Programmable Logic Controller, short for PLC, is a digital electronic device with a microprocessor. The device is a digital logic controller for automatic control, which can load control instructions into the memory at any time for storage and execution. The programmable controller is composed of internal CPU, instruction and data memory, input and output units, power supply modules, digital and analog units.
When the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution and output refresh three stages. Completing the above three stages can be called a scan cycle. During the entire operation, the CPU of PLC repeatedly executes the above three stages at a certain scanning speed.