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1 Features and System Configuration
CJ-series PROFIBUS Master Unit Operation Manual for NJ-series CPU Unit (W509)
1-1 Overview of PROFIBUS
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1-1-4 Bus Access Protocol
Device Profile
To enable the exchange of devices from different vendors, the user data has to have the same for-
mat. 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 pre-
cisely. Profiles have been defined for specific areas like drive technology, encoders, and for sen-
sors/actuators.
PROFIBUS DP-V1
PROFIBUS DP-V1 is an extension to the PROFIBUS DP protocol standard. It defines acyclic mes-
sage services between a PROFIBUS DP-V1 Master and a PROFIBUS DP-V1 slave device. These
acyclic message services allow exchange of extended parameter settings as well as extended diag-
nostics and alarm information, during regular I/O data exchange. PROFIBUS DP-V1 devices must
at least support PROFIBUS DP.
PROFIBUS DP-V1 services are designated as MSACn services (Master- Slave Acyclic, Class n), in
which n designates the Master Class (i.e. 1 or 2). The CJ1W-PRM21 supports PROFIBUS DP-V1
Class 1 and Class 2 Master functions as of Unit version 3.0.
OSI Layer 2: Buss 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.
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).
1-1-4 Bus Access Protocol
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