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Chapter 3 Deployment
Communication
Master with
master
Master-slave
procedure
3-4
The bus transfer protocol provides two alternatives for the access to the
bus:
Master communication is also referred to as token-passing procedure. The
token-passing procedure guarantees the accessibility
permission to access the bus is transferred between individual devices in
the form of a "token". The token is a special message that is transferred via
the bus.
When a master is in possession of the token it has the permission to
access the bus and it can communicate with any active or passive device.
The token retention time is defined when the system is configured. Once
the token retention time has expired, the token is passed to the following
master which now has permission to access the bus and may therefore
communicate with any other device.
Data communication between a master and the slaves assigned to it, is
conducted automatically in a predefined and repetitive cycle by the master.
You assign a slave to a specific master when you define the project. You
can also define which DP slaves are included and which are excluded from
the cyclic exchange of data.
Data communication between master and slave can be divided into a
parameterization, a configuration and a data transfer phase. Before a DP
slave is included in the data transfer phase the master checks whether the
defined configuration corresponds with the actual configuration. This check
is performed during the definition and configuration phase. The verification
includes the device type, format and length information as well as the
number of inputs and outputs. In this way a reliable protection from
configuration errors is achieved.
The master handles the transfer of application related data independently
and automatically. You can, however, also send new configuration settings
to a bus coupler.
When the status of the master is DE "Data Exchange" it transmits a new
series of output data to the slave and the reply from the slave contains the
Manual VIPA System 300S
HB140E - IM - RE_353-1DP01 - Rev. 14/47
of the bus. The
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