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Engineering Guide CDA3000
5 Communication and user modules
5.3
CAN-BUS
The CAN bus is a field bus which is in widespread use in automation. Its
data transfer was standardized in ISO 11898. However, most CAN net-
works work with custom conventions for the communication and interpre-
tation of user data.
Openness is attained through the use of the CANopen device profiles.
These profiles define the mode of communication (CiA/DS30x) and the
interpretation of the user data (CiA/ DS40x).
Figure 5.3
Topology of CAN
A CAN network is a multimaster network - that is to say, any station can
autonomously send messages on the bus which can in turn be received
by any other station on the bus.
Typically, however, transmissions are exchanged between two stations on
the bus.
The basic rule is: Any one can evaluate the information from an identifier
for its own ends. But only one station can have transmission rights for
each identifier.
Each transmission is assigned a priority by the selection of the identifier
for that transmission. The priority is antiproportional to the identifier
number - that is, a rise in the significance of the identifier results in fall in
the priority of the transmission. The monitoring of priorities and assign-
ment of access rights on the bus is controlled by hardware means by the
CAN controller.
Master 1
Priority 1
Master 2
Priority 2
Master 3
Priority 3
Master 4
Priority 4
