Siemens SIMATIC S7-400H System Manual page 153

The status of SFB instances of S7 communication contained in modified data blocks is
restored to the status prior to their initial call.
Note
When changing over to a CPU with modified configuration, the size of load memories in the
master and reserve may be different.
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.
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.
For information on changing the type of memory or on load memory expansions, refer to
section Failure and replacement of components during operation (Page 265).
When you initiate a link-up and update with the "Switchover to CPU with expanded memory
configuration" option in STEP 7, the system reacts as follows with respect to the handling of
memory contents.
RAM and load memory
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.
S7-400H
System Manual, 03/2012, A5E00267695-11
Link-up and update
12.3 Link-up and update sequence
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