Siemens SIMATIC S7-400H Manual page 80

Using I/O on the S7-400H
Note
If the DP master interface module can recognize failure of the complete DP master
system (e.g. in the case of a short circuit), only this event is reported ("Master
system failure coming" W#16#39C3). The operating system then no longer reports
individual station failures. This allows the switch-over process between the active
and slave channel to be accelerated.
Clearance of routing connections upon switchover of the active channel
When the active channel is switched over, all the routing connections to modules
and devices located downstream of the IM 153-2(FO) or IM 157 are cleared. You
have to re-establish these connections in the user program.
Duration of switch-over of the active channel
The maximum switch-over time is
DP error recognition time + DP switch-over time + switch-over time of DP slave interface
module
You can determine the first two addends from the bus parameters of your DP
master system in STEP 7. You define the last addend from the manuals of the DP
slave interface modules concerned ( Distributed I/O ET 200M and DP/PA bus
connection ).
Note
The above calculation also includes the processing time in OB 70 or OB 86. Note
that the processing for a DP station must take no more than 1 ms. If more
extensive processing is required, disconnect this from the direct processing of the
OBs mentioned.
Note that a change of signal can only be detected by the CPU if the signal duration
is greater than the specified switch-over time.
With a switch-over of the complete DP master system, the switch-over time of the
slowest component applies to all DP components. The DP/PA link normally
defines the transfer time and the associated minimum signal duration. We
therefore recommend you to connect DP/PA links to a separate DP master
system.
Switch-over of the active channel on link-up and update
On link-up and update with master/standby switch-over (see section 5.2.1) the
active and slave channels are switched over in all the stations of the switched I/O.
No OBs are started in this instance.
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S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04