This is important because if the controller has to react to an input event faster than the scan time, it may never detect the change. Scan times can vary from 1 millisecond to 20 milliseconds. The diagram below shows a model of the PLC Program Scan Cycle. Because inputs can change at any time, this process repeats over and over again while the PLC is in RUN mode. During the Program Scan Cycle, the PLC reads the inputs, solves the Ladder Logic, and energizes (or de-energizes) outputs. When a PLC executes a program, it must know, in real-time, when external devices controlling a process are changing.
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