In PROFINET, the 'Watchdog Time' of an IO Device is computed as Update Time multiplied by the Reduction Ratio (typically 3 or more); if no cyclic telegram is received within that window, the device drops its outputs to the safe state.

The calculation is straightforward: Update Time is the cyclic I/O refresh period (e.g. 8 ms), Reduction Ratio is the multiplying factor (3 by default), and Watchdog Time is the product (here 24 ms). If the Controller stays silent for that 24 ms, the device transitions to safe state. The ratio allows up to two missed telegrams to be tolerated before action is taken; a higher ratio is more tolerant of short disturbances but reacts more slowly when a real failure occurs.

Tune the Reduction Ratio against your network's measured jitter, not the default — keeping the factory 3 on a noisy backbone causes nuisance trips, while shrinking it on a clean network sharpens fault reaction without downside.