4 PLC Topics
4.2 System scale
4.3 User interface
5 PLC compared with other control systems6 Digital and analog signals
7 See also
9 External links
 HistoryThe PLC was invented in response to the needs of the American automotive manufacturingindustry. 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 dedicatedclosed-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 everyrelay.
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...