Siemens Simatic S7-1500R System Manual page 187

Commissioning
9.3 Commissioning procedure
Checking pairing in the RUN-Solo system state
Please observe the following rules if the redundant system is in the RUN-Solo system state:
● Do not immediately start replacing components.
● Do not immediately switch the failed CPU to the RUN. operating state.
First check the pairing status in the RUN-Solo system state.
Do not switch the failed CPU in the RUN-Solo system state to the RUN operating state.
This could result in an undefined system state for the redundant system. Both CPUs would
become primary CPUs.
If the S7-1500R/H redundant system is in the RUN-Solo system state, you must not
immediately switch the backup CPU to the RUN operating state.
Possible cause: No pairing between the two CPUs. Check the pairing status on the display
or on the basis of the diagnostics status or diagnostics buffer.
If there is no pairing, the redundancy connections have been interrupted. Follow the
procedure described in the section Checking before replacing components (Page 257).
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CAUTION
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA