ProSoft Technology PTQ-PDPMV1 - Manual (Page 220)

Hot Standby Support PTQ-PDPMV1 ♦ Quantum Platform
User Manual PROFIBUS DP Master Network Interface Module for Quantum
Page 220 of 306 ProSoft Technology, Inc.
August 12, 2014
Information for Determining the Watchdog Time TWD
PROFIBUS devices are able to activate a watchdog mechanism that monitors
each time interval of cyclic calls (data exchange) to make certain the PROFIBUS
Master is still active. The time measurement takes place in the PROFIBUS slave.
If the watchdog is activated and the time TWD (Watch Dog Time) since the last
cyclic call expires, the device leaves cyclic data exchange, goes into the original
state (Wait_prm) and sets the outputs to the secure state.
The value of the time TWD and the activation of the watchdog are transferred in
the parameterized telegram from the PROFIBUS Master to the PROFIBUS slave
at startup (transition to the data exchange). In general, dimensioning of the time
TWD is used-specific (not device-specific, not in the GSD). The value is bounded
below by cycle times.
As a rule, the configuration tool is used to enter the time TWD. There are
configuration tools in which the watchdog time is set 1x per PROFIBUS Master
and others in which the watchdog time is set individually for each PROFIBUS PA
station. This does not change with the value of the watchdog time.
For many tools, the time TWD is automatically calculated based on the cycle time
of the Master with a corresponding baud rate.
At higher baud rates on the PROFIBUS DP side (for example 12MBd), cycle
times on the PA side may be longer by a factor of 300. If parameters were set
directly for a PROFIBUS PA device at a baud rate (DP) for a time calculated for a
higher Master TWD, it would generally be less than the PA cycle and the device
would not enter into the data exchange.
Behavior of Segment Coupler 2
To ensure reliable operation of the PROFIBUS the following bus parameters
should be used:
 Transfer rate 45.45 kBd ... 12 MBd
 Watchdog time TWD = 5 s
 PROFIBUS DP Standard
Note: If there is a large number of PROFIBUS PA stations per channel of the SK2 segment
coupler, the watchdog time TWD should be verified. The limit value is about 32 stations, but
depends on the volume of data to be transferred.