Siemens SIMATIC S7-400H System Manual page 239

Response to failure
Duplex fiber-optic ring and bus system
Because S7 connections are used here (the connection ends at the CPU of the subsystem,
in this case CPUa1), an error in the fault-tolerant system (e.g. CPUa1 or CPa1) or in system
b (e.g. CP b) results in total failure of communication between the participating systems(see
previous 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 transfer is assumed to have been
successful. If there was an error message, the data transfer 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.
S7-400H
System Manual, 03/2012, A5E00267695-11
14.5 Communication via S7 connections
Communication
239