Information Technology Reference
In-Depth Information
21
CAN bus
21.1 Introduction
The Controller Area Network (CAN) protocol is an ISO-defined standard (ISO 11898) for
serial data communication at bit rates up to 1 Mbps. It was initially developed for the auto-
motive industry, and has the great advantage that it uses a common bus which reduces the
need for wiring harnesses. It has since outgrown this application. The standard includes a
physical layer and a data-link layer, which defines different message types, arbitration rules
for bus access and methods for fault detection and fault confinement.
Its basic features are:
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Differential transmission using twisted-pair cables.
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Arbitration - access to the bus is controlled by a non-destructive bitwise arbitration tech-
nique. In arbitration, every transmitter compares the level of the bit transmitted with the
level that is monitored on the bus. If these levels are the same then the unit will continue
to send.
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Small messages (only up to 8 bytes in length), each with an associated checksum. These
have fixed format messages, which transmit data and other information. Nodes only
transmit messages when the bus is free. The content of a message is identified by the
IDENTIFIER, which describes the meaning of the data, but does not provide for the des-
tination of the information. All the nodes on the network can decide whether they need to
read the data or not (multicast).
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No addressing structure - messages are broadcast on the common bus with a message
with a priority level and identification.
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Prioritisation of messages - each message has a defined priority. On a free bus, any node
can transmit their message. Two or more units which transmit a message at the same time
produce an error condition. The unit with a message of higher priority always gains ac-
cess to the bus over a lower priority node.
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Multicast reception with time synchronisation - all nodes are able to receive transmitted
data and can also quickly synchronise their clocks to the transmitted data.
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Error detection and signalling - powerful error handling scheme that allows for the re-
transmission of messages when they are not properly received. There is also automatic re-
transmission of corrupted messages as soon as the bus is idle again. The recovery time
from detecting an error until the start of the next message is at most 29 bit times, if there
is no further error.
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Multimaster - any device can gain control of the bus.
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Enhanced fault finding and fault isolation. Implementation of methods of fault finding
and removal of faulty nodes from the bus. There is also a distinction between temporary
