Siemens Simatic S7-1500R System Manual page 186

Pairing procedure
Proceed as follows to pair two CPUs
1. Create a redundancy connection between two CPUs. Connect the CPUs to the relevant
ports of the interfaces (for example for R-CPUs: X1 P2 R).
2. POWER ON both CPUs.
Loss of pairing
If pairing is already established, an inadmissible configuration version will lead to the loss of
pairing. Loss of pairing in RUN system state also causes the loss of synchronization
between primary CPU and backup CPU. The system switches to the RUN-Solo system
state.
The primary CPU switches to the RUN operating state and takes on sole control of the
process. The backup CPU switches to the STOP operating state.
In the event of the loss of pairing as a result of the failure of the primary CPU, the backup
CPU becomes the new primary CPU and takes on sole control of the process.
Note
An invalid configuration variant with more than two R/H CPUs can lead to a faulty system
state. Both R/H CPUs become primary CPUs.
Checking pairing state
You can find out how to check the success of pairing on the display and in STEP 7 in the
section Checking before replacing components (Page 257).
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA
Commissioning
9.3 Commissioning procedure
185