MOST bus: Ring fault diagnosis

The MOST (Media Oriented Systems Transport) network uses a ring bus for data communication between the various control modules. Signal transmission is by means of fiber-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 modules are directly connected to the K-CAN SYSTEM bus.

If, for example, the power supply or the diode of a control module is defective, it is no longer possible to communicate with the control modules 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 memory, etc.) of the Control Display, only the control module 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 memory entries in the control modules. A feature of the system faults is that they can be entered in a control module although the control module is OK. With the information regarding system faults in all MOST control modules, 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 modules.

Fault 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 module without voltage
  

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

• Transmit diode or receiver diode in a control module 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 module after the other. if you want to run the test for light output reduction for a special control module manually, then:

1. Switch on loud music.
   
2. Change to ”Control-module functions” of the special MOST control module (component activation: MOST bus light output reduction) and this control module reduce the light output (the light output is lowered for 5 seconds and then automatically reset by the control module to the normal value).
   
3. If the optical transmission from control module A to its successor (control module B) is OK, slight noise (”crackling”) can occur when the light output (from control module A) is lowered and automatically raised.
   
4. If the optical transmission from control module A to control module 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 module A, where the light output was reduced, and its successor (control module 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 fiber-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 module A, receiver diode control module B, fiber-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 modules 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 modules is switched off (battery disconnected) and then switched on again (battery reconnected), this switches the MOST control modules to ”ring fault mode”:

each MOST control module simultaneously sends a light signal to the next control module in the ring. In addition, each MOST control module checks whether it is receiving a light signal at its input. The control module 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 module that has stored the node position 0 and the node that precedes it in the MOST ring.

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

Thus, to locate a ring fault between two control modules, it is only necessary to identify the control module 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 modules are connected to the System K-CAN bus. It is not possible to communicate with the other control modules 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 module has stored the node position 0. A different mechanism has thus been implemented on the MOST control modules in order to be able to identify the two control modules between which there is a ring failure:

The control module that follows the control module with the node position 0 stores the node position 1, and the next one the node position 2, etc.

Example (control module Rear Display not fitted):The Audio System Controller control module 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 modules 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 module 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 module 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 module 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 module that has stored the node position 0 and the node that precedes it in the ring.)

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

Example:

MOST control modules vehicle equipment without Rear Display control module, 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 antenna tuner: 1

- Node position CD changer/iPod interface: 0

(Thus the ring fault is between the control module 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 module 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 module 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 module is specified. Remaining procedure:

1. Determine which MOST control modules are fitted in the vehicle. Whether a MOST control module is fitted in the vehicle can be determined as follows:
   
   • via black highlighted MOST control modules after short test or
     
   • via visual inspection in the vehicle
     
2. Beginning with the Control Display (using the MOST ring circuit diagram), count down the node position towards the Rear Display control module if fitted), instrument cluster, Audio System Controller etc. When node position 0 is reached, the ring fault then lies between the control module with node position 0 and the control module preceding it in the MOST ring. If the multimedia changer control module 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 module located before the control module 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 modules can be selected at Body, Audio Video Navigation (MOST Framework), MOST Functions, Function Framework, Control-module Power Supply (control module 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.