MOST bus: Ring fault diagnosis

For the MOST bus, there are inter-system fault code memory entries in the control units. A feature of the system faults is that they can be entered in a control unit although the control unit is OK. With the information regarding system faults in all MOST control units, conclusions can be drawn about the cause of the fault. The section below covers the system fault ”Network wake-up unsuccessful”. This fault can be entered in all MOST control units.

Fault code memory entry: Network wake-up unsuccessful

The fault ”Network wake-up unsuccessful” indicates a problem with the optical transmission. Insufficient light or no light is coming through at one position in the ring. Causes may be:

A distinction must be made as to whether the MOST ring is permanently or sporadically interrupted. To test, select and run the test module Ring fault diagnosis. The first step is that the stability of the MOST ring is checked.

If the MOST ring is sporadically interrupted, run the test for light output reduction.

If the MOST ring is permanently interrupted, continue the ring fault diagnosis.

Test of light output reduction

In the test program, the light output is automatically lowered for one MOST control unit after the other. if you want to run the test for light output reduction for a special control unit manually, then:

  1. Switch on loud music.
  2. Switch to ”Control Unit Functions” of the special MOST control unit (component activation: MOST bus light output reduction) and this control unit reduce the light output (the light output is lowered for 5 seconds and then automatically reset by the control unit to the normal value).
  3. If the optical transmission from control unit A to its successor (control unit B) is OK, slight noise (”crackling”) can occur when the light output (from control unit A) is lowered and automatically raised.
  4. If the optical transmission from control unit A to control unit B is not OK, the music goes off for a few seconds. This means that the defect lies between control unit A, where the light output was reduced, and its successor (control unit B) in the MOST ring.

Repeat operations 1 to 4 several times, as this method does not provide reliable identification, rather only an indication of a defect. Check the transmission path at which the music goes off for loose plug-in connections and kinks in the fibre-optic cable wiring harness. If the results of the visual inspection appear to be in order, the fault can only be accurately located (transmit diode control unit A, receiver diode control unit B, fibre-optic cable) using the optical test.

Ring fault diagnosis

In the case of a ring fault (defect in the MOST ring), the two MOST control units between which the defect in the ring can be found must be determined first. This is established using the ring fault diagnosis function.

Node position ”0”

If the power supply to the MOST control units is switched off and then on again, this switches the MOST control units to ”ring fault mode”:

each MOST control unit simultaneously sends a light signal to the next control unit in the ring. In addition, each MOST control unit checks whether it is receiving a light signal at its input. The control unit that does not detect a light signal at its input stores the relative node position 0 in its fault memory. Thus the ring fault is between the control unit that has stored the node position 0 and the node that precedes it in the MOST ring.

Determining the control unit with node position ”0”

Thus, to locate a ring fault between two control units, it is only necessary to identify the control unit that has stored the node position 0. If there is a ring fault in the MOST network, the system can only communicate with the Car Communication Computer (or Multi Audio System Controller) via diagnosis. This is possible because those two control units are connected to the K-CAN bus. It is not possible to communicate with the other control units because signals are only transmitted in one direction and there is a ring fault. That therefore means that it is not possible to establish which control unit has stored the node position 0. A different mechanism has thus been implemented on the MOST control units in order to be able to identify the two control units between which there is a ring failure:

The control unit located in the ring after the control unit with node position 0 stores node position 1; the next control unit in the ring stores 2 etc. This enables you to determine the position of the ring fault on the basis of the node position stored in the Car Communication Computer (or Multi Audio System Controller). This number can namely be read from the CAN bus. By counting downwards, backwards starting from the Car Communication Computer (or Multi Audio System Controller), you can determine the control unit with node position 0.

Special manner of counting for DVD changer: For the DVD changer control unit, when counting backwards from the Car Communication Computer (or Multi Audio System Controller), a jump of 2 must be made!

The ring fault diagnosis is run automatically in the test module. In the test module, the node position stored in the Car Communication Computer (or Multi Audio System Controller) control unit is specified. Further procedure if the position of the ring fault can not be determined automatically after determining the node position:

Procedure for locating the position of the ring fault using the node position:

  1. Determine the order of the control units in the ring. (To do so, select the menu item ”MOST ring control unit sequence” in the test module)
  2. From the Car Communication Computer (or Multi Audio System Controller) starting backwards (using the connection scheme for the MOST ring) count down to the node position. If the DVD changer control unit occurs when counting backwards, you have to subtract from node position 2! Refer to the special manner of counting for DVD changer). Otherwise, subtract one per control unit. When node position 0 is reached, the ring fault then lies between the control unit with node position 0 and the control unit preceding it in the MOST ring.
  3. Then check the power supply of the control unit located before the control unit with node position 0. If the power supply is OK, continue with the optical test on the MOST bus.