Siemens S7-1500 Function Manual page 18

Particularities of the retentive memory
Like for non-redundant CPUs, the retentive memory is a non-volatile memory. The retentive
memory saves a limited amount of data in the event of power failure.
Each of the CPUs of the redundant system S7-1500R/H has its own retentive memory. If one
of the CPUs changes to POWER OFF and the second CPU is still in RUN, the retentive data
of the CPU in RUN continues to be updated. If the remaining CPU also goes from RUN to
POWER OFF, the retentive memory of this CPU contains the most up-to-date retentive data.
Note the following for when you switch the CPUs back to RUN afterwards.
Note
Characteristics of retentive data after a STOP or POWER OFF of both CPUs
After a STOP or POWER OFF of both CPUs, switch the CPU with the more up-to-date data
to RUN first. The more up-to-date data is located in the CPU that controlled the process
before STOP or POWER OFF. This procedure gives you certainty that you are using the
most up-to-date retentive data. The precondition for this is that you did not change any data
using STEP 7 or the HMI while the CPUs were in STOP.
Redundancy ID
Unlike in a non-redundant CPU, an additional memory object is contained in the retentive
memory of the respective redundant CPU. Each CPU saves its respective redundancy ID in
this memory object. The redundancy IDs can assume the values 1 and 2. Different
redundancy IDs are required for redundant operation in order to clearly identify the CPUs
and assign the project data to the respective CPUs. For additional information on
redundancy IDs of the CPUs, refer to the Redundant System S7-1500R/H
(https://support.industry.siemens.com/cs/ww/en/view/109754833) system manual.
You can find an overview of the retentive behavior of the individual memory objects in
section Retentive behavior of the memory objects (Page 30).
Structure and Use of the CPU Memory
Function Manual, 10/2018, A5E03461664-AC
Memory areas and retentive memory
2.1 Memory areas
17