Controller Area Network (CAN-Bus): Notes

The CAN protocol was originally developed by Intel™ and Bosch in 1988 for use in the automotive industry. CAN provides a standardized, reliable and cost-effective communications network which allows vehicle manufacturers to combat the increasing size of vehicle wiring harnesses.

The CAN-Bus was introduced on BMW vehicles in 1993 in the 7 and 5 series vehicles with the M60 engine and automatic transmission. The CAN-Bus connected the DME (ECM) with the EGS (TCM) control units. This network allowed data to be transferred between DME and EGS at rate of up to 500 Kbps (Kilobits per second).

Fig 1: Identifying Example Of Early CAN-Bus On M60 Engine

Courtesy of BMW OF NORTH AMERICA, INC.

As shown in the above picture, the original CAN-Bus network contained only 2 control units or "subscribers". Since it's introduction, subscribers on CAN have increased as system needs dictated.

Beginning with the 1995 model year, new systems were added to the CAN-Bus. The introduction of the E38 750iL necessitated major changes to the CAN-Bus structure. EML and DSC were added as well as DME II for the M73 engine.

For the 1998 model year, the instrument cluster and the steering angle sensor were also added to expand the signal sharing capabilities of the vehicle.

When introduced, the CAN-Bus consisted of 2 copper wires and a third connection which served as a shield. The shield was needed to protect the CAN-Bus from electrical interference. Since the CAN-Bus uses relatively low voltage (approx 2.5), it is vulnerable to signal interruption from higher voltage circuits or aftermarket systems such as cell phones etc.

Fig 2: Identifying Tree Structure And Star Connector

Courtesy of BMW OF NORTH AMERICA, INC.

The shield on the CAN-Bus was only used until the 99 model year, after which the entire CAN-Bus network went to twisted pair wiring. Twisted pair configuration allows the same level of interference suppression and creates more flexibility in wiring due to the elimination of the extra shielding.

The two signal wires used in CAN are referred to as CAN-High and CAN-Low. Each wire carries the same information bi-directionally. The two wire configuration is used for redundancy in the event of failure.

Due to the linear structure of the network, the CAN-Bus is available for other modules in the event of a disconnected or failed control unit. This is referred to as a "Tree" structure with each control unit occupying a branch.

Currently, the CAN-Bus is used on all BMW vehicles and has been expanded to other systems. The introduction of the E65 brought about new variations of CAN. The new PT-CAN and K-CAN will be discussed in a later module.