With the continuous development of computer control technology, the application of programmable controller has been widely popularized and become an important part of automation technology. Programmable controllers first appeared in the United States. In 1968, General Motors (GM), an American automobile manufacturing company, put forward the requirements for developing a new controller, and proposed that the new generation controller should have the following ten conditions from the perspective of users:

(1) The program is simple and can be modified on site;

(2) Convenient maintenance, preferably plug-in type;

(3) The reliability is higher than that of relay control cabinet;

(4) The volume is smaller than the relay control cabinet;

(5) The data can be directly sent to the management computer;

(6) It can compete with relay control cabinet in cost;

(7) The input can be AC 115V (i.e. the grid voltage of the United States);

(8) The output is AC 115V and above 2a, which can directly drive the solenoid valve;

(9) When expanding, the original system only needs small changes;

(10) The user program memory capacity can be expanded to at least 4KB.

After the conditions were put forward, it immediately caused an upsurge of development. In 1969, American digital equipment company (DEC) developed the world's first programmable controller and applied it to the production line of general motors. At that time, it was called programmable logic controller (PLC), which was used to replace the relay to perform logic judgment, timing, counting and other sequential control functions. Then, Modicon company of the United States also developed a controller of the same name. In 1971, Japan introduced this new technology from the United States and soon developed the first programmable controller in Japan. In 1973, Western European countries also developed their first programmable controller.

With the development of semiconductor technology, especially microprocessor and microcomputer technology, after the mid-1970s, especially since the 1980s, PLC has widely used 16 bit or even 32-bit microprocessors as the central processor, and medium, large-scale or even super large-scale integrated circuits have been used in input and output modules and peripheral circuits, making new breakthroughs in the concept, design, performance price ratio and application of PLC. At this time, PLC is not only a logic judgment function, but also has the functions of data processing, PID regulation and data communication. It is more appropriate to call it programmable controller, which is called PC for short. However, in order to distinguish from the short name of PC of persona1 computer, it is generally called PLC (programmable logic controller).

PLC is the product of the combination of microcomputer technology and traditional relay contactor control technology. Its basic design idea is to combine the advantages of perfect, flexible and universal computer functions with the advantages of simple, easy to understand, easy to operate and cheap relay control system. The hardware of the controller is standard and universal. According to the actual application object, the control content is compiled into software and written into the user program memory of the controller. Relay control system has a history of hundreds of years. It is a control method that uses weak current signal to control strong current system. In complex relay control system, it is difficult to find and eliminate faults, which takes a long time and seriously affects industrial production. When the process requirements change, the components and wiring in the control cabinet need to be changed accordingly. The transformation period is long and the cost is high, so that the user would rather make a new control cabinet. PLC overcomes the shortcomings of complex wiring, low reliability, high power consumption, poor universality and flexibility of mechanical contacts in relay contactor control system, makes full use of the advantages of microprocessor, and conveniently connects the controller with the controlled object. Since PLC is composed of microprocessor, memory and peripheral devices, it should belong to a class of industrial control computers.

For users, the programmable controller is a non-contact equipment, and the production process can be changed by changing the program. Therefore, if the programmable controller is selected in the preliminary design stage, the design and debugging can be made simple and easy. From the perspective of manufacturers who manufacture programmable controllers, it is not necessary to specially design the controller according to the order requirements of users in the manufacturing stage, which is suitable for mass production. Because of these characteristics, the programmable controller was quickly welcomed by the industrial control industry and developed rapidly. At present, programmable controller has become a powerful tool for factory automation and has been widely used.