MOST bus
MOST Bus
The MOST (Media Oriented Systems Transport) bus uses a ring bus for data communication between the various control modules. 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 module 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 memory entries in a number of control modules.
- 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 module).
- The stability (MOST ring closed or interrupted) and the order of the control modules 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 modules in the ring is read out and the position of the ring fault displayed.
Deactivation, MOST 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.
The following 2 redundant fault memory entries for shutdown due to excess temperature are present:
- MOST control module: Shutdown excess temperature
This fault memory entry is entered in the head unit if the head unit has detected a shutdown due to excess temperature in another control module.
- ZGM: Shutdown excess temperature (example: central gateway module)
This fault memory entry is made in each individual MOST control module that runs a shutdown due to excess temperature.
Normally, a control module must not get too hot. If a control module overheats, under certain circumstances there will be a problem with the ventilation of the control module.
Service function for saving the target configuration
The target configuration of the MOST ring (= installed control modules in their order in the MOST ring) is stored in the head unit and in the central gateway module (ZGM). If a control module is retrofitted or the ZGM or head unit is replaced, the target configuration must be stored 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 modules between which the interruption exists.
Possible MOST control modules
MOST control modules that are always fitted:
- ZGM: Central gateway module
- KOMBI: Instrument cluster
- CIC: Car Information Computer
MOST control modules that are fitted depending on optional extras:
- DVDC: DVD changer
- AMPT: Top HiFi Amplifier
- TCU or ULF-SBX: Telematics control module or interface box
- ULF-SBX-H: Interface box High
- RSE: Rear Seat Entertainment
- 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 modules are programmed via Ethernet.
Intelligent Network Interface Controller (INIC)
With the ULF-SBX, a MOST control module with an Intelligent Network Interface Controller (INIC) was installed for the first time. All of the control modules 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 module. 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 module has failed or is running a reset.
No liability can be accepted for printing or other faults. Subject to changes of a technical nature