MOST bus

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

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 module is defective, it is no longer possible to communicate with the control modules on the MOST bus. An exception is the gateway control module of the head unit (Rad2, CHAMP, M-ASK, CCC or following). The control module 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 memory, etc.), the associated control modules 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 module becomes too hot, this control module switches off for up to 10 minutes. Furthermore, the entire MOST bus can switch off for 10 minutes. After the control module has cooled down, it is functional once again!

If the fault ”Cutoff excess temperature” is stored in a MOST control module, the control module 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 module has switched off due to excess temperature.
  
• The head unit and other MOST control modules have stored the fault: Then only the other MOST control modules 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 modules or MOST bus can be selected.