Siemens S7-1200 System Manual page 96

PLC concepts
4.1 Execution of the user program
Understanding event execution priorities and queuing
The number of pending (queued) events from a single source is limited, using a different
queue for each event type. Upon reaching the limit of pending events for a given event type,
the next event is lost. Refer to the topic about time error interrupt OBs (Page 88) for more
information regarding queue overflows.
Each CPU event has an associated priority. In general, the CPU services events in order of
priority (highest priority first). The CPU services events of the same priority on a "first-come,
first-served" basis.
Table 4- 13 OB events
Event
Program cycle
Startup
Time delay
Cyclic interrupt
Hardware interrupt
Time error
Diagnostic error
Pull or plug of modules
Rack or station failure
Time of day
Status
Update
Profile
The startup event and the program cycle event never occur at the same time because the startup event runs to
1
completion before the program cycle event starts.
You can have more than 50 hardware interrupt event OBs if you use the DETACH and ATTACH instructions.
2
You can configure the CPU to stay in RUN if the scan cycle exceeds the maximum scan cycle time or you can use the
3
RE_TRIGR instruction to reset the cycle time. However, the CPU goes to STOP mode the second time that one scan
cycle exceeds the maximum scan cycle time.
The priority for a new V4.0 CPU is 22. If you exchange a V3.0 CPU for a V4.0 CPU, the priority is 26, the priority that
4
was in effect for V3.0. In either case, the priority field is editable and you can set the priority to any value in the range 22
to 26.
Refer to the topic "Exchanging a V3.0 CPU for a V4.0 CPU (Page 995)" for more details.
96
Quantity allowed
1 program cycle event
Multiple OBs allowed
1 startup event
1
Multiple OBs allowed
Up to 4 time events
1 OB per event
Up to 4 events
1 OB per event
Up to 50 hardware interrupt events
1 OB per event, but you can use the same OB for multiple
events
1 event (only if configured)
1 event (only if configured)
1 event
1 event
Up to 2 events
1 event
1 event
1 event
2
3
System Manual, 03/2014, A5E02486680-AG
Default OB priority
1
4
1
4
3
8
18
18
22 or 26
4
5
6
6
2
4
4
4
S7-1200 Programmable controller