Work out plc scan time instantly with clear inputs, formula shown and shareable results.
Scan time is logic execution plus IO update plus housekeeping overhead. Worst-case response to an input is about two scans, because an input arriving just after the input scan waits a full cycle to be read and then another to be acted on — which is why fast events need interrupt inputs.
PLC scan
scan = (instructions/1000) x speed + modules x IO update time + overhead; worst-case response = 2 x scan
An input changing immediately after the input read is not seen until the next scan, and the output resulting from it is not written until the end of that scan.
Use a high-speed counter or interrupt input, or a latching input module. Normal scanned IO cannot reliably see a pulse shorter than one scan.