Siemens SIMATIC S5 Manual page 85

S5-115F Manual
OB 13: Time-driven program processing
The CPU 942F has OB 13 for time-driven program processing. The operating system processes the
time-interrupt OB at intervals specified by the user. The time-interrupt OB can interrupt the cyclic
control program but not the program servicing a process interrupt. If a time-interrupt OB is not
programmed, cyclic program scanning continues.
• Setting the call interval:
The call interval can be set as a multiple of 10 msec. (similar to setting scan monitoring). This
means times from 100 msec. to approximately 10 min. can be programmed. The default is
0 msec.
The call is suppressed by writing the value "0" into the appropriate system data word when
initializing the operating system.
• Interrupt points:
After every 20 msec. (max.), a program is called in the operating system, which checks to see if
a time-interrupt OB has to be processed. If a time interrupt request is pending in both sub-
units, the program branches to OB 13.
If the 20 msec. response time is also to be observed in the user program, an FB 254 must be
called after a maximum of 20 msec. user program run time. This FB synchronizes processing of
the OBs for servicing the time interrupt in both subunits ( 6.1.6).
The OB 13 call may be delayed:
Delay interval between FB 254 calls. The intervals between calls can vary:
Difference ± interval between FB 254 calls.
Example
Desired instant
100 msec.
200 msec.
100 msec.
200 msec.
When calculating the timing accuracy, note the following:
- The maximum absolute error is independent of the length of the interval involved.
The relative accuracy improves with increasing length of the interval involved.
Example
Interval
100 to 200 msec.
100 to 500 msec.
• Disabling the time-interrupt OB call:
The IA operation disables the call of all time-interrupt OBs. The RA operation enables it. One
call request can be stored during call disable. If no time/process interrupt processing is re-
quired, it is useful to program the IA operation, since this speeds up program scanning.
• Nesting depth:
Even when an OB for servicing a time interrupt is processed, the general block nesting depth
of 16 must not be exceeded.
EWA 4NEB 811 6149-02
Call Interval between calls
120 msec.
200 msec.
100 msec.
220 msec.
Length of interval
100 msec.
400 msec.
80 msec.
120 msec.
Absolute difference
40 msec.
40 msec.
Introduction to STEP 5
Difference
40 msec.
Relative error
40 %
10 %
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