Siemens SIMATIC S7-1500 System Manual page 171

Basics of program processing
14.2 CPU overload behavior
Discarding similar events or fetching them later
Below, the term "similar events" refers to events from a single source, such as start events
for a specific cyclic interrupt OB.
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.
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. If the
value of the "Events to be queued" parameter is low, this ensures that an overload situation
is mitigated rather than aggravated.
If the maximum number of start events is reached in the queue for a cyclic interrupt OB
(Cyclic interrupt), for example, each additional start event is only counted and subsequently
discarded. During the next scheduled execution of the OB, the number of discarded start
events is made available (in the start information) in the "Event_Count" input parameter. You
can then react appropriately to the overload situation. The CPU then sets the counter for lost
events to zero.
If the CPU, for example, first discards a start event for a cyclic interrupt OB, its further
behavior depends on the OB parameter "Report event overrun into diagnostics buffer": If the
check box is selected, the CPU enters the event DW#16#0002:3507 once in the diagnostics
buffer for the overload situation at this event source. Additional diagnostics buffer entries of
the event DW#16#0002:3507 that refer to this event source are suppressed until all events
from this source have been post-processed.
Threshold mechanism for time error OB request
The cyclic interrupt OB parameter "Enable time error" is used to specify whether the time
error interrupt should be called when a specific overload level is reached for similar events.
You can find the OB parameter "Enable time error" in the properties of the OB in the
"Attributes" category.
If yes (check box selected), you use the OB parameter "Event threshold for time error" to
specify the number of similar events in the queue at which the time error interrupt should be
called. If this parameter has the value 1, for example, the event DW#16#0002:3502 is
entered once in the diagnostics buffer and the time error interrupt is requested when the
second event occurs. Additional diagnostics buffer entries of the event DW#16#0002:3502
that refer to this event source are suppressed until all events from this source have been
post-processed.
In the event of an overload, you therefore have the option of programming a reaction well
before the limit is reached for similar events and thus before the events are discarded.
The following value range applies to the "Event threshold for time error" parameter:
1 ≤ "Event threshold for time error" ≤ "Events to be queued".
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S7-1500 Automation System
System Manual, 02/2014, A5E03461182-AB