Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 262

Supplementary information
15.9 Communication via S7 connections
With this configuration and redundant operation, the fault-tolerant system is connected to the
standard system via bus1. 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 15-9
262
Example of linking standard and fault-tolerant systems in a redundant bus system
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB