PLC History

"PLC development began in 1968 in response to a request from an US car manufacturer (GE). The first PLCs were installed in industry in 1969. "
The Hydramatic Division of the General Motors Corporation specified the design criteria for the first programmable controller in 1968. Their primary goal was to eliminate the high costs associated with inflexible, relaycontrolled systems.

The specifications required a solid-state system with computer flexibility able to
(1) survive in an industrial environment,
(2) be easily programmed and maintained by plant engineers and technicians, and
(3) be reusable. Such a control system would reduce machine downtime and provide expandability for the future.

Some of the initial specifications included the following:
• The new control system had to be price competitive with the use of relay systems.
• The system had to be capable of sustaining an industrial environment.
• The input and output interfaces had to be easily replaceable.
• The controller had to be designed in modular form, so that subassemblies could be removed easily for replacement or repair.
• The control system needed the capability to pass data collection to a central system.
• The system had to be reusable.
• The method used to program the controller had to be simple, so that
it could be easily understood by plant personnel.

Communications abilities began to appear in approximately 1973. They could also be used in the 70’s to send and receive varying voltages to allow them to enter the analog world.

The 80’s saw an attempt to:
standardize communications with manufacturing automation protocol (MAP), reduce the size of the PLC, and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers.

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, instruction lists, C and structured text all at the same time.