CAN bus primer
The CAN bus (Controller Area Networking) was defined in the late 1980 by Bosch, initially for
use in automotive applications.
It has been found to be very useful in a wide variety distributed industrial systems and is
becoming popular in avionics applications due its robustness and ease of use.
The connection uses two wires which are twisted around each other. This forms a “balanced
transmission line”. It helps to reduce emissions and also makes the link more robust against
external interferences.
The CAN bus is always implemented as a single cable allowing only short stubs to connect to
equipment along the route. Never implement a CAN bus as a “star” or other wiring topology.
The CAN bus requires termination resistors at each end of the bus. These are to be 120 ohm
resistors. 1/4W or 1/8W resistors are usually used here. The resistors must be installed at
each end of the bus, not in the center or anywhere else.
For short CAN runs (less than three meters) it is possible to install a single resistor of lesser
value (not less than 60 ohms) at any location in the cable run.
The two wires are referred to “CAN High” and “CAN Low”. These must connect to the
corresponding lines at the devices. Never swap these connections (I.e. Never connect CAN H
to CAN L at any device) as the CAN bus will not be able to function.
Never run the CAN bus connection inside a wire harness next to sensitive connection such as
audio or signal wires. Never run the CAN bus next to RF cables.
Making twisted wire
It is very easy to make your own twisted wire. Simply take two equally long wires (for example
5 meters) in parallel and tie one end (both wires) to a fixture (a door handle works well). Insert
the other end (both wires) into a drill. Stretch the wires so they are straight. Run the drill for a
few short bursts at slow speed and you have a created a perfect twisted pair !
Shielded, twisted wires
It is possible to purchase shielded, twisted wire. This can be used in applications where there
may be electrical noise issues. In this case we advise to connect the shield to ground AT A
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