MOST bus

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.

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

Car Communication Computer or Multi Audio System Controller

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

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

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 ”Deactivation, excess temperature” is stored in a MOST control module, the control module 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 module has switched off due to excess temperature.
  
• The Multi Audio System Controller (or Car Communication Computer) 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, 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 modules or MOST bus can be selected.