Siemens SIMATIC PCS 7 Configuration Manual page 96

Configuration of the hardware (AS and I/O)
6.2 Special settings for H systems
Calculating monitoring times for updating the reserve CPU
To avoid influencing the ongoing process during the critical update phase, four monitoring
times are provided that can cancel the update phase where a monitoring time is exceeded.
A dialog box is opened via the "Calculate" button where the monitoring times are calculated
using the hardware configuration and the information on the user program. The minimum
peripheral device hold time results in all cases from the quantity of DP slaves configured.
Information on the user program is described in the online help.
Note
The default user program value settings (0,0,1024) provide the default monitoring times (8,000
ms, 3,200 ms, 800 ms, 70 ms). These values are already optimized for typical user cases
meaning that no new calculation is required.
In the event that monitoring times which are set too low prevent redundant operation during a
plant conversion (e.g. through configured DP slaves that cannot be reached, the update will
take longer than 800 ms), the times can be increased and updated during operation.
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Compendium Part A - Configuration Guidelines (V8.2)
Configuration Manual, 08/2016, A5E38166869-AA