Terminal Resistor

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An electrical conductor through which current passes always has an ohmic, an inductive and a capacitive impedance. When data is transmitted from point "A" to point "B" over a CAN line, the total sum of that impedance has an effect on the transmission of that data.

The higher the transmission frequency, the greater the effect of the inductive and capacitive impedance. The result of this can be that when the signal reaches the end of the data transmission line, it is unidentifiable. For that reason, the data line is "modified" by terminal resistors so as to preserve the original signal.

The inductive impedance is produced by phenomena such as the coil effect of the wire. The capacitive impedance for example is produced by the effect of routing the wire parallel to the vehicle body. The terminal resistors on a bus system vary. They are generally dependent on the following parameters:

Frequency of data transmission on the bus system.
Inductive/capacitive load on the transmission channel.
Length of the data transmission cable (The longer the cable is, the greater is the inductive component).

The control units are categorized either as basic control units or other control units. Categorization is performed on the basis of the impedance level. Basic control units are those which always have to be fitted in the car regardless of equipment level or market. Other control units are those that are dependent on the options fitted.

The terminal resistor values for the K-CAN system are 820 Ohm for "Basic" controllers and 12K Ohms for all other control units.

Fig 1: Identifying Terminal Resistor For K-CAN System

Courtesy of BMW OF NORTH AMERICA, INC.

For the E65 the Basic controllers are:

K-CAN S: LM, IHKA, CAS, CD, ZGM, KOMBI.

K-CAN P: CAS, TMFAT, TMBFT, HKL

The resistors are located inside of the control units.

Wake-Up on the K-CAN

The control units on the K-CAN network are "awakened" via the bus. For that reason, it has been possible to dispense with the previous function of terminal 15 as wake-up line. The wake-up message is passed directly to the control unit output stage by the CAS module receiver. The output stage switches terminal 30 on and the unit is woken up.