The factory computer named after its inventors' 84th project
In 1968 a General Motors transmission plant wanted to stop rewiring walls of relays every time a production line changed. A small firm in Bedford, Massachusetts won the contract with its 84th project, the Modicon 084. When GM finally retired one unit, it had run almost twenty years without interruption.
Before programmable logic controllers, factories ran on relays, cam timers, drum sequencers and dedicated control loops, all physically wired together. Altering a process meant rewiring and redocumenting, and tracking down faults was tedious. General-purpose computers of the era were no better suited, needing specialist programmers and carefully controlled rooms with clean power and steady temperatures.
GM Hydramatic's request for an electronic replacement grew out of a white paper by engineer Edward R. Clark, and Bedford Associates built the winning machine in 1969. The team formed a company, Modicon, short for modular digital controller, and Dick Morley, one of the engineers, is often called the father of the device. Modicon also devised Modbus to let its controllers talk, and that protocol became an open standard for linking industrial equipment. The brand passed to Gould Electronics in 1977 and later to Schneider Electric, which displays one of the first 084 units in North Andover.
The machines were meant for engineers rather than programmers, so their main language, ladder logic, mimics the wiring diagrams of the relay panels it replaced. It remains popular because the controller scans its logic in a predictable, repeating order, and the ladder format makes timing problems easy to spot. Early programs were stored on cassette tapes, and the oldest units used magnetic-core memory.
The name itself shifted. Such devices were first known as programmable controllers, or PCs, until IBM's Personal Computer arrived in 1981 and claimed the abbreviation. Odo Josef Struger, who helped create Allen-Bradley's controller, is credited with coining PLC to separate the two. By the mid-1980s Allen-Bradley had overtaken Modicon to lead the North American market.
Source: Programmable logic controller
