Siemens SIMATIC S7-400H Manual page 164

Modifications to the System while in Operation
Reaction if monitoring times are exceeded
If one of the monitored times exceeds the maximum value configured the update is
interrupted and no change of master takes place. The fault-tolerant system
remains in Solo mode with the previous master CPU and in certain conditions
attempts to perform the change of master later. For more details refer to
Section 5.3.
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S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04