Siemens SIMATIC S7-400H System Manual page 238

Communication
14.5 Communication via S7 connections
With this configuration and redundant operation, the fault-tolerant system is connected via
bus1 with the standard system. This applies no matter which CPU is the master CPU.
For linked fault-tolerant and standard systems, the availability of communication cannot be
improved by means of a dual electrical bus system. To be able to use the second bus
system as redundancy, a second S7 connection must be used and managed accordingly in
the user program (see next figure).
Figure 14-8
On a plant bus configured as duplex fiber-optic ring, communication between the partner
systems is maintained if the duplex fiber-optic cable breaks. The systems then communicate
as if they were connected to a bus system (linear structure); see following figure.
Figure 14-9
238
Example of linking standard and fault-tolerant systems in a redundant bus system
Example of linking of standard and fault-tolerant systems in a redundant ring
System Manual, 03/2012, A5E00267695-11
S7-400H