PLC Scan

The PLC program is executed as part of a repetitive process
referred to as a scan. A PLC scan starts with the CPU reading
the status of inputs. The application program is executed using
the status of the inputs. Once the program is completed, the
CPU performs internal diagnostics and communication tasks.
The scan cycle ends by updating the outputs, then starts over.
The cycle time depends on the size of the program, the number
of I/Os, and the amount of communication required.