Siemens SIMATIC CPU 410 System Manual page 125

Figure 8-2
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.
Calculating the maximum inhibit time for priority classes > 15 (T
The maximum inhibit time for priority classes > 15 is determined by 4 main factors:
● As shown in Figure 12-2, all the contents of data blocks modified since last copied to the
standby CPU are once again transferred to the standby CPU on completion of the
update. The number and structure of the DBs you write to in the high-priority classes is a
decisive factor in the duration of this operation, and thus in the maximum inhibit time for
priority classes > 15. Relevant information is available in the remedies described below.
● In the final update phase, all OBs are either delayed or inhibited. To avoid any
unnecessary extension of the maximum inhibit time for priority classes > 15 due to
unfavorable programming, you should always process the time-critical I/O components in
a selected cyclic interrupt. This is particularly relevant in fail-safe user programs. You can
define this cyclic interrupt in your configuration. It is then executed again right after the
start of the maximum inhibit time for priority classes > 15, provided you have assigned it a
priority class > 15.
● In link-up and update operations with master/standby changeover (see section Link-up
sequence (Page 346)), you also need to changeover the active communication channel
on the switched DP slaves and switched IO devices on completion of the update. This
operation prolongs the time within which valid values can neither be read nor output. How
long this process takes is determined by your hardware configuration.
● The technological conditions in your process also decide how long an I/O update can be
delayed. This is particularly important in time-monitored processes in fail-safe systems.
Note
For details, refer to Manual
300 Automation Systems, Fail-safe Signal Modules
internal execution times of fail-safe modules.
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
Correlation between the minimum I/O retention time and the maximum inhibit time for
priority classes > 15
S7-400F and S7-400FH Automation Systems
Link-up and update
8.3 Time monitoring
)
P15
. This applies in particular to the
S7-
and Manual
125