Siemens SIMATIC S7-400H System Manual page 35

Synchronization modules
The synchronization modules are used to link the two CPUs. They are installed in the CPUs
and interconnected by means of fiber-optic cables.
There are two types of synchronization modules: one for distances up to 10 meters, and one
for distances up to 10 km between the CPUs.
A fault-tolerant system requires 4 synchronization modules of the same type. For more
information on synchronization modules, refer to section Synchronization modules for
S7-400H (Page 341).
Fiber-optic cable
The fiber-optic cables are used to interconnect the synchronization modules for the
redundant link between the two CPUs. They interconnect the upper and lower
synchronization modules in pairs.
You will find the specifications of fiber-optic cables suitable for use in an S7-400H in
section Selecting fiber-optic cables (Page 349).
S7-400H
System Manual, 03/2012, A5E00267695-11
S7-400H setup options
3.3 The S7–400H basic system
35