Programmable logic controller, whose English full name is programmable logic controller, referred to as PLC, is a digital operation of the electronic system, specially designed for industrial applications. It uses programmable memory, which is used to store the instructions of logic operation, sequence control, timing, counting and arithmetic operation, and controls various types of mechanical equipment or production process through digital or analog input and output interfaces.

PLC is a kind of digital operation electronic device specially designed for application in industrial environment. It is mainly composed of the following parts: central processing unit (CPU): it is the control center of PLC, the core of PLC and plays the role of nerve center. Each set of PLC has at least one CPU.

The structure and working principle of PLC

Memory: it is the memory for storing system software; User program memory is used to store PLC user program applications; Data memory is used to store the intermediate state and information of PLC program execution, which is equivalent to PC memory.

Input and output interface (I / O module): it mainly completes the interface between PLC and electrical circuit. The I / O module integrates the I / O circuit of PLC. The input register reflects the state of input signal and the output point reflects the state of output latch. Communication interface: its main function is to realize data exchange (Communication) between PLC and external equipment. Power supply: the power supply of PLC provides working power for PLC circuit, which plays a very important role in the whole system.

Programmable logic controller (PLC) works in the way of sequential scanning and continuous circulation. That is to say, when PLC is running, according to the program compiled and stored in the user program memory according to the user’s control requirements, PLC makes periodic cycle scanning according to the instruction step number or address number. If there is no jump instruction, Then the user program is executed one by one from the first instruction until the end of the program, and then the first instruction is returned to start the next round of scanning. In each scanning process, the sampling of the input signal and the refreshing of the output state are completed.


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