Cpu Overload Behavior - Siemens SIMATIC S7-1500 Automation System System Manual

Basics of program processing

13.2 CPU overload behavior

13.2 CPU overload behavior
Principle of CPU overload behavior
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.
When an event occurs, the execution of the associated OB is triggered. Depending on the
OB priority and the current processor load, in most cases there is a time delay before the OB
is executed. The same event can therefore occur once or several times before the OB
belonging to the preceding event is processed. Such a situation is handled by the CPU as
follows: In the order of their occurrence, the operating system positions the events into the
queue associated with their priority.
To control temporary overload situations, you can limit the number of pending events that
are linked from the same source. The next event is discarded as soon as the maximum
number of pending start events 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.
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 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, for example, the maximum number of start events is reached in the queue for a cyclic
interrupt OB, 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 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. Additional diagnostic 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.
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S7-1500 Automation System
System Manual, 01/2013, A5E03461182-01