Siemens SIMATIC S7-1500 System Manual page 394

The communication connections in the primary CPU are temporarily ended. You can no
longer delete, load, generate or compress blocks in the user program. You can no longer run
any test or commissioning functions.
Table 12- 8
④ Copying the work memory
The primary CPU stores a consistent snapshot of its memory contents and some system
memory contents (backup CPU dump) at the next cycle control point: Process image, bit
memory, SIMATIC time/count functions, temporary local data, data block contents.
After the snapshot, the primary CPU immediately resumes user program execution.
Communication connections are reestablished and asynchronous instructions are started.
The primary CPU copies the consistent snapshot to the backup CPU and continues operating
in parallel. Data blocks, the process image, etc. are immediately overwritten with current data
from the primary CPU.
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF
Preparing the copying of the work memory
Primary CPU
Commissioning
12.4 Operating and system states
Backup CPU
393