MOST bus: fibre-optic cable communication fault

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, aerial tuner, ..., 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 ”Fibre-optic cable, communication fault”. This fault can be entered in all MOST control units.

Fault code memory entry: Fibre-optic cable communication fault

The fault ”Fibre-optic cable communication fault” indicates a problem with the optical transmission. Insufficient light is coming through at one position in the ring. Causes may be:

• Fibre-optic cable: wiring harness defective (e.g. too strongly kinked),
  

• Output power/reception sensitivity of a diode is too low
  

• Connector not correctly inserted
  

• Voltage fluctuations (e.g. on powering up a control unit)
  

If the fault is stored, the running system triggers a reset and starts up once again. This switches off music briefly; the display screen of the Control Displays continues to run.

If this fault is entered in a control unit, it is first of all not possible to determine where the defect lies in the MOST ring. The fault lies between the control unit that was first to store the fault ”Fibre-optic cable communication fault” and the control unit preceding it in the MOST ring. The following procedure is required to localize the fault:

1. Starting in MOST ring order (see circuit diagram) with the control unit instrument cluster, check whether the ”Fibre-optic cable, communication fault” is stored.
   Note:
   Following the quick test, the fault-pattern selection contains the all stored vehicle faults. This enables you to determine quickly which control unit was first to store the ”Fibre-optic cable, communication fault”.
   The fault lies between the control unit that was first to store the ”Fibre-optic cable communication fault” (= control unit B) and the control unit preceding it in the MOST ring (= control unit A)
   
2. In the case of voltage dips below 9 V, it can occur that the fault is stored ”incorrectly”. For this reason, perform the following test as a countercheck:
   
   • Switch to ”Control Unit Functions” of control unit B and erase the fault memory
     
   • Switch to the ”Control Unit Functions” of control unit A and reduce the light output
     
   • Starting in MOST ring order (see circuit diagram) with the control unit 'instrument cluster', check whether the ”Fibre-optic cable, communication fault” is stored.
     
   • If control unit B is again the first to store the ”Fibre-optic cable communication fault”, it can be assumed that the continuing defect lies between control units A and B.
     
3. Check control units A and B for loose plug-in connections and check the fibre-optic cable for kinks in the 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 testing and programming unit. The test program for the optical test on the MOST bus can be found under function selection: Body, Bus Functions, MOST Functions and Optical Test.