As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time. 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, 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 world's first PLC into commercial production.
When production requirements changed so did the control system. 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, possibly hundreds or thousands, of individual relays. 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 desired outcome. 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. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones. -5- ALLEN BRADLEY VOL1 UEUNEED027A In the mid1970's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU.
The AMD 2901 and 2903 processors were quite popular in Modicon and AllenBradley 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.