MOST bus

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.

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. Exceptions are the gateway control unit Car Communication Computer (or Multi Audio System Controller) and the Head Up display control unit. Those control units are connected to MOST data bus the K-CAN.

Car Communication Computer or Multi Audio System Controller

Either the control unit Car Communication Computer or Multi Audio System Controller is fitted in the vehicle. These two control units are the interface between the buses MOST and K-CAN. For the rest of troubleshooting (read fault code memory, etc.), at the BMW diagnosis system for the

• Car Communication Computer all the control unit names with the abbreviation CCC must be selected (e.g. CCC-GW)
  
• Multi Audio System Controller all the control unit names with the abbreviation M-ASK must be selected (e.g. M-ASK-GW)
  

In comparison to the Multi Audio System Controller, the Car Communication Computer has two slots at the front. This external characteristic can be used to distinguish the tow control units.

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 ”Deactivation, excess temperature” is stored in a MOST control unit, the control unit concerned and the Multi Audio System Controller (or Car Communication Computer) also store the fault. This means there are two possibilities:

• Only the Multi Audio System Controller (or Car Communication Computer) has stored the fault: Then this control unit has switched off due to excess temperature.
  
• The Multi Audio System Controller (or Car Communication Computer) 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, however, the Multi Audio System Controller (or Car Communication Computer) can have switched 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.