A PLC (programmable logic controller) is an industrial digital computer designed for the control and automation of manufacturing processes or robotic devices. Programmable Logic Controllers are used where reliability control, simple programming and fault diagnosis is necessary. They were originally built to replace electromechanical relay systems in industrial automation.
A CPU (central processing unit) forms part of the PLC system. PLC CPUs are a microprocessor-based control system and act as the electronic circuitry within an industrial computer. They are usually built-in carry out instructions from the computer program via programming languages, performing basic mathematical, logical, control, input and output operations.