Siemens SIMATIC S7-400H Manual page 183

Note
Fault-tolerant connections occupy two connection resources on CP b1 for the
redundant connections. One connection resource each is assigned to CP a1 and
CP a2.
Redundancy
block diagram
Figure 8-6
Response to failure
Double faults in the fault–tolerant system (in other words, CPUa1 and CPa 2) and
a single fault in the standard system (CPb1) result in a complete failure of
communication between the systems involved (refer to Figure 8-6).
Fault-Tolerant S7 Connections
Interruptions of a partial connection result in a prolonging of the runtime for
ongoing communication jobs via fault-tolerant S7 connections.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Fault-tolerant system a
CPU CP
a1 a1
OLM
Fault-tolerant
system a
CPUa1
CPa1
CPUa2 CPa2
Example of redundancy with fault-tolerant system and fault-tolerant CPU
Communication Functions
Standard system with fault-tolerant CPU
CPU CP
b1 b1
OLM
OLM
Standard system with fault-tolerant CPU
Bus 1
CPb1 CPUb1
Bus 2
System bus as opti-
cal two-fiber ring
8-13