PLC History

A Very Brief History of the PLC

The advent of the PLC began in the 1960′s and 1970′s to replace traditional “hard-wired” controls, and has since become the predominant choice for industrial controls. Before PLCs, much of machine control relied on contacts and relays providing hard-wired “logic” for machine controls. Changes to the logic were labor intensive and costly.

In 1968, GM’s Hydramatic division specified the design criteria for what would become the first programmable logic controller. They requested a solid-state system that would:

  • survive the industrial environment
  • be easily programmed by plant engineers and technicians, and
  • be easily reprogrammed and re-used

The winning proposal came from Bedford Associate – which introduced the MOdular DIgital CONtroller (MODICON). The MODICON is still a popular brand of PLC today, but is owned by Schneider Electric. Other prevalent PLC brands today are: Allen-Bradley, Siemens, Omron, and GE.

The Automotive Industry was a major early adopter of programmable logic controllers (PLC). They wanted a programming method that could be easily understood by their existing controls engineers and technicians. The result of this desire was a programming language called Relay Ladder Logic (or “ladder logic”).

The layout of Ladder Logic is very similar to reading the diagrams for hard wired relay controls. Ladder Logic is still one of the most popular “language” for programming PLCs, but many others have developed over the years.