A PLC’s CPU (central processing unit) is essentially a scanning device that runs a loop through every I/O point. The PLC (programmable logic controller) was developed to replace physical relays and timers with a purpose-built small computing unit designed for industrial environments. Although a great improvement over manual control, this type of automation structure had a major disadvantage: to change the function (or logic) of the control system, the system had to be literally re-wired. Banks of relays appeared in semi-automated factories, notably automobile plants. Industrial automation was originally implemented with discrete relays and timers, which were connected with copper wire. Compare PLCs and Ladder Logic with Opto 22 Hardware and Software
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