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The following figure shows the correlation between the minimum I/O retention time and the
maximum inhibit time for priority classes > 15.
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
● In the final update phase, all OBs are either delayed or inhibited. To avoid any
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 10/2013, A5E32631667-AA
Correlation between the minimum I/O retention time and the maximum inhibit time for
priority classes > 15
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.
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.
8.3 Time monitoring
)
P15
Link-up and update
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