Siemens SIMATIC PC RI25/45 Technical Description page 94

Monitoring Module
Note
If you modify the watchdog time after the watchdog has been activated (that
is during watchdog execution), the watchdog is retriggered.
Marginal In order to secure the recognition of a fail-state from an interrupt-evaluating
Conditions
unit which is connected to the relay interface, the SafeCard guarantees an
error hold time of at least 500 ms. This also applies if the PC is reset, for
example, after the watchdog has been executed.
4-8
SIMATIC PC RI25/45, Technical Description
C79000-G7076-C808-01