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The hard-wired nature of these components made it difficult for design engineers to alter the automation process. Changes would require rewiring and careful updating of the documentation. Troubleshooting was a tedious process.<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 |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>
The PLC provided several advantages over earlier automation systems. It was designed to tolerate the industrial environment better than systems intended for office use, and was more reliable, compact, and required less maintenance than relay systems. It was easily
===Modicon===
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