CAN-Bus 7 (Hardware)
| ||Logical one||Logical zero|
|Two out of three are sufficient if necessary.|
Instead of just one wire there are three wire combinations. The signal between CAN_L and ground is mostly just the reverse of the signal between CAN_H and ground. Large interferences may weaken just one of the
two signals; the other one is strengthened automatically. Receiving information from the Controller Area Network Bus, not only the difference between CAN_H and CAN_L are taken into consideration, but also the
voltage course against ground.
|While repairing pay attention to the twisting.|
For one manufacturer, CAN_H and CAN_L each have their particular color. They must be twisted in certain lengths. For the repair, the maximum length without twist is predefined. If this is not met, disturbances on the
Bus are possible. Also, two twisted wires may not be joined together at the same spot. In this case, one or both of them have to be joined one or two twists further.
|Look for anti-reflecting resistance in the control unit.|
To avoid reflections on the Bus, the control units have to be equipped with end resistances. In earlier times, 50 ohms were necessary for local area networks. These could also be measured when everything was
completely switched off. In the car there are fewer reflections, because the wires are not that lengthy. The resistances are sometimes distributed over the control units. The largest resistance (of e.g. 66 ohms) is
switched by the control unit, which is always present regardless of the vehicle's accessories. Other features do not add more than 3 ohms. Thus there always is protection against reflections. This arrangement is
chosen, because a control unit may also switch off the resistance. Resistances are consumers who should burden the net under normal operating conditions or in the case of disturbances as briefly as possible. In
any case, for vehicles they are found seldom at the end of the wire system. 05/07