The elapsed time for one defined cyclic pass of a controller’s program, with the associated processing included by that controller’s scan-time definition.
Are scan time, task interval and I/O update time the same?
No. Scan duration describes a pass through cyclic processing; a periodic-task interval specifies when a task is scheduled. I/O update timing concerns movement of input or output data. Identify process-image updates, execution and interruptions separately for the actual controller.
Ask which task and timing definition a quoted value describes. A configured period is not evidence that every execution completes in that duration, and one controller-wide number does not describe every possible signal path.
In a simple polling example, a level observed only at 0 and 10 ms misses a pulse present from 3 to 5 ms. This assumes no latch, event capture or separate hardware counter; those functions can change the outcome.
Controllers may offer event or interrupt processing, or dedicated counting hardware, in addition to cyclic logic. Identify the actual capture path and any input qualification ahead of it. A faster nominal scan does not describe the complete path.
Retain the controller and input model, task type, capture path and measured operating conditions. Communications, interruptions, task priorities and pre-emption can affect the timing context. An average scan under one condition does not establish worst-case pulse capture.
Check recorded counts against known product events at the intended line conditions. Capture depends on the configured input path and qualification rules, as well as cyclic execution speed.