MOST Bus
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 head unit (dependent on equipment fitted).
Service information for MOST system analysis
- Due to the ring structure of the MOST, a component fault often affects the complete system. A component fault thus generates fault memory entries in a number of control units.
- The MOST system analysis has been restructured and simplified.
- The following test procedures are available:
- MOST system analysis
- Deactivation, MOST excess temperature
- Service function for saving the desired configuration
MOST system analysis
- The MOST system analysis runs during the quick test. It analyses all communication faults with MOST control units.
- The "MOST system analysis" test procedure also assesses the stability (MOST ring broken or unbroken) and the order of the control units in the MOST ring.
- The ring break diagnosis is integrated in the MOST system analysis. For that purpose the order of the MOST control units in the ring is read out and the location of the ring break indicated.
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.
- Head unit: high temperature cut-out (e.g. basic head unit HU-B)
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, it can sometimes be due to a problem with the ventilation of the control unit.
Service function for saving the desired configuration
The specified configuration of the MOST ring (= control units fitted and the order they are arranged in the MOST ring) is stored in the head unit. If a control unit is retrofitted or the head unit is replaced, the specified configuration must be saved again. The desired configuration is carried out with the programming (ISTA/P). Then, if a break in the ring occurs, the specified configuration can be used to determine between which control units the break is located.
Possible MOST devices
MOST devices that are always fitted:
- Instrument panel: instrument cluster (high-spec version only, no MOST with basic version)
- HU-H, HU-B, CIC, CHAMP: Headunit
MOST devices that are fitted depending on optional extras:
- DVDC: DVD changer
- CBX-MEDIA and CBX-ECALL: Combox multimedia and Combox emergency call
- VM: Video module
- AMPT: Top HiFi amplifier
The arrow shows the direction of the light in the MOST ring.
NOTICE: programmed via Ethernet.
The MOST devices are programmed via Ethernet.
Intelligent Network Interface Controller (INIC)
Integrated in the MOST control units is an Intelligent Network Interface Controller (INIC). The INIC is the transmitter and receiver 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.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.