Introduction to 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.

PLC hardware

Introduction to PLC hardware


Keeps checking the PLC controller to avoid errors. They perform functions including logic operations, arithmetic operations, computer interface and many more.


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.