Cpu Overload Behavior - Siemens Simatic ET200pro Operating Instruction

5.2

CPU overload behavior

Requirements
For the event scenarios considered in the following section, it is assumed that you have
assigned an OB to each event source and that these OBs have the same priority. The
second condition, in particular, is only for the sake of a simplified representation.
Principle of CPU overload behavior
An occurring event triggers the execution of the associated OB. Depending on the OB
priority and the current processor load, a time delay may occur before the OB is executed
when there is an overload. The same event can therefore occur once or several times before
the user program processes the OB belonging to the preceding event. The CPU treats such
a situation as follows: The operating system queues the events in the queue associated with
their priority in the order of their occurrence.
To control temporary overload situations, you can limit the number of queued events that
originate from the same source. The next event is discarded as soon as the maximum
number of pending triggers of a specific cyclic interrupt OB, for example, is reached.
An overload occurs when events which originate from the same source occur faster than
they can be processed by the CPU.
More detailed information is provided in the following sections.
Discarding similar events or fetching 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
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 triggers is reached in the queue for a cyclic interrupt OB (Cyclic
interrupt), for example, each additional trigger is only counted and subsequently discarded.
During the next scheduled execution of the OB, the CPU provides the number of discarded
triggers 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, for example, first discards a trigger for a cyclic interrupt OB, its further behavior
depends on the OB parameter "Report event overflow into diagnostic 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 until all events from this source have been
post-processed.
CPU 1516pro-2 PN (6ES7516-2PN00-0AB0)
Operating Instructions, 09/2016, A5E35873416-AA
Basics of program execution
5.2 CPU overload behavior
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