Siemens SIMATIC S7-400H System Manual page 110

Link-up and update
9.4 Time monitoring
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. disable 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
The following applies to the calculation of the minimum I/O retention time:
● with central I/O: T
● with distributed I/O: T
When using central and distributed I/Os, the resultant minimum I/O retention time is:
T
PH
The following figure shows the correlation between the minimum I/O hold time and the
maximum disable time for priority classes > 15.
Figure 9-5
Note the following condition:
50 ms + minimum I/O hold time ≤ (maximum disable time for priority classes > 15)
It follows that a high minimum I/O hold time can determine the maximum disable time for
priority classes > 15.
110
= 30 ms
PH
PH
where T = maximum target rotation time
TRmax
all DP master systems of the fault-tolerant station
= MAX (30 ms, 3 x T
TRmax
Correlation between the minimum I/O hold time and the maximum disable time for
priority classes > 15
)
PH
= 3 x T
TRmax
)
System Manual, 09/2007, A5E00267695-03
S7-400H