MOST Bus 1

MOST Bus

MOST bus
The MOST (Media Oriented Systems Transport) bus uses a ring bus for data traffic between the various control units. Signal transmission is by means of optical fibre. 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 the ring is broken, diagnosis communication is only possible with the central gateway module (ZGM). This is possible because the control unit is directly connected to the K-CAN. The headunit also a K-CAN connection, but no gateway function. For this reason, if the MOST bus is interrupted it is no longer possible to communicate with the headunit.

Service information for MOST system analysis
- Due to the ring structure on the MOST, a component fault often affects the complete system. A component fault thus generates fault entries in a number of control units.
- The MOST system analysis has been restructured and simplified.
- The following procedures are available as of F01:
- MOST system analysis
- Deactivation, MOST excess temperature
- Service function for saving the desired configuration

MOST system analysis
- All relevant faults in the MOST are displayed in the vehicle test (no longer a summary for a virtual control unit).
- The stability (MOST ring closed or interrupted) and the order of the control units in the MOST ring are evaluated in the test module "MOST system analysis".
- The ring break diagnosis is integrated in the MOST system analysis (as of F01). Here, the order of the MOST devices in the ring is read out and the position of the ring fault displayed.

Deactivation, MOST 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.
The following 2 redundant fault entries for shutdown due to excess temperature are present:
- MOST device: Switch-off, excess temperature
This fault entry is entered in the headunit if the headunit has detected a shutdown due to excess temperature in another control unit.

- ZGM: Disengagement due to excess temperature (e.g. Central Gateway Module)
This fault entry is entered in every single MOST device that shuts down in response as a consequence of excess temperature.

Normally, a control unit must not get too hot. If a control unit overheats, under certain circumstances there will be a problem with the ventilation of the control unit.

Service function for saving the desired configuration
The desired configuration of the MOST ring (= installed control units in their order in the MOST ring) is stored in the headunit and in the central gateway module (ZGM). If a control unit is retrofitted or the ZGM or headunit is replaced, the desired configuration must be saved again. The desired configuration is carried out with the programming (ISTA/P). In the event of a ring fault, it is then possible to use the desired configuration to determine the control units between which the open circuit exists.

Possible MOST devices
MOST devices that are always fitted:
- ZGM: Central gateway module
- Instrument panel: Instrument panel
- CIC: Car Information Computer

MOST devices that are fitted depending on optional extras:
- DVDC: DVD changer
- AMPT: Top HiFi amplifier
- TCU or ULF-SBX: Telematics control unit or interface box
- ULF-SBX-H: Interface box High
- RSE: Rear seat entertainment system
- VM or SDARS: Video module or satellite tuner







The arrow shows the direction of the light in the MOST ring.

NOTICE: MOST port has been eliminated.

As of F01, MOST port has been eliminated. The MOST devices are programmed via Ethernet.

Incoming changes possible to MOST devices
- AMPT sequence changed on the MOST network
- SDARS integrated in the headunit (e.g. CIC)
- Optic-fibre cable connectors now eliminated







Intelligent Network Interface Controller (INIC)
With the ULF SBX, a MOST device with an Intelligent Network Interface Controller (INIC) was installed for the first time. All of the control units developed for the F01 (e.g. RSE, ZGM, KOMBI, CIC) have a built-in INIC. The INIC is the transmit and receive module in the MOST device. The INIC itself is a fully functional, autonomous network node. The network node can operate the fundamental network functions without interaction with its application. This means that the MOST ring remains functional even if a control unit has failed or is running a reset.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.