CAN Bus - A Brief Guide

[fordfan]

CAN Bus


We hear it mentioned alot on this forum, so what is it?

CAN stands for controller area network. In this day and age many vehicle systems are computer controlled, the CAN Bus is the way by which messsages are shared between the control units. The CAN Bus comprises of two wires twisted together at around 40 twists per metre, these are known as CAN High and CAN Low. The signals are digital which means that they are 0 or 1 and are represented by a square wave (for diagnostic purposes the low voltage is in reality a little over 0). CAN High has the signal where 0 is off and 1 is on, CAN Low is the inverse, 1 for off and 0 for on, the messages are derived from the difference in signal between the CAN High and CAN Low, as they are twisted together any interference will affect the High and Low equally, meaning that the difference will still be the same.

On the Skodas there are basically two CAN systems (more can be added as required by the vehicle deigners), Powetrain CAN and Convenience CAN. The Powetrain CAN will comprise of units that control the engine, gearbox, brakes, steering etc., the convenience CAN will comprise of units that control the door locks, windows, mirrors, radio, air conditioning etc.

The powertrain CAN is around 5 times faster than the the convenience CAN, as engine data needs to be constantly updated, and the convenience CAN mostly
processes user inputs (pushing a button). Control units that use a lower speed CAN are cheaper to produce than those that use higher speeds. These CANs
cannot talk to each other directly, so are interfaced through the Gateway, which is often found in the dash panel. The vehicle daiagnostics plug is also connected to the Gateway.

Data from the engine sensors is sent to the engine control unit in analogue form (varying voltage), any data that needs to be sent from ECU goes through a CAN controller (which converts the signal to digital), a transceiver and is broadcast to every control unit on the network.

A CAN sgnal is made up of 108 or 126 bits (a bit is either 1 or a little over 0), the signal is split up as follows:

Start 1 bit
Status or Identifier 11 bits, 29 bit identifiers are now beginning to be used, hence 108 or 126 bit signals
Unused 1 bit
Check or Control 6 bits
Data 64 bits
Safety 16 bits
Confirmation 2 bits
End 7 bits


When the signal is received it goes through the transceiver on the receiving control unit, the CAN controller for conversion and finally gets used by the control unit, which then, after a safety check, sends an aknowledgement to the originating control unit. If this data is required by the convenience CAN (it may be a vehicle speed signal that the radio and the door locks use) it will go through the Gateway onto the other CAN network.


Why is it used?

It enables vehicle manufacturers to enhance their vehicles for little extra cost. By having the ability to share a signal from one sensor such as an ABS sensor the speedometer works, the power steering adapts its weighting, the doors automatically lock, the wheels don't lock up under heavy braking, the traction control system works and the raido volume automatically adjusts. Without the CAN Bus a separate sensor and the associated wiring would be needed for each of these functions.