NORD Drivesystems BU0020 - Manual (Page 51)

5 PROFIBUS technology and protocol
BU 0020 GB-2612 51
Fig. 8: DP slave properties (left)
Fig. 9: Access monitoring setting (right)
Note
Under certain circumstances, disabling of access monitoring can result in the outputs of
the affected slaves not being set to "0" in case of errors. It is therefore strongly
recommended that the access monitoring is only disabled for test purposes during
commissioning.
During system planning, the access monitoring time is automatically calculated for the entire
PROFIBUS network by the software tool STEP 7. Access monitoring should not be confused with the
Timeout function in parameter P513. These are different functionalities.
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5.5 PROFIBUS master
PROFIBUS masters are field devices, which take the initiative for the exchange of data with field
devices which operate as slaves. A master has the sole access right to slaves on the bus and
determines the data transfer on the bus. The master may send out messages without request if it has
an access right (token). In the case of several masters in a bus structure, only the master which
currently has access rights to the bus may send messages. In contrast to the DP slaves, masters are
designated as active participants, which have bus access rights for a limited period of time (token-
holding time).
All data which a PROFIBUS master requires for the exchange of data with the slaves (e.g. I/O area),
must be created before the system is started and loaded into the master ( GSD file).
The main tasks of a master are:
- exchange of data with the planned slaves
- coordination of bus access
- execution of error handling
- providing the user with slave data
SRD Send and request data with acknowledge. In a message cycle, the master sends output
data to a slave and as a response receives the input data back in the same cycle.