Programmable logic controller
A programmable logic controller (PLC) or programmable controller is a digital
computer used for automation of electromechanical processes, such as control of
machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are
used in many industries and machines. Unlike general-purpose computers, the PLC is
designed for multiple inputs and output arrangements, extended temperature ranges,
immunity to electrical noise, and resistance to vibration and impact. Programs to
control machine operation are typically stored in battery-backed or non-volatile
memory. A PLC is an example of a real time system since output results must be
produced in response to input conditions within a bounded time, otherwise unintended
operation will result.
The PLC was invented in response to the needs of the American automotive
manufacturing industry. Programmable logic controllers were initially adopted by the
automotive industry where software revision replaced the re-wiring of hard-wired
control panels when production models changed.
Before the PLC, control, sequencing, and safety interlock logic for
manufacturing automobiles was accomplished using hundreds or thousands of relays,
cam timers, and drum sequencers and dedicated closed-loop controllers. The process
for updating such facilities for the yearly model change-over was very time
consuming and expensive, as electricians needed to individually rewire each and
every relay.
In 1968 GM Hydramatic (the automatic transmission division of General
Motors) issued a request for proposal for an electronic replacement for hard-wired
relay systems. The winning proposal came from Bedford Associates of Bedford,
Massachusetts. The first PLC, designated the 084 because it was Bedford Associates'
eighty-fourth project, was the result. Bedford Associates started a new company
dedicated to developing, manufacturing, selling, and servicing this new product:
Modicon, which stood for MOdular DIgital CONtroller. One of the people who
worked on that project was Dick Morley, who is considered to be the "father" of the
PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later acquired
by German Company AEG and then by French Schneider Electric, the current owner.
One of the very first 084 models built is now on display at Modicon's
headquarters in North Andover, Massachusetts. It was presented to Modicon by GM,
when the unit was retired after nearly twenty years of uninterrupted service. Modicon
used the 84 moniker at the end of its product range until the 984 made its appearance.
The automotive industry is still one of the largest users of PLCs.
Early PLCs were designed to replace relay logic systems. These PLCs were
programmed in "ladder logic", which strongly resembles a schematic diagram of relay
logic. This program notation was chosen to reduce training demands for the existing
technicians. Other early PLCs used a form of instruction list programming, based on a
stack-based logic solver.
Modern PLCs can be programmed in a variety of ways, from ladder logic to
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