PROFIsafe 227
Calculating the watchdog time
It is not always easy to calculate the worst-case delay time of “black channel”
components. See AC500-S Safety User Manual (3ADR025091M0207 [English]) for a
proposed method of tracing the actual PROFIsafe cycle times in a real system.
You must then set F_WD_Time about 30% higher than the worst-case value in
variable tResponseTimeMS (in the AC500-S safety program) for the given safety
device.
If you use this approach for the FSO module, you can set the PROFIsafe cycle time
and the corresponding watchdog time F_WD_Time as short as possible for the given
system.
If the longest recorded PROFIsafe cycle time (minimum F_WD_Time) is, for example,
40 ms, a suitable value for F_WD_Time is:
F_WD_Time = 40 ms x 1.3 = 52 ms.
If you can calculate F_WD_Time instead, use the values given in the table on
page 224. DAT time for the FSO module is 50 ms and the FENA-21 module adds
3 ms to both Bus times.
For example, if HAT is 6 ms and Bus times are 4 ms until FENA, F_WD_Time is:
F_WD_Time = (50 ms + (3 ms + 4 ms) + 6 ms + (4 ms + 3 ms)) x 1.3 = 91 ms.
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