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PLC History

In the late 1960’s PLCs were first introduced The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates Bedford Proposed something called a Modullar digital controller(MODICON) to a major U.S car manufacturer .Other companies at the time proposed computer based schemes , one of which was based upon the POP-8. The MODICON 084 brought the world’s first PLC into commercial production.

When production requirement changed so did the control system . This becomes very expensive when the change is frequent . Since relay are mechanical devices they used also have a limited life time which required strict adhesion to maintain schedule. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possible hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices. These relays would be individually wired together in a manner that would yield the outcome. Were there problems? You bet

These “new controllers” also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming chances easily performed. They also had tosurvive the harsh industrial environment. That’s a lot to ask! The answer were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state once.

In the mid70’s the dominant PLc technologyes were sequencer state machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLC’s. Conventional Microprocessors Lacked the power to quickly solve PLC logic in all but the smallest PLCs.

Communication abilities began to appear in approximately 1973.the first such system was Modicon’s Modbus. The PLC could Now talk to other PLC.