The rise of programmable controllers is inseparable from the requirements of the development of modern industrial automation production in the United States. PLC originated in the 1960s. At that time, General Motors of the United States, in order to solve the problems of time-consuming circuit modification of the relay sequence control system and difficult maintenance and maintenance when the factory production line was adjusted. Before the emergence of programmable logic controllers, general control, sequence control and safety interlock logic control in the automobile manufacturing industry must rely entirely on numerous relays, timers and specialized closed-loop controllers to achieve. They are huge in size and have serious noise. Not only does the annual maintenance work consume a lot of manpower and material resources, but also the wiring maintenance of the relay-contactor system has high requirements for the proficiency of the maintenance personnel.
In response to these problems, General Motors Corporation of the United States issued an open tender to the society in 1968, requiring the design of a new system to replace the relay system, and proposed the famous "General Ten" bidding index. Subsequently, according to this idea, the American Digital Equipment Corporation (DEC) successfully developed the first PDP-14 controller in 1969, which was used and successful in the automatic assembly line of automobiles. Since the system was mainly used for sequential control and could only perform logical operations, it was named Programmable Logic Controller (PLC).
The earliest PLC only had the simple logic on/off function, but compared with the traditional relay control method, it has the advantages of easy modification, installation, diagnosis and less space.
In the early 1970s, PLC introduced microprocessor technology, which enabled PLC to have arithmetic operation functions and multi-bit digital signal output/input functions, and can directly write programs with ladder diagram symbols. The use of this new technology has produced huge repercussions in the industry. Japan introduced this technology from the United States in 1971, and soon developed its own DCS-8 programmable logic controller. Germany and France also had their own technology from 1973 to 1974. China successfully developed its first programmable logic controller in 1977, but the core of the microprocessor used was MC14500. In the mid-1970s, PLC functions were added with technologies such as remote communication, analog output and input, and NC servo control. After the 1980s, the PLC high-speed communication network function was introduced, and some special output/input interfaces, man-machine interface, high-function function instructions, data collection and analysis capabilities were added at the same time.