Siemens SIMATIC S7-400H System Manual page 148

Link-up and update
12.3 Link-up and update sequence
For information on the required steps based on the scenarios described above (alteration of
the hardware configuration, or of the type of load memory), refer to section Failure and
replacement of components during operation (Page 265).
Note
Even though you have not modified the hardware configuration or the type of load memory
on the reserve CPU, there is nevertheless a master/reserve changeover and the previous
master CPU changes to STOP.
Switchover to CPU with expanded memory configuration
You may have expanded the load memory on the reserve CPU. The memory media for
storing load memory must be identical, i.e. either RAM cards or FLASH cards. The contents
of FLASH cards must be identical.
During the link-up, the user program blocks (OBs, FCs, FBs, DBs, SDBs) of the master are
transferred from the load memory and work memory to the reserve. Exception: If the load
memory modules are FLASH cards, the system only transfers the blocks from work memory.
For information on changing the type of memory or on load memory expansions, refer to
section Changing the CPU memory configuration (Page 329).
NOTICE
Assuming you have implemented a different type of load memory or operating system on
the reserve CPU, this CPU does not go into RUN, but rather returns to STOP and writes a
corresponding message to the diagnostic buffer.
Assuming you have not expanded load memory on the reserve CPU, this CPU does not go
into RUN, but rather returns to STOP and writes a corresponding message to the
diagnostic buffer.
The system does not perform a master/reserve changeover, and the previous master CPU
remains in RUN.
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System Manual, 03/2012, A5E00267695-11
S7-400H