Siemens Simatic S7-1500R System Manual page 212

④
Copying the work memory
The backup CPU establishes connections to the IO devices in the PROFINET ring.
At the next cycle control point, the primary CPU saves a consistent snapshot of the contents
of its work memory and certain system memory contents (process image, bit memory,
SIMATIC timer/counter functions, temporary local files and data block content).
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.
The display of the primary CPU indicates the operating state.
Table 9- 8
The backup CPU is busy accepting the data before it can also process the user program.
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA
Copying the work memory
Primary CPU
Commissioning
9.4 Operating and system states
Backup CPU
211