Siemens SIMATIC S7-400H Manual page 65

In the final phase of the update all the OBs are delayed or blocked. To avoid the
max. blocking time for priority classes > 15 being unnecessarily extended as a
result of unfortunate symbolic programming, modify the time-critical I/O
components in a selected watchdog interrupt. This is particularly relevant in
the case of fail-safe user programs. You can specify this watchdog interrupt in
the configuration; it will then be executed again after the start of the max.
blocking time for priority classes > 15, but only if you have assigned it a priority
class > 15.
On link-up and update with master/standby switch-over (see section 5.2.1) then
on conclusion of the update the active communication channel in the switched
DP slaves must be switched over. This extends the time for which no valid
values can be read or output. The duration of this process is determined by
your hardware configuration.
The technological circumstances of your process give rise to requirements
in respect of how long the I/O update can be deferred. This is particularly
important in the case of time-monitored processes in fail-safe systems.
Note
Other particular circumstances when using fail-safe modules may be found in the
manuals under S7-400 F and S7-400 FH Programmable Controllers; S7-300
Fail-Safe Systems and Programmable Controllers; Fail-Safe Signal Modules . This
applies in particular to module-internal run times in fail-safe modules.
1. For each DP master system determine from the bus parameters in STEP 7
– T
TR
– DP switch-over time (referred to below as T
2. For each DP master system determine from the Technical Data for the
switched DP slaves
– the maximum switch-over time for the active communication channel
(referred to below as T
3. From the technological specifications for your system determine
– the maximum permissible time period for which your I/O modules are not
updated (referred to below as T
4. From your user program determine
– the scan time of the highest priority or selected (see above)
watchdog interrupt (T
– the run time of your program in this watchdog interrupt (T
5. For each DP master system this gives rise to
T (DP master system) = T
P15
1]
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
for the DP master system
SLAVE_UM
)
WA
+ T
)
DP_UM
).
).
PTO
– (2 T + T + T
PTO TR WA
+ T )
DP_UM SLAVE_UM
Link-up and Update
)
PROG
PROG
[
5-21