Communications Via S7 Connections – One-Sided Mode - Siemens SIMATIC S7-400H Manual

Communication Functions
Note
The communication functions START and STOP act on exactly one CPU or on all
CPUs of the fault-tolerant system (for more details refer to the Reference Manual
System Software for S7-300/400, System and Standard Functions).
8.5.1 Communications via S7 Connections – One-Sided Mode
Availability
Availability is similarly enhanced by using a redundant system bus for
communications from a fault-tolerant system to a standard system.
If the system bus is configured as an optical two-fiber ring, communications
between the systems involved continue to exist in the event of the two-fiber optic
cable rupturing. The systems then communicate as if they were connected to a
bus system (line), see Figure 8-9.
When fault-tolerant systems and standard systems are interconnected, the
availability of communications cannot be enhanced by means of a twin electrical
bus system. In order to be in a position to use the second bus system as a
redundant bus, you have to use a second S7 connection, which has to be
managed accordingly in the user program (refer to Figure 8-10).
8-16
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07