Siemens SIMATIC S7-400H System Manual page 165

Figure 11-5
Reaction to failure
If a two-fiber ring is used, only a double error within a fault-tolerant system (e.g. CPUa1 and
CPa2 in one system) leads to total failure of communication between the systems involved
(see first figure).
If a double error (e.g. CPUa1 and CPb2) occurs in the first case of a redundant electrical bus
system (see second figure), this results in a total failure of communication between the
systems involved.
In the case of a redundant electrical bus system with CP redundancy (see third figure), only
a double error within a fault-tolerant system (e.g. CPUa1 and CPUa2) or a triple error (e.g.
CPUa1, CPa22 and bus2) will result in a total failure of communication between the systems
involved.
Fault-tolerant S7 connections
Any disruption of subconnections while communication requests are active over fault-tolerant
S7 connections leads to extended delay times.
S7-400H
System Manual, 09/2007, A5E00267695-03
Example of fault-tolerant system with additional CP redundancy
11.5 Communications via fault-tolerant S7 connections
Communication
165