Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 267

Figure 15-14 Example of connecting a fault-tolerant system to a single-channel third-party system via
Response to failure
Double errors in the fault-tolerant system (i.e., CPUa1 and IM 153) and a single fault in the
third-party system lead to a total failure of communication between the systems involved.
This can be seen in the previous figure.
The point-to-point CP can also be inserted centrally in "Fault-tolerant system a". However, in
this configuration even the failure of the CPU, for example, will cause a total failure of
communication.
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB
PROFINET IO with system redundancy
Supplementary information
15.9 Communication via S7 connections
267