ProSoft Technology PTQ-PDPMV1 - Manual (Page 221)

PTQ-PDPMV1 ♦ Quantum Platform Hot Standby Support
PROFIBUS DP Master Network Interface Module for Quantum User Manual
ProSoft Technology, Inc. Page 221 of 306
August 12, 2014
Determining the Parameter TWD
The watchdog time for the value above is a number based on experience, in
other words one that usually works. If it turns out that the watchdog time is too
long (slaves are not switching into the secure state quickly enough) or too short
(slaves are switching into the secure state without the Master ever having failed)
this must be factored into the calculation. Depending on the configuration tool
you are using, you can
 set parameters for only one watchdog time TWD for the entire PROFIBUS
system. In this case, the greatest delay time must be used as the basis for
determining TWD.
 set parameters for a watchdog time TWD for each individual slave.
The time TWD that is set (parameter) must be greater than the longest delay
time TV_max that will occur. This is composed of a number of elements as
follows:
TV_max = TCycle_DP + TCycle_PA_channel
where:
TCycle_PA_channel = the cycle time of the PROFIBUS PA channel
TCycle_DP = Cycle time of the PROFIBUS DP
Note: Pepperl+Fuchs recommends three times the PROFIBUS PA cycle time.
7.4.3 Details for calculating the TWD parameter
The PA cycle time TCycle_PA_channel depends on
1 The number n of stations on a channel
2
The effective data length LΣ (average of the total of input and output data of
all devices [number of bytes (unit less)]):
Cycle time can be calculated in an approximate manner as
TCycle_PA_channel = n * (0.256 ms * LΣ + 12ms) + 40 ms
Note: For more information on calculating cycle time and other related data for the SK1 or SK2,
refer to PEPPERL & FUCHS, Instructions Manual Segment Coupler SK1 and SK2.
The above information references PEPPERL & FUCHS, Instructions Manual Segment Coupler
SK1 and SK2. For additional information about the SK1 or SK2, please contact PEPPERL &
FUCHS.