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==Invention and early development==
PLC originated in the late 1960s in the automotive industry in the US and were designed to replace relay logic systems.<ref>{{Harvnb|Parr|1998|p=438}}</ref> Before, control logic for manufacturing was mainly composed of [[relay]]s, [[cam timer]]s, [[Drum sequencer (controller)|drum sequencers]], and dedicated closed-loop controllers.<ref>{{
The hard-wired nature made it difficult for design engineers to alter the process. Changes would require rewiring and careful updating of the documentation. If even one wire were out of place, or one relay failed, the whole system would become faulty. Often technicians would spend hours troubleshooting by examining the schematics and comparing them to existing wiring.<ref>{{Cite web|url=https://www.controlsystemsandautomation.com/learn/plc/plc-programming-basics-i/|title=PLC Programming Basics Part I|date=2019-07-23|website=Control Systems & Automation|language=en-US|access-date=2020-02-23}}</ref> When general-purpose computers became available, they were soon applied to control logic in industrial processes. These early computers were unreliable<ref>{{Harvnb|Laughton|Warne|2002|p=16/3}}: "The first industrial computer application was probably a system installed in an oil refinery in Port Arthur USA in 1959. The reliability and mean time between failure of computers meant that little actual control was performed."</ref> and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality.<ref>{{Harvnb|Parr|1998|p=437}}</ref>
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