MOST Bus
MOST Bus
The MOST (Media Oriented Systems Transport) bus 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 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 head unit 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 head unit.
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 code memory entries in a number of control units.
- The MOST system analysis has been restructured and simplified.
- The following test modules are available as of F01:
- MOST system analysis
- Deactivation, MOST excess temperature
- Service function for saving the target 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 fault diagnosis is integrated in the MOST system analysis (as of F01). Here, the order of the MOST control units 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 code memory entries for shutdown due to excess temperature are present:
- MOST control unit: Deactivation, excess temperature
This fault code memory entry is entered in the head unit if the head unit has detected a shutdown due to excess temperature (i.e. overheating) in another control unit.
- ZGM: Disengagement due to overheating (e.g. Central Gateway Module)
This fault code memory entry is entered in every single MOST control unit that shuts down in response as a consequence of overheating.
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 target configuration
The target configuration of the MOST ring (= installed control units in their order in the MOST ring) is stored in the head unit and in the central gateway module (ZGM). If a control unit is retrofitted or the ZGM or head unit is replaced, the target configuration must be saved again. The target configuration is carried out with the programming (ISTA/P). In the event of a ring fault, it is then possible to use the target configuration to determine the control units between which the interruption exists.
Possible MOST control units
MOST control units that are always fitted:
- ZGM: Central gateway module
- KOMBI: Instrument panel control unit
- CIC: Car Information Computer
MOST control units 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.
NOTE: MOST direct access has been eliminated.
As of F01, MOST direct access has been eliminated. The MOST control units are programmed via Ethernet.
Incoming changes possible to MOST control units
- AMPT sequence changed on the MOST network
- SDARS integrated in the head unit (e.g. CIC)
- Fibre-optic cable connectors now eliminated
Intelligent Network Interface Controller (INIC)
With the ULF-SBX, a MOST control unit 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 control unit. 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.
No liability can be accepted for printing or other errors. Subject to changes of a technical nature