Typically the PLC CPU scans the ladder from top left to bottom right and reads and executes the condition of physical I/O’s. But the physical switches and coils of relay logic are replaced with PLC’s memory location which are represented as Inputs (I) and Outputs (O) in ladder logic. Ladder logic shares similar look and feel of relay logic. Relay Logic has a Start switch, Stop switch, Control Relay, Relay Coil (CR1) and a Motor (Mtr). Comparison between Relay Logic and Ladder Logic.īelow is a very simple motor control relay logic and it’s corresponding ladder logic. To this day, ladder logic remains the most popular method to program a PLC. This eliminated the need to teach electricians, maintenance technicians and engineers how to program. The first generation PLCs were programmed with a technique that was based on relay logic wiring schematics. The decision to use ladder logic as the main programming method was very strategic as it did not need much time to retrain engineers to adapt to this. It mimics the relay logic (combination of switches, relays, coils and contacts). Relay Ladder Logic is one of the international standards and is the main programming method used to program PLC’s. The Relay Ladder Logic is the user programmed control algorithm. relay, motors, valves etc.) connected to the outputs of the PLC.
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