PLC is a digital operation electronic device specially designed for industrial application. It uses programmable memory to store instructions for performing logical operations, sequential operations, timing, counting and arithmetic operations, and can control various types of machines or production processes through digital or analog input and output. PLC and its related peripheral equipment should be designed according to the principle that it is easy to form a whole with industrial control system and expand its functions.
At present, the programmable logic controller used in industry is equivalent to or close to the host of a compact computer, and its advantages in expansibility and reliability make it widely used in various industrial control fields. There are always a large number of PLC controllers in computer direct control system, centralized distributed control system DCS and field bus control system FCS. There are many manufacturers of PLC, such as Siemens, Schneider, Mitsubishi and Delta. Almost all manufacturers involved in industrial automation will have their own PLC products.