MOST bus

The MOST (Media Oriented Systems Transport) network uses a ring bus to enable data communication between the various control units.

Data 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, 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 the gateway control unit of the head unit (Rad2, CHAMP, M-ASK, CCC or following). The control unit is connected both to the MOST data bus and the K-CAN.

Head unit

The head unit is the interface between the data buses MOST and K-CAN. For the rest of troubleshooting (read fault code memory, etc.), the associated control units must be selected at the BMW diagnosis system. One of the following head units is fitted in the vehicle.

• Radio 2, (RAD2, RAD2 gateway)
  
• Central Head Unit and Multimedia Platform, (CHAMP-BO, CHAMP-GW)
  
• Multi Audio System controller, (M-ASK-BO, M-ASK-GW, M-ASK-NAV)
  
• Car Communication Computer,(CCC-ANT, CCC-ASK, CCC-A, CCC-BO, CCC-GW)
  

Distinction between the head units

In comparison with the M-ASK and CHAMP, the CCC has two slots at the front. This external characteristic can be used to distinguish the CCC and M-ASK.

Deactivation, excess temperature

If a control unit becomes too hot, this control unit switches off for up to 10 minutes. Furthermore, the entire MOST bus can switch off for 10 minutes. After the control unit has cooled down, it is functional once again!

If the fault ”Cutoff excess temperature” is stored in a MOST control unit, the control unit concerned and the head unit store the fault. This means there are two possibilities:

• Only the head unit has stored the fault: Then this control unit has switched off due to excess temperature.
  
• The head unit and other MOST control units have stored the fault: Then only the other MOST control units have switched off due to excess temperature. In addition, the head unit can have switched itself off.
  

Diagnosis

At the function selection (Body, Bus Functions, MOST functions), other test modules for analysis of the MOST control units or MOST bus can be selected.