Device Types; Bus Access Protocol - Omron CJ1W-PRT21 Operation Manual

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Device Types

PROFIBUS distinguishes between master devices and slave devices. The CJ1W-PRT21 is a PROFI-
BUS slave device.
Master 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.
 DPM1 and DPM2
There are two types of master devices: DP master class 1 (DPM1) and DP master class 2 (DPM2).
A DPM1 is a central controller which exchanges information with the decentralised stations (i.e. DP
slaves) within a specified message cycle.
DPM2 devices are programmers, configuration devices or operator panels. They are used during
commissioning, for configuration of the DP system, or for operation and monitoring purposes.
Slave Devices
Slave devices are peripheral devices. Typical slave devices include input/output devices, valves,
drives, and measuring transmitters. They do not have bus access rights and they can only acknowl-
edge received messages or send messages to the master when requested to do so. Slave devices are
also called passive devices.
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Bus Access Protocol

 OSI Layer 2: 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.
 Medium Access Control
The Medium Access Control (MAC) specifies the procedures which determine when a device is per-
mitted to transmit data. A token passing procedure is used to handle the bus access between mas-
ter 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 this is not recommended.
 Polling Procedure
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).
CJ-series PROFIBUS Slave Unit Operation Manual for NJ-series CPU Unit (W510)
1 Features and System Configuration
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