Programmable logic controller (Programmable Logic Controller, PLC) is a digital operation controller with microprocessor for automatic control. It can load control instructions into memory for storage and execution at any time. PLC is composed of CPU, instruction and data memory, input / output interface, power supply, digital to analog conversion and other functional units. The reason why it is named programmable logic controller is because the previous programmable logic controller only has the function of logic control. Later, with the development, these computer modules with simple functions have already included logic control, timing control, analog control , Multi-machine communication and other functions, the name was also changed to Programmable Controller (Programmable Controller).The programmable logic controller used in modern industry has been equivalent to the mainframe of a compact computer. Its advantages in scalability and reliability make it widely used in various industrial control fields. Whether it is a computer direct control system, a centralized decentralized control system (DCS), or a fieldbus control system (FCS), there are a large number of various types of PLC controllers. There are many PLC manufacturers, such as Siemens, Schneider, Mitsubishi, Delta, etc., almost all manufacturers involved in the field of industrial automation, will have PLC products to provide.