Information Technology Reference
In-Depth Information
21.8.4
Explain the concept of dominant and recessive levels, and how these can be used
to determine if two or more devices are communicating at the same time.
21.9 Notes from the author
As with the WorldFIP bus, the CAN bus is a well-designed network, based on techniques
learned from computer networks. It is a serially connected bus, where all nodes have access
to the network, and collisions between nodes are detected within a very short time. This al-
lows devices to have a relatively equal share of the bandwidth of the bus. As automobiles are
noisy environments, the CAN bus is a rugged bus which copes well with errors, and also
devices which are not operating correctly.
The relatively high bit rates of the CAN bus allows a great deal of information to be
passed between instruments and their controllers. To prevent major problems, the bus can be
organized into segments, so that a major fault in one are a does not greatly affect other
areas. A failure of any of the controllers can lead to major problems, so secondary control-
lers can be made to monitor the operation of the primary, and can remove the primary con-
troller from the bus if they are not operating correctly. Another method is to allow localized
control when the primary control is not functioning properly.
Power dissipation is also a major factor in cars as devices must be ready to respond
quickly to events, but not to dissipate much power when they are idle. Thus, the CAN bus has
methods to allow devices to sleep if they are inactive and then is awoken when a specific
event occurs.
The car of the future, based on the CAN bus, would have little need for complex wiring
harnesses, and would simply require the daisy chaining of devices onto the common bus. The
connector used can be matched to the environment, such as heavy-duty connector for robust
situations, or a light connector for ease of connection/disconnection.
As the CAN bus has been designed with a thought for the seven-layered OSI model, which
is used in computer networks, there is great potential for using standard computer network
protocols, such as TCP/IP. Thus will allow CAN busses to connect straight into the Internet,
and allow for remote control and remote data acquisition over the Internet, or over a local
or wide area network. The data could be protected using data encryption techniques. So,
maybe one day you could log into the Internet and switch on the air conditioning in your car
before you even leave your house.
