Siemens Simatic S7-1500R System Manual page 196

Note
Role change during loading
Beware of a possible role change between primary and backup CPU shortly before, during
or after loading.
A role change can occur during loading if the primary CPU fails (power failure, hardware
defect) or is in STOP and:
• You switch the backup CPU to RUN operating state during this time using the mode
selector, the display or a communication command.
or
• You POWER ON the backup CPU during this time and the configured startup behavior in
STEP 7 is that the CPU automatically switches to RUN after POWER ON.
In the event of a role change, the new primary CPU starts up with the old project data. The
new project data in the backup CPU is overwritten with the old project data during
synchronization of the two CPUs for redundant operation.
Downloading project data to the backup CPU
By default, the project data is downloaded to the primary CPU. In the SYNCUP system state,
the project data is then transferred from the primary CPU to the backup CPU.
You can also download the project data to the backup CPU. This makes sense if the backup
CPU is to be primary CPU with its project data upon a restart.
Basic procedure:
1. Download the project data to the backup CPU. The primary CPU continues to control the
process.
2. Switch the primary CPU to the STOP operating state after loading.
3. Switch the backup CPU to the RUN operating state. It becomes the new primary CPU
and controls the process on its own with the newly loaded user program in the RUN-Solo
system state.
4. Switch the new backup CPU to the RUN operating state.
Result: The system state is RUN-Redundant.
Note
Please note the following during download to the backup CPU:
If the project uses retentive data, the backup CPU runs with its data which may be outdated.
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA
Commissioning
9.3 Commissioning procedure
195