The Birth of the PLC

The Original Challenge 

The early history of the PLC is fascinating.  Imagine if you will a fifty foot long cabinet filled with relays whose function in life is to control a machine.  Wires run in and out of the system as the relays click and clack to the logic.  Now imagine there is a problem or a small design change and you have to figure it all out on paper and then shut down the machine, move some wires, add some relays, debug and do it all over again.  Imagine the labor involved in the simplest of changes.  This is the problem that faced the engineers at the Hydra-matic division of GM motors in the late 1960's.

Fortunately for them the prospect of computer control was rapidly becoming a reality for large corporations as themselves.  So in 1968 the GM engineers developed a design criteria for a "standard machine controller".  This early model simply had to replace relays but it also had to be:

  • A solid-state system that was flexible like a computer but priced competitively with a like kind relay logic system.
  • Easily maintained and programmed in line with the all ready accepted relay ladder logic way of doing things.
  • It had to work in an industrial environment with all it's dirt, moisture, electromagnetism and vibration.
  • It had to be modular in form to allow for easy exchange of components and expandability.

The Race is On 

This was a tall order in 1968 but four companies took on the challenge.

  1. Information Instruments, Inc. (fully owned by Allen-Bradley a year later).
  2. Digital Equipment Corp. (DEC)
  3. Century Detroit
  4. Bedford Associates

Bedford Associates, run by Richard Morley, won the contract and quickly formed a new company around the technology called MODICON after Modular Digital Control.  By June of 1969 they were selling the first viable Programmable Controller the "084" (their 84th project) which sold over one thousand units.  These early experiences gave birth to their next model the "184" in 1973 which set Modicon as the early leader in programmable controllers.

Not to be outdone, the powerhouse Allen-Bradley (all ready known for it's rheostats, relays and motor controls) purchased Information Instruments in 1969 and began development on this new technology.  The early models (PDQ-II and PMC) were deemed to be too large and complex.  By 1971 Odo Struger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin 1774 PLC which would make them successful for years to come.  Allen-Bradley termed their new device the "Programmable Logic Controller" (patent #3,942,158) over the then accepted term "Programmable Controller".  The PLC terminology became the industry standard especially when PC became associated with personal computers.

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