MOST bus: Ring fault diagnosis

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:

• 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, select and run the test module Ring fault diagnosis. The first step is that the stability of the MOST ring is checked.

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

If the MOST ring is permanently interrupted, continue 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 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 a few seconds. 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 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 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 and then on again, 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.

Determining the control module with node position ”0”

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 located in the ring after the control module with node position 0 stores node position 1; the next control module in the ring stores 2 etc. This enables you to determine the position of the ring fault on the basis of the node position stored in the Car Communication Computer (or Multi Audio System Controller). This number can namely be read from the CAN bus. By counting downwards, backwards starting from the Car Communication Computer (or Multi Audio System Controller), you can determine the control module with node position 0.

Special manner of counting for DVD changer: For the DVD changer control module, when counting backwards from the Car Communication Computer (or Multi Audio System Controller), a jump of 2 must be made!

The ring fault diagnosis is run automatically in the test module. In the test module, the node position stored in the Car Communication Computer (or Multi Audio System Controller) control module is specified. Further procedure if the position of the ring fault can not be determined automatically after determining the node position:

Procedure for locating the position of the ring fault using the node position:

1. Determine the order of the control modules in the ring. (To do so, select the menu item ”MOST ring control module sequence” in the test module)
   
2. From the Car Communication Computer (or Multi Audio System Controller) starting backwards (using the circuit diagram for the MOST ring) count down to the node position. If the DVD changer control module occurs when counting backwards, you have to subtract from node position 2! (Refer to the special manner of counting for DVD changer). Otherwise, subtract one per control module. 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.
   
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 module for checking power supply:Switch to the function selection on the diagnosis system. The test module for checking the power supply of the MOST control modules can be selected At Body, Bus Functions, MOST Functions and MOST Control Units Power Supply.
     
   • Test module for optical test:Switch to the function selection on the diagnosis system. The test module for the optical check of the MOST bus can be selected at Body, Bus Functions, MOST Functions and Optical Test.