User Manual

PLC concepts made easy
4.3 Execution of the user program
Easy Book
56 Manual, 03/2014, A5E02486774-AF
Consider the following two cases where interrupt events trigger a cyclic OB and a time delay
OB. In both cases, the time delay OB (OB201) has no process image partition assignment
and executes at priority 4. The cyclic OB (OB200) has a process image partition assignment
of PIP1 and executes at priority 2. The following illustrations show the difference in execution
between non-interruptible and interruptible execution modes:
Figure 4-1 Case 1: Non-interruptible OB execution
Figure 4-2 Case 2: Interruptible OB execution
Note
If you configure the OB execution mode to be non
-interruptible, then a time error OB cannot
interrupt OBs other than program cycle OBs. Prior to V4.0 of the S7
-1200 CPU, a time error
OB could interrupt any executing OB. With V4.0, you must configure OB execution to be
interruptible if you want a time error OB (or any
other higher priority OB) to be able to
interrupt executing OBs that are not program cycle OBs.
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 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.