Siemens Simatic S7-1500 Function Manual page 18

Events to be queued
The OB parameter "Events to be queued" is used to specify how many similar events the
operating system places in the associated queue and therefore post-processes. If this
parameter has the value 1, for example, exactly one event is stored temporarily.
If the maximum number of similar start events is reached in the queue, each additional start
event is only counted and subsequently discarded. During the next scheduled processing of
the event, the CPU provides the number of discarded start events in the "Event_Count" input
parameter (in the start information). You can then react appropriately to the overload
situation. The CPU then resets the counter for lost events to zero.
Note
Post-processing of cyclic events is often not desirable, as this can lead to an overload with
OBs of the same or lower priority. Therefore, it is generally advantageous to discard similar
events and to react to the overload situation during the next scheduled OB processing. A low
value of the "Events to be queued" parameter mitigates an overload situation.
To ensure that the CPU processes the OB of at least one queued event, the minimum
number of events to be queued is "1". The maximum number of events that can be queued
is "12".
Report event overflow into diagnostic buffer
If the CPU first discards a start event of a cyclic interrupt OB, for example, its further
behavior depends on the OB parameter "Report event overflow into diagnostic buffer". If you
have selected the check box, the CPU enters the event DW#16#0002:3507 in the diagnostic
buffer for the overload situation at this event source. If an overload situation occurs again
(overflow counter changes from 0 to 1), another diagnostic buffer entry is made at the next
OB end.
Enable time error
The cyclic interrupt OB parameter "Enable time error" is used to specify whether the CPU is
to call a time error OB when a specific overload level is reached for similar events. You use
the OB parameter "Enable time error" to program a reaction to an overload before the limit
for similar events is reached. The reaction occurs before the CPU discards similar events.
By default, the "Enable time error" parameter is not set.
Event threshold for time error
Select the "Enable time error" check box to enable the "Event threshold for time error" OB
parameter. You use the "Event threshold for time error" OB parameter to specify how many
similar events in the queue are permitted before the CPU calls a time error OB.
The following value range applies to the "Event threshold for time error" parameter:
1 ≤ "Event threshold for time error" ≤ "Events to be queued".
Cycle and response times
Function Manual, 10/2018, A5E03461504-AD
Program execution
2.2 Overload behavior
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