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5
Overview of PROFIBUS
Section 1-1
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 pro-
cedure automatically.
Polling Procedure
The polling or master-slave procedure permits the master, currently in pos-
session 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 mas-
ter-slave communication relationship, and a DPM2 master can take the initia-
tive to communicate with DPM1 master devices 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
In the case of broadcast communication a master device sends an unac-
knowledged message to all other devices (masters and slaves).
Multicast Communication
In the case of multicast communication a master device sends an un-acknowl-
edged message to a predetermined group of slave devices.
1-1-5
Diagnostic functions
Extensive Diagnostics
Extensive diagnostic functions defined in PROFIBUS DP enable the fast loca-
tion of error at slave devices. Diagnostic messages are transmitted over the
bus and collected at the master. Three diagnostic message types are defined:
Device Related
Diagnostics
• Messages concerning the general operational status of the whole device,
e.g. over temperature, low voltage.
Module Related
Diagnostics
• Messages indicating that an error is present in a specific I/O range of a
device, e.g. an 8-bit output module.
Channel Related
Diagnostics
• 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