing but a setofinstructions shown in the below figure.

Ladder Logic
Ladder Logic for Controlling Water Level of Overhead Tank

Ladder logic is like ‘Relay Logic’ but in ‘soft form’. ‘Soft form’ means physically relay coil and its contacts are not present but still, it works in the same manner. It can be programmed using software like TIA Portal (for Siemens PLC) and work exactly like relay logic as shown in the below figure.

Relay vs. Ladder Logic

When you press the Start push-button, the controller identifies the input, processes according to the logic made and provides the output. By pressing the stop push-button you can stop the pump. If the overhead tank sensor is sensed the pump stops automatically and won’t start even if you press start push button again. Take a look at the short video.

Not only this, if you want to stop the pump after the stipulated time, you can also do that just by using OFF Delay Timer instruction. You need not purchase the timer and connect it in a panel.

Timer Instruction in PLC

In addition to this, PLC allows you to do arithmetic functions also like addition, subtraction, division, and multiplication.

Arithmetic Function in PLC

You can count the objects with the help of an inbuilt counter function.

There are a lot of other functions that PLC can do. The overhead tank level control example is a very simple one. Likewise, one can do complex programming having more than thousands of inputs and outputs.


You may be thinking that PLC is like a computer.

No, it is not the computer but the architecture of PLC is the same as the personal computer. The difference is that PLCs can work in an industrial environment. They have special input/output interfaces and control programming languages. Today’s PLC has various math and logic functions. PLCs are the first choice in process industries. They are getting smaller in size with faster CPUs.


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