It is a real-time system that is different from a personal computer. In a traditional relay-based motor control system, whenever the design is changed, the entire system is almost re-produced, which is not only time consuming but also laborious. At the same time, the relay has contact contacts. The disadvantages of bad, wear, and bulky, therefore resulting in increased costs, low reliability, and difficult maintenance. To improve these shortcomings, the United States DEC first published in 1969: Programmable Controller.
The program controller was called Programmable Logic - Controller (abbreviated as PLC) in the initial stage of its publication. The first purpose was to replace the relays, so that the sequence logic of the relay logic and other functions such as timing or counting was mainly controlled. Therefore, it is also called a sequence controller. Its structure is also like a microcomputer, so it can also be called a microcomputer programmable controller (MCPC), until 1976, the United States Motor Manufacturing Association formally given the name Programmable Controller, can program controller, referred to as PC, due to the current personal computer (Personal Computer) is very common, and often used in conjunction with the programmable controller, in order to distinguish between the two, it