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FieldConnex® Basic Segment Coupler
Commissioning
The T
WD
is usually entered via the configuration tool. The watchdog setting in the
DP/PA gateway is transparent. In some configuration tools, the response monitor is preset once
for the PROFIBUS master. In other tools, the response monitor is preset individually for each
PROFIBUS PA participant. The response monitor value does not change in any way.
Many tools calculate the T
WD
automatically with a corresponding baud rate based on the cycle
time of the master.
If the baud rates at the PROFIBUS DP end are high (e.g., 1.5 MB/s), the cycle times of the PA
end are up to 300 x longer. If a PROFIBUS PA device is directly programmed with a T
WD
calculated at a higher master baud rate (DP), this is usually shorter than the PA cycle and the
device does not exchange data.
In order to ensure that the DP/PA gateway operates reliably, it is recommended that you set the
following bus parameters:
■
Program only one T
WD
for the entire PROFIBUS system. Determine the longest delay
period in order to define the T
WD
.
■
Program one T
WD
for each individual slave.
The preset (programmed) time T
WD
must be greater than the maximum occurring delay period
T
V_max
. This is determined as follows:
T
V_max
= T
cycle_DP
+ T
cycle_PA_channel
with T
cycle_PA_channel
= cycle time of the PROFIBUS PA channel
T
cycle_DP
= cycle time of PROFIBUS DP
Fuchs recommends a value 3 x the PROFIBUS PA cycle time.
The PA cycle time T
cycle_PA_channel
depends on the following aspects:
■
Number of devices attached to the bus on 1 channel n
■
User data length L
The user data length L is a unitless variable calculated as an average quantity of user data input
and output (in bytes) for all devices.
The following is an approximate calculation for the cycle time:
T
cycle_PA_channel
= n * (0.256 ms * L + 12 ms) + 40 ms
