Siemens SIMATIC S7-400H Manual page 225

Notice
The following should be taken into consideration when using redundant I/O
modules that you have installed as one–way modules on the user level (see
Chapter 7.5):
During link–up and synchronization following a change to the system, the I/O
modules of the master CPU used up until this point may disappear from the
process image for a short time before the (changed) I/O modules of the "new"
master CPU are completely entered into the process image.
This may lead to the false impression during the initial updating of the process
image following a change to the system, that the redundant I/O modules are
completely down or there are redundant I/O modules. The redundancy status can
therefore only be correctly evaluated when the process image has been
completely updated.
This phenomenon does not occur with modules that have been released for
redundant operation (see Chapter 7.4).
Preparations
To minimize the time during which the fault-tolerant system has to run in single
mode, you should perform the following steps before making the hardware
change:
• Make sure that the memory components of the CPUs are sufficient for the new
configuration and the new user program. If necessary, first expand the memory
components (see Section 11.7).
• Make sure that modules that are plugged in but not configured do not have any
effect on the process.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Modifying the System During Operation
11-5