Communication
Bus access
10
Networking via PROFIBUS−DP
10.3
10.3.1
L
10.3−1
EDSPM−TXXX−9.0−11/2009
10.3 Communication
10.3.1 Bus access
The transmission protocol offers two bus access procedures.
The master communication is also referred to as token passing procedure. The
token passing procedure makes sure that the bus access authorisation is
assigned. The bus access authorisation is given by means of a "token". The token
is a special telegram transmitted via the bus.
If a master has a token, it can communicate with all of the other bus nodes. The
token hold time is defined during system configuration. Once the token hold time
has elapsed, the token is passed on to the next master which is then in possession
of the bus access authorisation and can communicate with all of the other nodes.
The data transfer between the master and the slaves assigned to it is automatically
controlled by the master and takes place in a fixed and recurring sequence. The
slaves are assigned to a master during configuration. In addition, it can be defined
which slaves participate in the cyclic process data transfer.
Before master and slave can communicate, the configuration and the parameter
setting are checked for errors after startup.
The following are checked: type, format information, length information and the
number of inputs and outputs.
If the parameters are valid, the slave changes over to the DataExchange (DE) state.
The master can now transmit output data to the slave and receive the current input
data from the slave.
While the process data transfer is in progress, the master can transmit new
parameter data to the slave.
Master « Master
Master « Slave
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