Siemens SIMATIC S7-1500 System Manual page 395

Commissioning
12.4 Operating and system states
The display of the primary CPU indicates the copying progress.
Table 12- 9
The backup CPU is busy accepting the data before it can also process the user program.
⑤ Making up backup CPU lag
In phase ⑤, the backup CPU catches up with the primary CPU.
The backup CPU sends a status message on its program progress to the primary CPU at each
cycle control point.
394
Copying the work memory
Primary CPU Backup CPU
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF