Siemens SIMATIC S7-400H System Manual page 168

Communication
11.5 Communications via fault-tolerant S7 connections
Figure 11-8
Reaction to failure
Double errors in the fault-tolerant system (in other words, CPUa1 and CPa2) and the failure
of the PC result in a total failure of communication between the systems involved (see
previous figures).
PC / PG as engineering system (ES)
To be able to use a PC as an engineering system, you need to configure it under its name as
a PC station in HW Config. The ES is assigned to a CPU and is capable of executing the
STEP 7 functions on that CPU.
If the CPU fails, communication between the ES and the fault-tolerant system is also no
longer possible.
168
Example of redundancy with a fault-tolerant system, redundant bus system, and CP
redundancy on PC
System Manual, 09/2007, A5E00267695-03
S7-400H