History of PLC

 PLC history is most exciting question at recent time. In late 1960, PLC was first introduced for replacing Modular Digital Controller (MODICON) which is complicated relay based machine control. This is named as MODICON 84 at that time which is distributed to US major company.

As we all know relay is mechanical device which has less life. As time passed control system improve its technique but as requirement its size is mind boggling.

In the mid 70’s the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in MODICON and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903 (ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.

Communications abilities began to appear in approximately 1973. The first such system was MODICON's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!

The 80’s saw an attempt to standardize communications with General Motor's manufacturing automation protocol (MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay!

The 90’s have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the

1980's.The latest standard (IEC1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams,

Instruction lists, C and structured text all at the same time! PC's are also being used to replace

PLCs in some  applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.This is how new generation started.