Siemens SIMATIC S7-400H Manual page 191

Installing and starting the fault-tolerant system
We recommend you to perform the following steps when installing and starting up
the fault-tolerant system.
1. Save the configured fault-tolerant system to a flash memory card.
2. When installing the synchronization submodules, make sure the rack numbers
are set correctly.
3. Do not connect the synchronization submodules with fiber-optic cables.
4. Plug the flash memory card into the central processing unit concerned and
start it. Then the Stop LED flashes (reset request).
5. Perform a manual reset for both central processing units.
6. Connect the two central processing units with fiber-optic cable.
7. Start the two central processing units.
The fault-tolerant system then works in Redundant system mode.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Separate Operation
B-5