Omron GRT1 - 27-11-2006, Operation Manual

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PROFIBUS Technology Appendix A
A-4 Bus Access Protocol
The PROFIBUS bus access protocol is implemented by OSI 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 device 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. Each master executes this procedure automatically.
The polling or master-slave procedure permits the master, currently in possession of the token, to access its
assigned slaves. The figure below shows a possible configuration The configuration shows three active devices
(masters) and six passive devices (slaves).
The three masters form a logical token ring. When an active device 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 devices
in a master-slave communication relationship, and a DPM2 master can take the initiative to communicate with
DPM1 master devices 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: a master device sends an unacknowledged message to all other devices (mas-
ters and slaves).
• Multicast communication: a master device sends an un-acknowledged message to a predetermined group
of devices (masters and slaves).
A-5 Diagnostic functions
Extensive diagnostic functions defined in PROFIBUS DP enable the fast location of errors at slave devices.
Diagnostic messages are transmitted over the bus and collected at the master. Three levels of diagnostic mes-
sages are defined:
• Messages indicating the general operational status of the whole device, e.g. over temperature, low voltage.
• Messages indicating an error in a specific I/O range of a device, e.g. an 8-bit output module.
• Messages indicating an error at a given input or output, e.g. short circuit on Output 5.
DPM1 DPM2 DPM1
Token Passing
Polling
PROFIBUS
Passive stations
Slave devices
Active stations
Master devices