width BMW 7 SERIES LONG 2006 E66 MOST Bus Diagnosis Workshop Manual

4
MOST Bus Diagnosis
Principle of a Multimedia Network
An important feature of a multimedia network is that it transports not only control data and
sensor data, e.g. like the CAN bus and I bus (instrumentation bus). A multimedia network
can also carry digital audio and video signals and graphics as well as other data services.
Advantages of the Multimedia Network
All data can be transported across a shared network. This offers the following benefits:
 Additional signal wiring harnesses are eliminated.
 The only addition many control units need is the power supply.
 As each participant (= each control unit) has access to all the
data, cost-intensive assemblies for signal distribution can be
eliminated.
Different data formats also have different requirements for transmission regarding both
mechanism (synchronous or asynchronous data) and the required band-width
(baud or bits/second). The MOST format is able to meet these requirements to a satisfac-
tory extent.
Audio
Video
RGB
Data Services
Control
KT-9387
Information transmitted on MOST network

5
MOST Bus Diagnosis
Functional Description
Data Transport
MOST currently offers a band-width of 22.5 Mbits/s . In the next generation, the band-width
will be increased to 50 and then later to 150 Mbits/s (as of approx. 2002).
In order to meet the different requirements of the applications regarding data transport,
each MOST message is divided into three parts:
 Control data
 Asynchronous data: e.g. navigation system, arrow
representation
 Synchronous data: e.g. audio, video signals
The control data controls the functions and devices in the network. The information can be
compared to CAN bus data.
The control data has a band-width of 700 Kbit/s. That corresponds to around 2 27
70
00
0

mes-
sages per second.
For the data transmission of synchronous and/or asynchronous data, there is a total of 60
bytes. The limit is variable: e.g. 20 bytes of synchronous data and 40 bytes of asynchro-
nous data.
A message over the MOST
Control Data
700 kB/s
2700 Messages
Asynchronous
Data
e.g Navigation (arrow
display)
Synchronous data
Audio
Video
Data Transport on the MOST
Different data is “packaged”
and then sent as one mes-
sage to be “unwrapped” by
next device on the MOST
ring.
KT-9388
44.1 MHz