PLC History　　　　In the late 1960s 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 A ociates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the worlds first PLC into commercial production. 　　　　When production requirements changed so did the control system. This becomes very expe ive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, po ibly 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 ma er that would yield the desired 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 changes easily performed. They also had to survive the harsh industrial environment. That"s a lot to ask! The a wers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.　　　　In the mid70"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 microproce ors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microproce ors 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. 　　　　Communicatio  abilities began to a ear in a roximately 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 communicatio  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 communicatio  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 i tead 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 (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, i truction lists, C and structured text all at the same time! PC"s are also being used to replace PLCs in some a licatio . The original company who commi ioned the MODICON 084 has actually switched to a PC based control system. 　　　　What will the 00"s bring? Only time will tell.