Siemens SIMATIC S7-1500 System Manual page 396

The display of the primary CPU indicates the lag of the backup CPU.
Table 12- 10 Making up backup CPU lag
Once the backup CPU has caught up, both CPUs switch to the RUN-Redundant operating
state. The backup CPU establishes connections to the IO devices (only with system
redundancy S2 and R1) and the communication links are available again. Both CPUs process
the user program synchronously.
Note
High load during SYNCUP
The higher load during SYNCUP can prolong the program cycle.
If SYNCUP is taking too long and the displays are not showing any progress, you can abort
SYNCUP and optimize your user program. You can find more information under "SYNCUP
system state aborts".
Note
Setting a sufficiently long maximum cycle time
Configure a sufficiently long maximum cycle time for the CPUs. A temporary increase in the
cycle time can occur upon a system state transition SYNCUP → RUN-Redundant.
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF
Primary CPU
Commissioning
12.4 Operating and system states
Backup CPU
395