Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 278

Supplementary information
15.10 Communication via fault-tolerant S7 connections
Response to failure
Double errors in the fault-tolerant system, e.g., CPUa1 and CPa2, or failure of the PC station
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 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 STEP 7
functions on that CPU.
If this CPU fails, communication between the ES and the fault-tolerant system is no longer
possible.
278
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB