There are many definitions of PLC. The International Electrotechnical Commission (IEC) defines PLC as: PLC (programmable logic controller) is an electronic system of digital operations designed for use in industrial environments. It uses a programmable memory to store instructions to perform logical operations, sequential control, timing, counting and arithmetic operations, and to control various types of machinery or production processes through digital and analogue inputs and outputs. Programmable controllers and their related equipment should be easy to form a whole with the industrial control system, easy to expand its function of the principle of design.

PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability, and simple programming. The position of PLC in industrial automation control, especially in sequential control, cannot be replaced in the foreseeable future.