6
MOST Bus Diagnosis
Optical Bus
The MOST bus is a plastic optical waveguide. The MOST bus is coded in green in the E65
(Repair cables are black in color). The light wavelength is 650 nm (red light). The MOST
bus requires the following converter components:
Optical transmitter
Optical receiver
Each control unit of the MOST framework contains a transmitter and a receiver. The trans-
mitter and receiver have been developed by BMW. The low closed circuit (rest) current
properties of the transmitter and receiver enable optical wake-up by the MOST bus.
Optical Transmitter
A driver is fitted in the transmitter. The
driver energizes an LED (light-emitting
diode).
The LED transmits light signals on the
MOST bus (650 nm light, i.e. red visible
light). The repeat frequency is 44.1
MHz.
The sensing frequency on a CD player and for audio is 44.1 MHz; this means than no addi-
tional buffer is required, yet another reason why this bus system is so efficient for multi-
media.
Optical Receiver
The receiver receives the data from the
MOST bus. The receiver consists of:
An LED
A pre-amplifier
A wake-up circuit
An interface that converts the optical signal into an electrical signal
The receiver contains a diode that converts the optical signal into an electrical signal. This
signal is amplified and further processed at the MOST network interface.
Transmitter
Light LED Driver
Receiver
Light
LED
Pre-
Amp. Wake
-up
43-07-31
43-07-30
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
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