Siemens SIMATIC S7-400H System Manual page 176

Using I/Os in S7–400H
13.3 Using single-channel switched I/O
If one channel has already failed, and the remaining (active) channel also fails, then there is
a complete station failure. This starts OB 86 (event W#16#39C4).
Note
If the DP master interface module can detect failure of the entire DP master system (due to
short-circuit, for example), it reports only this event ("Master system failure entering state"
W#16#39C3). The operating system no longer reports individual station failures. This feature
can be used to accelerate the changeover between the active and passive channel.
Duration of a changeover of the active channel
The maximum changeover time is
DP error detection time + DP changeover time + changeover time of the DP slave interface
You can determine the first two values from the bus parameters of your DP master system in
STEP 7. You can obtain the last value from the manuals of the relevant DP slave interface
module
NOTICE
When using fail-safe modules, always set a monitoring time for each fail-safe module that is
longer than the changeover time of the active channel in the fault-tolerant system. If you
ignore this rule, you risk failure of the fail-safe modules during the changeover of the active
channel.
NOTICE
The above calculation also includes the processing time in OB 70 or OB 86. Make sure that
the processing time for a DP or PNIO station is no longer than 1 ms. In situations requiring
extensive processing, exclude this processing from direct execution of the OBs mentioned.
Note that the CPU can only detect a signal change if the signal duration is greater than the
specified changeover time.
When there is a changeover of the entire DP master system, the changeover time of the
slowest component applies to all DP components. A DP/PA link or Y link usually
determines the changeover time and the associated minimum signal duration. We therefore
recommend that you connect DP/PA and Y links to a separate DP master system.
When using fail-safe modules, always set a monitoring time for each fail-safe module that is
longer than the changeover time of the active channel in the fault-tolerant system. If you
ignore this, you risk failure of the fail-safe modules during the changeover of the active
channel.
Changeover of the active channel during link-up and updating
During link-up and update with master/standby changeover (see section Link-up sequence
(Page 147)), the active and passive channels are changed over on all stations of the
switched I/O. At the same time OB 72 is called.
176
(distributed I/O device ET 200M DP/PA bus link
or ).
System Manual, 03/2012, A5E00267695-11
S7-400H