You must have heard a lot of PLCs and how they can provide great machine control and safety. But, do you really have any idea that how they work and what they are? Well, if you don’t have any idea then nothing to worry about. We are here to give you some basic and necessary information about how PLCs work. It is no rocket science, it is just the basics of computers. You must have computers at home, and you know all the terminology like CPU, input devices and output devices. You must be familiar with the concept of RAM and ROM. How the computer memory works? And, you know the difference between the hardware and software. How computers can communicate? So, it is all about the computer and PLCs are also computers. They have the central processing unit.

But, here comes the differences. PCs are for home and personal use. A person can use it for many purposes and you can take it anywhere. But, PLC is designed to be used in the rugged and harsh places like industries and factories. Where the conditions are not so favorable. Though, the PLC is much flexible. It can interface with input and output devices and other computers.

How it works?

Well, there are four sections of PLC. Which are the backplane and power supply, the CPU, the input and output section and the program section. The backplane is here to provide the connections. It provide the physical assembly for the PLC input output and the bus connections between all parts.


CPU is the central core of the PLC and it maintain the functionality. CPU has four different operating modes. It can operate on programming mode, run mode, stop mode and reset mode.

When you are transferring the program logic from the laptop to PLC, you will download it through this mode. When it will download the complete controlling program, it is then placed in run mode so it can execute the program.

Run mode is the mode when the PLC is running the full operation. It will read the inputs and will set the output according to the program.

Stop mode is there to put all the outputs to stop. When you require the need of some more programming in the middle of the process, you can use this mode and program it according to your needs.

Reset mode is there to reset the PLC from operating conditions back to switch on positions.