Renu Electronics GWY-500, User Manual

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Getting Started
15
Doc No: UMAN\GWY-500-B\0106
REVISION 4
3.1.3 PROFIBUS-DP Characteristics
3.1.3.1 Bus Access Protocol
Layer 2 The bus access protocol is implemented by layer 2. This protocol also includes data
security and the handling of the transmission protocols and messages.
Medium Access Control 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).
Token passing 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. Each master executes this procedure automatically. A user can only
change the target rotation time, but is not recommended.
Polling procedure 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
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, and a DPM2 master can take the initiative to communicate with DPM1
master stations in a master-master communication relationship.
Multi-peer communication 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 (masters and slaves).
Multicast communication: An active station sends an unacknowledged message to a
predetermined group of stations (masters and slaves).