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 there is a ring fault in the MOST network, however, the diagnosis system can only communicate with the instrument cluster and the Control Display. This is possible because those two control modules are directly connected to the K-CAN SYSTEM bus.
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 here are of course the instrument cluster and the Control Display.
Transmission direction in the MOST ring
Transmission of messages is from the Control Display towards the CD changer/iPod interface, aerial tuner, ..., instrument cluster, ... and back to the Control Display.
Control Display
The Control Display is divided into the functions CD GW Control Display (Gateway) and CD Control Display. The gateway is the interface between the MOST and K-CAN SYSTEM buses. For the remaining troubleshooting (read fault memory, etc.) of the Control Display, only the control module name CD GW Control Display (Gateway) needs to be selected at the tester!
Due to their high standby current, the MOST control modules must be disconnected from the power supply in ”sleep mode”. If, for example, there is a fault on the bus that continuously wakes up the MOST bus, the Control Display automatically wakes up the CAN bus. This results in all the buses in the vehicle waking up. However, this leads to battery discharge. For this reason, it is of great benefit to know who initiated the wake-up! In order to find out which MOST control module wakened the MOST bus, you can read out the status ”Wake up” from all MOST control modules. If you want to determine the status ”Wake up” of an individual MOST control module, you have to switch to the control-module functions and select the ”Wake-up status” mode under diagnosis requests.
Possible statuses include:
The control module with the status ”control module woke up” is the cause of the wake-up. This has led to identification of which CU caused the fault, but not to any indication regarding why.
Example:
CD-GW Control Display (Gateway) delivers the status ”control module woke up”. It can then be that the Control Display woke up due to
Meaning of ”sleep”:the internal power supply is switched off
Meaning of ”wake”:the internal power supply is switched on.