MOST bus: Ring fault diagnosis

The MOST (Media Oriented Systems Transport) network uses a ring bus for data communication between the various control units. Signal transmission is by means of fibre-optic cable. Data transmission on the ring bus takes place in one direction only. On the MOST ring, messages can only be transmitted provided the bus ring is complete and fully functional. If there is a ring fault in the MOST network, however, the diagnosis system can only communicate with the instrument cluster and the Control Display. This is possible because those two control units are directly connected to the K-CAN SYSTEM bus.

If, for example, the power supply or the diode of a control unit is defective, it is no longer possible to communicate with the control units on the MOST bus. Exceptions here are of course the instrument cluster and the Control Display.

Transmission direction in the MOST ring

Transmission of messages is from the Control Display towards the CD changer / iPod interface, ..., instrument cluster, and back to the Control Display.

Control Display

The Control Display is divided into the functions CD GW Control Display (Gateway) and CD Control Display. The gateway is the interface between the MOST and K-CAN SYSTEM buses. For the remaining troubleshooting (read fault code memory, etc.) of the Control Display, only the control unit name CD GW Control Display (Gateway) needs to be selected at the tester!

Diagnosis MOST bus

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:

• Control unit without voltage
  

• Fibre-optic cable defective (e.g. kinked too strongly)
  

• Transmit diode or receiver diode in a control unit defective
  

• Connector not correctly inserted
  

A distinction must be made as to whether the MOST ring is permanently or sporadically interrupted. To test, switch on the radio and check whether music can be heard. If no music can be heard, the MOST ring is permanently interrupted.

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

If the MOST ring is permanently interrupted, run 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 around 5 to 10 seconds and the Control Display restarts. 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 and restarts the Control Display 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 (battery disconnected) and then switched on again (battery reconnected), 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.

Example:The Audio System Controller control unit has stored the relative node position 0. That means that the ring fault is between the Audio System Controller and the control unit that precedes the Audio System Controller on the MOST ring bus.

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 ring, however, the diagnosis system can only communicate with the instrument cluster and the Control Display. This is possible because those two control units are connected to the System 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 that follows the control unit with the node position 0 stores the node position 1, and the next one the node position 2, etc.

Example (control unit Rear Display not fitted):The Audio System Controller control unit has stored the relative node position 0. The instrument cluster therefore stores the relative node position 1 and the Control Display (Gateway), the relative node position 2. Node position 2 can be read out by the Control Display (Gateway). Thus, on the basis of the node position stored on the Control Display (Gateway) and the details of the MOST control units fitted on the vehicle, it is possible to locate the position of the fault in the ring by simply counting backwards from the Control Display (Gateway):

- node position of Control Display (Gateway): 2

- node position of instrument cluster: 1

- node position of Audio System Controller: 0

(Thus the ring fault is between the control unit that has stored the node position 0 and the node that precedes it in the ring.

Counting method for multimedia changer and navigation system: When counting backwards for the Multimedia Changer control unit and the navigation system from the Control Display, jump of 2 must be made! This is best illustrated with the aid of another example:

Assumption: Rear Display control unit and headphone interface not fitted

The Control Display (Gateway) has stored the node position 9. We thus count back as follows:

- node position of Control Display (Gateway): 9

- node position of instrument cluster: 8

- node position of Audio System Controller: 7

- node position of telephone (or Flexible Bus Interface): 6

- node position of multimedia changer: 4 (counts as 2)

- node position of video module: 3

- node position of navigation system: 1 (counts as 2)

- node position of voice input unit: 0

(Thus the ring fault is between the control unit that has stored the node position 0 and the node that precedes it in the ring.

Counting method depending on the MOST control unit equipment:You count the relative node position backwards from the Control Display until you reach the control unit with the relative node position 0. In order to be able to perform the count correctly, you must first know which control units are fitted on the MOST ring.

Example:

MOST control units vehicle equipment without Rear Display control unit, headphone interface, navigation and voice input. The Control Display (Gateway) has stored the node position 9. We thus count back as follows:

- node position of Control Display (Gateway): 9

- node position of instrument cluster: 8

- node position of Audio System Controller: 7

- node position of telephone (or Flexible Bus Interface): 6

- node position of multimedia changer: 4 (counts as 2)

- node position of video module: 3

- node position of amplifier: 2

- node position of aerial tuner: 1

- Node position CD changer/iPod interface: 0

(Thus the ring fault is between the control unit that has stored the node position 0 and the node that precedes it in the ring.)

Deviation to the above concept!

Assumption 1: Rear Display control unit and headphone interface not fitted

The above rule differs if the Control Display (Gateway) has stored 0 to 2 as its node position:

Assumption 2: Rear Display control unit and headphone interface fitted

The above rule differs if the Control Display (Gateway) has stored 0 to 4 as its node position:

Locating the position of the ring fault using the node position: The ring fault diagnosis is run automatically in the test program. In the test program, the node position that is stored in the Control Display (Gateway) control unit is specified. Remaining procedure:

1. Determine which MOST control units are fitted in the vehicle. Whether a MOST control unit is fitted in the vehicle can be determined as follows:
   
   • via black highlighted MOST control units after quick test or
     
   • via visual inspection in the vehicle
     
2. Beginning with the Control Display (using the MOST ring connection scheme), count down the node position towards the Rear Display control unit if fitted), instrument cluster, Audio System Controller etc. 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. If the multimedia changer control unit or the navigation system occur when counting backwards, you have to subtract from node position 2! (See also 'Counting method for multimedia changer and navigation system')
   
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.
   
   • Test programs for checking power supply:At the testing device, switch to function selection. The test programs for checking the power supply of the MOST control units can be selected at Body, Audio Video Navigation (MOST Framework), MOST Functions, Function Framework, Control-unit Power Supply (control unit e.g. instrument cluster).
     
   • Test program for optical test:At the testing device, switch to function selection. The test program for the optical check of the MOST bus can be selected at Body, Bus Functions, MOST Functions and Optical Test.