Siemens SIMATIC ET 200AL System Manual page 1271

Parameter assignment/addressing
3.3 Network settings
User-defined
The PROFIBUS subnet will only work properly if the parameters for the profile have been
synchronized. Select the "User-defined" profile if none of the other profiles "match" for
operation of a PROFIBUS device and if you have to adapt the bus parameters for your
special layout.
You cannot configure all theoretically possible combinations with the user-defined profile
either. The PROFIBUS standard prescribes some parameter limits depending on other
parameters. It is, for example, not permitted that a responder responds (Min Tsdr) before the
initiator is able to receive the frame (Trdy). These standard specifications are also checked
in the "User-defined" profile.
Note
User-defined settings
Use user-defined settings only if you are familiar with the PROFIBUS parameters. It is
usually better to work with the "DP" profile.
Contact Customer Support (Page 91) if you have any questions.
The bus parameters that were last valid on the PROFIBUS subnet are automatically set as
user-defined. If the "DP" bus profile was valid for the subnet, for example, the bus
parameters for "DP" are set in the "User-defined" bus profile. You can modify the parameters
based on these settings.
The monitoring times are not automatically recalculated in the "User-defined settings" setting
so that the uniformity of the set values is not changed without your knowledge, for example,
to configure other configuration tools.
You can calculate the monitoring times Ttr and watchdog based on the parameters you have
set. To do so, click on the "Recalculate" button.
See also
Additional network stations (Page 51)
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PROFIBUS with STEP 7 V13
Function Manual, 12/2014, A5E03775446-AC