Siemens SIMATIC S7-400H - Figure 12-5 Correlation between the Minimum I;O Retention Time and the Maximum Inhibit Time for

Link-up and update
12.4 Time monitoring
S7-400H
160 System Manual, 03/2012, A5E00267695-11
Configuration of the monitoring times
When configuring monitoring times, always make allowances for the following dependencies;
conformity is checked by STEP 7:
Max. cycle time extension
> max. communication delay
> (max. inhibit time for priority classes > 15)
> min. I/O retention time
If you have configured different monitoring times in the CPUs and perform a link-up and
update operation with master/standby changeover, the system always applies the higher of
the two values.
Calculating the minimum I/O retention time (T
PH
)
The following applies to the calculation of the minimum I/O retention time:
● With central I/O: T
PH
= 30 ms
● For distributed I/O (PROFIBUS DP): T
PH
= 3 x T
TRmax
Where T
TRmax
= maximum target rotation time
of all DP master systems of the fault-tolerant station
● For distributed I/O (PROFINET IO): T
PH
= T
wd_max
with T
wd_max
= maximum cyclic interrupt time (product of WD factor and update time) of a
switched device in all IO subsystems of the fault-tolerant station
When using central and distributed I/Os, the resultant minimum I/O retention time is:
T
PH
= MAX (30 ms, 3 x T
TRmax
, T
wd_max
)
The following figure shows the correlation between the minimum I/O retention time and the
maximum inhibit time for priority classes > 15.
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Figure 12-5 Correlation between the minimum I/O retention time and the maximum inhibit time for
priority classes > 15
Note the following condition:
50 ms + minimum I/O retention time ≤
(maximum inhibit time for priority classes > 15)
It follows that a high minimum I/O retention time can determine the maximum inhibit time for
priority classes > 15.