MOST bus: control module sequence

Transmission direction and control module sequence in the MOST ring

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. Here, the transfer of messages starts and ends at the Multi Audio System Controller (or Car Communication Computer).

With the corresponding MOST equipment, the ring-shaped connection of the control modules is implemented using a light station. This light station is located in the luggage compartment on the left above the wheel. As the light station means that the control modules can be connected differently with one another on replacement and in the plant, there is no fixed control module sequence in the vehicle. The system circuit diagram thus only shows the maximum MOST equipment, not the right order of control modules. The order of control modules can, however, be determined automatically or manually.

Automatic determination of the control module sequence

The test module ”control module sequence” enables automatic determination of the control module sequence. This automatic determination only provides the right results if the configuration of the MOST ring is correctly stored in the Multi Audio System Controller (or Car Communication Computer).

If the automatic determination returns implausible results, there are two possibilities:

• Determine the control module sequence manually (see below).
  
• Store the configuration of the MOST ring once again. To do so, switch to the function selection on the diagnosis system. Then call up the test module at Service functions -> Body -> MOST framework -> Save MOST ring configuration. This procedure may only be run if the MOST ring is closed. For this reason, check the ring stability first when you call up the test module. Then call up the test module ”control module sequence” once again.
  

Manual determination of the control module sequence

The order of the control modules in the MOST bus can be determined by a visual check in the vehicle.

• The Multi Audio System Controller (or Car Communication Computer) is connected to the CAN bus and MOST bus.
  
• The next MOST control module in the direction of the light is the CD changer, if fitted.
  
• The next MOST control module is the Head Up display, if fitted.
  
• For the order of the other MOST control modules, look at the labeling of the optical cables at the optic-fiber cable station. The input cable of the light station is the cable labeled with ”MASK”/”CCC”, ”CDC” or ”HUD”. Each input of the station has a direct optical connected to the opposite output. This means that the order of the control modules can be determined by means of the following labeling:
  
  • KHI or KHM (= Headphones Interface)
    
  • DVDC (= DVD Changer)
    
  • AMP (= Amplifier)
    
  • VM (= Video Module)
    
  • TEL (= Telephone)
    
  • TCU (= Telematic Control Unit)
    
  • ULF (= Universal Charging and Hands-Free Unit)
    
  • JNAV (= Navigation System Japan)
    
  • SDARS (= Satellite Tuner (US))
    
  • The output of the light station is labeled with ”FLASH”. This cable leads to the flash adapter and then to the Multi Audio System controller (or Car Communication Computer). This closes the ring.