PLC late 60 in the United States, comes first, and was called the programmable logic controller PLC (Programmable Logic Controller), aims to replace the relay, to perform a logical sequence control functions such as judgment, timer and counter. PLC‘s basic design idea is to computer functions, flexible, general advantages and relay control system easy to combine, the advantages of convenient operation, low price, the hardware is standard and universal. According to the actual application, software writes controllers will control the content within the user program memory. Controller and control easy connection.
Semiconductor technology, especially the development of the microprocessor and microcomputer technology, after the middle of the 70 ‘s, PLC has been widely used microprocessor CPU, input/output modules and peripheral circuits are used in medium and large scale or even super large-scale integrated circuits, PLC logic is no longer functional, but also with data processing, PID control and data communication functions.
Programmable controller is a digital operation of the electronic system, specifically designed for application in industrial environments. It features a programmable memory, internal memory which you used to perform logical operations, sequence control, timing, calculations and operations such as arithmetic instructions, and digital and analog inputs and outputs, and control various types of machinery or production processes. PLC is computer technology combined with traditional relay–contact control technology product, it overcomes the mechanical contact relay contact controlling system terminal complex, high reliability and low power consumption, disadvantage of universality and flexibility, making full use of advantages of microprocessor.
Programmable controller for users, is a contactless device, changes to the program can be changed in the production process, so can be used in the preliminary design stage using programmable controller, then determine the process during the implementation phase. The other hand, from the perspective of manufacturing production manufacturers of programmable controller in the manufacturing stage not required according to the orders of specially designed controllers, suitable for mass production. Because of these characteristics, soon after the advent of programmable controller was welcomed by the industrial sector, and rapid development. At present, the PLC has become a powerful tool for factory automation, has been widely used.