Siemens SIMATIC S7 System Manual page 44

PLC concepts
3.1 Execution of the user program
Each CPU event has an associated priority, and the event priorities are classified into priority
groups. The following table summarizes the queue depths, priority groups and priorities for
the supported CPU events.
Note
You cannot change the priority or the priority group assignments or the queue depths.
In general, events are serviced in order of priority (highest priority first). Events of the same
priority are serviced on a "first-come, first-served" basis.
Event Type (OB)
Program Cycle
Startup
Time Delay
Cyclic
Edges
HSC
Diagnostic Error
Time Error event/
MaxCycle time
event
2xMaxCycle time
event
Special cases for the startup event
1


After the execution of an OB has started, processing of the OB cannot be interrupted by the
occurrence of another event from the same or lower priority group. Such events are queued
for later processing, allowing the current OB to finish.
However, an event from a higher priority group will interrupt the current OB, and the CPU
then executes the OB for the higher-priority event. After the higher-priority OB finishes, the
CPU executes the OBs for any other events queued in this higher priority group, based on
the priority within that group. When no other events are pending (queued) in this higher
44
1 program cycle event
Multiple OBs allowed
1 startup event
Multiple OBs allowed
4 time delay events
1 OB per event
4 cyclic events
1 OB per event
16 rising edge events
16 falling edge events
1 OB per event
6 CV = PV events
6 direction changed events
6 external reset events
1 OB per event
1 event
1 time error event
1 MaxCycle time event
1 2xMaxCycle time event
The startup event and the program cycle event will never occur at the same time because the
startup event will run to completion before the program cycle event will be started (controlled by
the operating system).
No events are allowed to interrupt the startup event. Events that occur during the startup event
are instead queued for later processing after the startup event is finished.
Quantity
1 (default)
200 or greater
100 (default)
1
200 or greater
200 or greater
200 or greater
200 or greater
200 or greater
82 only
80 only
No OB called
Valid OB Queue
Numbers Depth
1
1
8
8
32
16
8
8
-
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
Priority Priority
Group
1 1
1
2 3
4
5
6
9
3 26
3 27