B&r X20CP1584 Central Processing Unit CPU OVP, New

PLC originated in the late 1960s in the automotive industry in the USA and were designed to replace relay logic systems.^[1] Before, control logic for manufacturing was mainly composed of relays

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 times technicians would spend hours troubleshooting by examining the schematics and comparing them to existing wiring.^[2] When general-purpose computers became available, they were soon applied to control sequential and combinatorial logic in industrial processes.^{[citation needed]} These early computers required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality.^{[citation needed]}