7
MOST Bus Diagnosis
Control Unit/Control Unit Connection
The MOST ring is composed of optical point-to-point connections between 2 control units.
Each control unit has a network interface. The network interface consists of:
An opto-electrical converter (optical waveguide receiver, already mentioned).
An opto-electrical converter (optical waveguide transmitter, already mentioned).
A MOST transceiver (interface between the optical waveguide receiver/transmitter and
the electronic network driver).
A network driver, the so-called NetServices.
The NetServices run on a microcontroller (main computer in the control unit)
On the application level, a control unit in the MOST framework contains stand-alone func-
tion units, so-called function blocks. Examples of function blocks include:
Tuners
Amplifiers
CD players
A control unit can contain several function blocks at one time, e.g. the AVT contains the
functions:
Antenna
Amplifier
Tuner
MOST Fiber Optic cable
Example
SVSNavigation
Network interface
Network interface
NetServices SWNetServices SW
Most
transceiverMost
transceiver
Optical
receiverOptical
receiver Optical
transmitterOptical
transmitter
KT-9397
1
1 22
10
MOST Bus Diagnosis
CD Changer Audio (CDC)
The CD changer is a slave control unit in the MOST framework.
Navigation System (NAV 01)
The control unit of the navigation system has controller tasks and slave functions in the
MOST framework.
Slave Control Units
The following control units are slave control units:
Kombi (control unit of the instrument cluster)
AVT
LOGIC7
SVS Speech processing system
Telephone
MMC
Component Locations
Located in the dashboard assembly:
Control Display
CD Changer
ASK
Kombi
OPPS Connector
Located in the luggage compartment, rear left:
Logic 7
SVS
NAV
MMC
Telephone
Located in the C pillar left side:
AVT
12
MOST Bus Diagnosis
Diagnosis
It is important to remember that on the MOST network, messages can only be transmitted
provided the bus ring is complete and fully functional. If there is a ring fault in the MOST
network, the diagnostic system only communicates with the instrument cluster and the
Control Display because both of these modules are directly connected to the K-CAN
System Bus.
The fiber optic signals on the MOST network always travel in one direction and only in one
direction. Signals always originate at the Control Display and travel to the CD changer, AVT,
Logic 7 (if equipped), SVS, NAV, MultiMediaChanger (if equipped), Telephone, ASK, Kombi
and back to the Control Display.
The MOST bus allows intersystem fault memory entries in the individual control modules.
A feature of the system faults is that faults may be entered in a control module although the
control module is OK. Conclusions may be drawn about the cause of the fault, using the
fault information stored in all the control modules.
The possible system faults are:
Optical wave guide communication fault (All MOST Control Module) FC 111
A Control Module does not switch a light off (All MOST Control Modules)
Network wake-up unsuccessful (Control Display (Gateway), ASK, Telephone Only)
FC E18D
Ring fault diagnosis run (Control Display (Gateway) and Kombi Only) FC E190
The Control Display functions are split between acting as a Gateway and Displaying infor-
mation. The Gateway function serves as the interface between the MOST and the K-CAN
System buses. Although the Control Display is one control module, two control module
names are displayed in the DIS Plus:
CD Control Display (Gateway)
CD Control Display
The faults stored in the Control Display are distributed between the Control Display
Gateway and Control Display according to the function of the fault.
