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, 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 directly connected to the K-CAN data 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. An exception is of course the gateway control unit Car Communication Computer (or Multi Audio System Controller).
Transmission direction in the MOST ring
Messages are transmitted from the Car Communication Computer (or Multi Audio System Controller) towards the CD changer, telephone, ..., and back to the Car Communication Computer (or Multi Audio System Controller).
Car Communication Computer or Multi Audio System Controller
Either the control unit Car Communication Computer or Multi Audio System Controller is fitted in the vehicle. These two control units are the interface between the buses MOST and K-CAN. For further troubleshooting (reading fault code memory, etc.), the control unit name must be selected at the tester CCC-GW or M-ASK-GW!
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, 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:
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.
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.
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 that follows the control unit with the node position 0 stores the node position 1, and the next one the node position 2, etc.
Counting method for multimedia changer and Japan navigation system: For the multimedia changer control unit and the Japan navigation system, when counting backwards from the Car Communication Computer (or Multi Audio System Controller), a jump of 2 must be made!
Counting method depending on the MOST control unit equipment: You count the node position backwards from the Car Communication Computer (or Multi Audio System Controller) until you reach the control unit with the 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.
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 stored in the Car Communication Computer (or Multi Audio System Controller) control unit is specified. Remaining procedure: