Siemens Simantic ET200SP System Manual page 154

Basics of program execution
7.2 CPU overload behavior
Discarding similar events or handling them later
Below, the term "similar events" refers to events from a single source, such as triggers 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
The post-processing of cyclic events is often undesirable, 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 defused 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 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 sets the counter for lost
events to zero.
If the CPU discards a start event for a cyclic interrupt OB the first time, its further behavior
depends on the OB parameter "Report event overflow 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. The CPU suppresses additional diagnostics
buffer entries of the event DW#16#0002:3507 that refer to this event source 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 you enable the time error OB (option box set), you define, with the "Event threshold for
time error" OB parameter, the number of similar events in the queue that cause the user
program to call the time error OB. If this parameter has the value 1, for example, the CPU
enters the event DW#16#0002:3502 once in the diagnostics buffer and requests the time
error OB when the second event occurs. The CPU suppresses additional diagnostics buffer
entries of the event DW#16#0002:3502 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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Distributed I/O system
System Manual, 12/2016, A5E03576849-AG