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1-4
Section
PROFIBUS-DP characteristics
5
1-4
PROFIBUS-DP characteristics
1-4-1 Bus Access Protocol
The bus access protocol is defined by layer 2. This protocol also includes data
security and the handling of the transmission protocols and messages.
The Medium Access Control (MAC) specifies the procedures, which determine
when a station is permitted to transmit data. A token passing procedure is used
to handle the bus access between master devices, and a polling procedure is
used to handle the communication between a master device and its assigned
slave device(s).
The token passing procedure guarantees that the bus access right (the token) is
assigned to each master within a precisely defined time frame. The token
message, a special message for passing access rights from one master to the
next master, must be passed around the logical token ring - once to each master
- within a specified target rotation time.
The polling or master-slave procedure permits the master, which currently owns
the token, to access its assigned slaves. The picture below shows a possible
configuration.
Token passing and Slave polling mechanisms
The configuration shows three active stations (masters) and six passive stations
(slaves).
The three masters form a logical token ring. When an active station receives the
token message, it can perform its master role for a certain period of time. During
this time it can communicate with all assigned slave stations in a master-slave
communication relationship. In addition a DPM2 master can take the initiative to
communicate with DPM1 master stations in a master-master communication
relationship.
In addition to logical peer-to-peer data transmission, PROFIBUS-DP provides
multi-peer communication (broadcast and multicast).
Broadcast communication:
an active station sends an unacknowledged
message to all other stations (master and slaves).
Multicast communication:
an active station sends an unacknowledged
message to a predetermined group of stations
(master and slaves).
Layer 2
Medium Access Control
Token passing
Polling procedure
Multi-peer
communication