PLC, short for Programmable Logic Controller, is a digital operational electronic system specifically designed for use in industrial environments. ‌1

PLC uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, and controls various types of mechanical equipment or production processes through digital or analog input and output. It has the characteristics of reliable operation and programmability, and is widely used in the field of industrial automation.

PLC has multiple classification methods:

1. Classified by structural form:

Integrated PLC: The power supply, CPU, I/O interface and other components are centralized in one chassis, with a compact structure, suitable for small PLCs.
Modular PLC: divides each component into multiple modules, such as CPU module, I/O module, etc., suitable for large and medium-sized PLCs, easy to expand and maintain.

2. Classified by function:

Low end PLC: It has basic logic operation, timing, counting and other functions.
Mid range PLC: equipped with analog input/output, remote I/O and other functions.
High end PLC: It has functions such as complex calculations and large-scale process control.

3. Classified by I/O points:

Small PLC: I/O points less than 256.
Medium sized PLC: The number of I/O points ranges from 256 to 2048.
Large PLC: I/O points greater than 2048.