In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system. If a discrete input is energized, the memory bit is a 1, and a \"normally open\" contact controlled by that bit will pass a ...
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. ...