Siemens SIMATIC S7 F, Manual

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Safety Mechanisms
Fail-Safe Systems
A5E00085588-03
3-7
3.7 Fail-Safe User Times
Time values generated in the Safety Program with the F_TP, F_TON and F_TOFF
blocks are monitored by means of safety mechanisms of the CPU. To do this, two
mutually independent time counters are compared. As long as the discrepancy
between the two counters is less than 10 ms within a time period of 50 s, the time
is considered correct. If the discrepancy is larger, a hardware fault is assumed and
the Safety Program is disabled.
The maximum inaccuracy of user times can be calculated on the basis of the
following table:
User Times From To Max. Inaccuracy
10 ms 50 s ± 5 ms
> 50 s 100 s ± 10 ms
... ... ...
> n* 50 s (n+1)*50 s ± (n+1)*5 ms
The actual inaccuracy is considerably less than this. Also note the time inaccuracy
that occurs due to processing in the cyclic interrupt scan cycle.