Siemens SIMATIC S7-400H Manual page 52

Link-up and Update
Note
If you have not changed either the hardware configuration or the type of load
memory on the standby CPU a master/standby switch-over is still carried out and
the previous master CPU switches to STOP mode.
Switch to CPU with extended memory configuration
You may have made the following memory modifications on the standby CPU:
– enlargement of the main memory and/or
– enlargement of the load memory – in so doing you must have load memory
module of the same type, i.e. either RAM cards or FLASH cards; in the case
of FLASH cards the content must agree.
On link-up the user program blocks (OBs, FCs, FBs, DBs, SDBs) of the master
are copied from the load memory and the main memory to the standby (exception:
if the load memory modules are FLASH cards only the blocks from the main
memory are copied).
The steps to be performed in the above-mentioned scenarios (enlargement of the
main memory, enlargement of the load memory) are described in Chapter 10.
Note
If you have changed the type of load memory or the operating system on the
standby CPU this will not switch to RUN mode but instead will relapse into STOP
mode with a corresponding diagnostics buffer entry.
If you have enlarged neither the main memory nor the load memory on the
standby CPU this will not switch to RUN mode but instead will relapse into STOP
mode with a corresponding diagnostics buffer entry.
No master/standby switch-over will take place and the CPU that has been the
master CPU up to that point will remain in RUN mode.
Switch to CPU with modified operating system
On the standby CPU you have replaced the operating system with a newer
operating system to match the operating system of the master CPU.
On link-up the user program blocks (OBs, FCs, FBs, DBs, SDBs) of the master
are copied from the load memory and the main memory to the standby (exception:
if the load memory modules are FLASH cards only the blocks from the main
memory are copied).
The steps to be performed when replacing the operating system are described in
Chapter 10.
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S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04