Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 263

If the plant bus is configured as a duplex fiber-optic ring, the communication of the systems
involved is maintained if a break of the two-fiber fiber-optic cable occurs. The systems then
communicate as if they were connected to a bus system (linear structure); see following
figure.
Figure 15-10 Example of linking of standard and fault-tolerant systems in a redundant ring
Response to failure
Duplex fiber-optic ring and bus system
S7 connections are used here in which the connection ends on the CPU of the subsystem,
here CPUa1. For this reason, an error in the fault-tolerant system, e.g., CPUa1 or CPa1, as
well as an error in system b, e.g., CP b, will result in a total failure of the communication
between the two systems involved. This can be seen in the preceding figures.
There are no bus system-specific differences in the response to failure.
Linking standard and fault-tolerant systems
Driver block "S7H4_BSR": You can link a fault-tolerant system to an S7-400 / S7-300 using
the "S7H4_BSR" driver block. For more information, contact Siemens by e–mail:
function.blocks.industry @siemens.com
Alternative: SFB 15 "PUT" and SFB 14 "GET" in the fault-tolerant system: As an alternative,
use two SFB 15 "PUT" blocks over two standard connections. First call the first block. If
there was no error message when the block executed, the transmission is considered to
have been successful. If there was an error message, the data transmission is repeated via
the second block. If a connection cancelation is detected later, the data is also transferred
again to exclude possible information losses. You can use the same method with an SFB 14
"GET".
If possible, use the mechanisms of S7 communication for communication.
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB
Supplementary information
15.9 Communication via S7 connections
263