It was easily extensible with additional I/O modules, while relay systems required complicated hardware changes in case of reconfiguration. It tolerated the industrial environment better than computers and was more reliable, compact and required less maintenance than relay systems. The PLC provided several advantages over earlier automation systems.
These early computers were unreliable and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality. When general-purpose computers became available, they were soon applied to control logic in industrial processes. Often technicians would spend hours troubleshooting by examining the schematics and comparing them to existing wiring. If even one wire were out of place, or one relay failed, the whole system would become faulty. Changes would require rewiring and careful updating of the documentation. The hard-wired nature made it difficult for design engineers to alter the automation process. Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.
PLC originated in the late 1960s in the automotive industry in the US and were designed to replace relay logic systems.
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