Device Types; Bus Access Protocol - Omron PRT1-SCU11 Operation Manual

Overview of PROFIBUS
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Device Types

Master Devices
Class 1 Master (DPM1)
Class 2 Master (DPM2)
Slave Devices
Device Profile
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Bus Access Protocol

OSI Layer 2: Bus Access
Protocol
Medium Access Control
Token Passing
Polling Procedure
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PROFIBUS distinguishes between master devices and slave devices.
Master devices determine the data communication on the bus. A Master can
send messages without an external request, as long as it holds the bus
access right (the token). Masters are also referred to as active devices in the
PROFIBUS standard.
There are two types of master devices:
A PROFIBUS-DP Class 1 Master (DPM1) device is a central controller, which
exchanges information with the decentralized devices (i.e. DP slaves) within a
specified message cycle.
PROFIBUS-DP class 2 Master (DPM2) devices are programmers, configura-
tion devices or operator panels. They are used during commissioning, for con-
figuration of the DP system, or for operation and monitoring purposes.
Slave devices are peripheral devices. Typical slave devices include input/out-
put devices, valves, drives, and measuring transmitters. They do not have bus
access rights and they can only acknowledge received messages or send
messages to the master when requested to do so. Slave devices are also
called passive devices. The PROFIBUS-DP Gateway Unit is a slave device.
To enable the exchange of devices from different vendors, the user data has
to have the same format. The PROFIBUS-DP protocol does not define the
format of user data, it is only responsible for the transmission of this data. The
format of user data may be defined in so called profiles. Profiles can reduce
engineering costs since the meaning of application-related parameters is
specified precisely. Profiles have been defined for specific areas like drive
technology, encoders, and for sensors / actuators.
The PROFIBUS bus access protocol is implemented by OSI layer 2. This pro-
tocol also includes data security and the handling of the transmission proto-
cols 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 proce-
dure 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 pro-
cedure automatically.
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-
Section 1-1