Siemens SIMATIC S7-400H System Manual page 164

Communication
11.5 Communications via fault-tolerant S7 connections
Note
The number of connection resources required on the CPs depends on the network you are
using.
If you implement an optical two-fiber ring (see figure below), two connection resources are
required per CP. In contrast, only one connection resource is required per CP if a double
electrical network (see figure after next) is used.
Figure 11-3
Figure 11-4
164
Example of redundancy with fault-tolerant system and redundant ring
Example of redundancy with fault-tolerant system and redundant bus system
System Manual, 09/2007, A5E00267695-03
S7-400H