Programmable Controller, a member of computer family, is designed to use in the field of industry control. The early Programmable Controller is named as Programmable Logic Controller, simply PLC. It is used to substitude for relay to realize logic control. With the development of technology, the function of this device has surpassed the scope of logic control. Hence, this device today has been called Programmable Controller, simply PC. In order to avoid the confusion with Personal Computer, it is named as PLC, Programmable Logic Controller.
In 1960s,the automatic control system in auto production line is basically composed of relay control device,which was constantly redesigned and installed when cars remodelled. With the development of production, the car updated cycle became more and more short. Thus, relay control device needed to be re-designed and re-installed usually. It is a waste of time, labor and material.Even impeding the shortage of update cycle. To change the situation, General Motors invited bids in 1969, asking for replacement of relay control device with new one. In 1969, the DEC of U.S.A. had developed a PLC, trying and gaining the success in the Gm automatic assemble line.PLC has developed rapidly since it come out. To standardize its production and development, America National Electrical Manufactory Association first named formally Programmable Controller.In 1984 after four years' invesigation. And definiting it as following: PC is a digital electric device, using programmmable memory to store command. It executes functions of logic, sequence, counting and calculation, and controls mecanical and working program through digital and input/output module. Later in 1987, International Electrotechnical Commission had consented to its definition.All in words, PLC is a compute designed for application in industry environment, possessing rich input and output interface and powerful drive capability. In practical application, its hardware is in need of device selection while software is asked for designing and formation according to command.
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