MOST bus: control unit sequence

Transmission direction and control unit sequence in the MOST ring

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. The transfer of messages starts and ends at the head unit.

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

Automatic determination of the control unit sequence

The test module ”control unit sequence” enables automatic determination of the control unit sequence. This automatic determining only delivers the right results if the configuration of the MOST ring in the head unit is correctly stored.

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

• Determine the control unit 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 ring -> 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.
  

Manual determination of the control unit sequence

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

• The head unit is connected on the CAN bus and MOST bus.
  
• For the order of the other MOST control units, look at the labelling of the optical cables at the optic-fibre cable connector. The input cable of the optic-fibre cable connector is the cable labelled with ”RAD2”/”MASK”/”CCC”. Each input of the connector has a direct optical connection to the opposite output. This means that the order of the control units can be determined by means of the following labelling:
  
  • CDC (= CD changer)
    
  • AMP (= amplifier)
    
  • VM (= video module)
    
  • TEL (= Telephone)
    
  • TCU (= Telematic Control Unit)
    
  • ULF (= universal charging and hands-free unit)
    
  • JNAV (= navigation system Japan)
    
  • DAB (= digital tuner)
    
  • IBOC (= digital tuner (US))
    
  • KHI (= headphone interface)
    
  • HUD (= Head Up display)
    
  • MMC (= DVD changer)
    
  • SDARS (= satellite tuner (US))
    
  • The output of the optic-fibre cable connector is labelled with ”FLASH”. This cable leads to the MOST direct access point and then to the head unit. This closes the ring.