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CYCLE PARTS
7.1.3.
CAN LINE
CAN line technology (Controller Area Network) is used to
connect the different electronic devices of the vehicle, so
they operate like a network of computers (Internet).
The use of a CAN line made it possible to significantly
simplify the layout of the electric system and reduce the bulk
of the wiring harness.
In addition, the CAN line avoids the duplication of sensors
because both processing units installed on the vehicle
(instrument panel and ECU) use the inputs from the same
sensors.
BENEFITS OFFERED BY CAN TECHNOLOGY
•
Less wires: the CAN line uses a pair of wires to
transmit information between the different nodes.
•
Faults Confination: nodes are capable of isolating
faults and avoid system breakdown.
•
Noise immunity: this is achieved through differential
signalling. Information is transmitted over the pair of
wires and the receiving device reads the difference
between the signals on the two wires. Any interference
due to external sources will affect both signal equally
and the difference between their voltages remains
constant.
•
Transfer Rate: messages are transferred at a bit rate of
about 250 kbps (the nodes receive data every 20 ms,
i.e. 50 times per second).
CAN PROTOCOL (CONTROLLER AREA NETWORK)
The CAN line uses the CSMA/CD communications protocol
(Carrier Sense Multiple Access /w Collision Detection)
"Carrier Sense" means that a node will determine whether
the BUS link shared by all connected devices is busy before
using the link to transmit a message. When the BUS link is
idle, multiple nodes are allowed access at the same time
(Multiple Access). When two nodes start transmission at the
same time, a collision occurs. The nodes sense a collision
(Collision Detection) and begin a process of arbitration to
determine which message has higher priority (messages are
unaffected by arbitration and the higher priority message is
given priority so there is no delay).
The CAN protocol is based on messages rather than
addresses. Messages are divided into several portions
(frames), and each frame carries different information:
message priority, data, error detection, acknowledgement of
receipt, etc. All nodes in the network receive all messages
sent over the BUS (with acknowledge or error frames) and
each node determines whether a given message is to be
processed or discarded. In addition, any node can request
information from other nodes (RTR = Remote Transmit
Request).
7 - 14
TUONO 1000
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