MOST bus: Network wake-up unsuccessful

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, the system can only communicate with the Car Communication Computer (or Multi Audio System Controller) via diagnosis. This is possible because those two control modules are directly connected to the K-CAN data 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. An exception is of course the gateway control module 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 module Car Communication Computer or Multi Audio System Controller is fitted in the vehicle. These two control modules are the interface between the buses MOST and K-CAN. For further troubleshooting (reading fault memory, etc.), the control module name must be selected at the tester CCC-GW or M-ASK-GW!

Diagnosis MOST bus

For the MOST bus, there are intersystem 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:

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. Switch to ”Control-Module Functions” of the special MOST control module (component activation: MOST bus light output reduction) and this control module will 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.

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 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 modules are connected to the 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.

Counting method for multimedia changer and Japan navigation system: For the multimedia changer control module 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 module equipment: You count the node position backwards from the Car Communication Computer (or Multi Audio System Controller) until you reach the control module with the 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.

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 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:
  2. Beginning with the Car Communication Computer (or Multi Audio System Controller) (using the MOST ring circuit diagram) count down the node position towards the rear display control module, telephone 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 Japan navigation system occur when counting backwards, you have to subtract from node position 2! see also Counting method for multimedia changer and Japan 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.