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ISO 15765

CAN Bus is a communications system used in the automotive industry to link control units for data exchange.

The CAN bus is simply a pair of wires, often twisted around each other, running around the vehicle. The only components connected to the CAN bus are the electronic control units, referred to as nodes. Other components, such as sensors, motors, light bulbs, switches, etc. are wired only to the electronic control units.

A simple example of the CAN bus operations would be to look at the wheel speed sensor data. The wheel speed sensor connects to an electronic control unit, usually ABS, which will transmit the wheel speed sensor data on to the CAN bus in  a specific data format, often proprietary to the manufacturer.  The other electronic control units which are interested in the wheel speed sensor data such as the instrument cluster and the engine management system can then read and process the wheel speed sensor data as required.

An electronic control unit may communicate to another by transmitting its data on the CAN bus. The other electronic control units may either listen to the incoming data, ignore it or reply to the data message if required.

The CAN bus itself consists of two wires, CAN-H and CAN-L, which will either have the same voltage when idle (about 2.5V), or a voltage difference of 2V when a signal is placed on the CAN bus. When a signal is placed on the CAN bus the CAN-H line is at a higher voltage than the CAN-L line.

To prevent data transmission problems, the CAN bus is terminated at either end of the two-wire network with resistors of 120 Ohms.

A simple check to see if the CAN bus is in use in a vehicle, and accessible via the OBD socket, is to connect a resistance meter across pin 6 and pin 14. Due to the combined resistance of the two termination resistors at 120 Ohms each the overall resistance should be read as 60 Ohms.

The CAN bus speed may be as fast as 1 MHz, although the OBD-II ISO 15765 specification specifies two speeds which may be used for on-board diagnostics, 250 KHz and 500 KHz.

Each electronic control unit will have its own CAN identity code, like an address.  In fact an electronic control unit may respond to several CAN identity codes. If an electronic control unit is to communicate to another it will need to know the CAN identity code of the recipient. This is designed into the system early on and care is taken not to duplicate CAN identity codes.

The CAN identity code may consist of either 11 bits or 29 bits in the address, depending on which CAN bus system is in operation. 

A vehicle which uses CAN bus for on-board diagnostics can only respond to an OBD-II request from a tester which uses CAN bus.  From model year 2008 vehicle manufacturers must use the OBD protocol specified in ISO 15765, also known as Diagnostics On CAN.

Further information coming soon.


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      This page and web site contents was created by Sinclair Easton 14th May 2006.  

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