NORD Drivesystems BU0220 - Manual (Page 86)

PROFIBUS DP – Supplementary manualfor NORD Frequency Inverter SK 2xxE
86 Subject to technical amendments BU 0220 GB-0912
8.3.2.1 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 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
The following data transfer services are defined for the PROFIBUS DP:

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.

SDN: Send data with no acknowledge, allows the transmission of broadcast telegrams
(unacknowledged telegrams).
The PROFIBUS DP master cyclically reads the input information from the connected DP slaves and cyclically
writes the output information to the DP slaves.
PROFIBUS DP masters exist as

modules within a PLC

CPU module with integrated PLC

standard PC modules

Stand-alone boards
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Confirmation with data
Command, data (if necessary)
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Command, data (if necessary)
DP
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DP
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