transmission BMW 7 SERIES LONG 2005 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

11
MOST Bus Diagnosis
Communication Direction in MOST structure
Important!!! The component sequence of the MOST controllers in the ETM is incor-
rect when it comes to signal transmission direction. The correct sequence is indi-
cated above!
The signal transmission direction of the MOST in a vehicle
with full equipment takes place starting at the Control
Display and travels serially towards the CD changer,
Antenna tuner, Hi-fi amplifier, Speech Processing Module,
Navigation, Multi-media Changer, telephone, Audio
System Controller, Instrument Cluster and again back to
the Control Display.
Counts as 2Counts as 2

13
MOST Bus Diagnosis
Optical Wave Guide Communication Fault
This fault (FC 111) indicates a problem with optical transmission. Insufficient light is being
received by one of the modules in the ring. The loss of light may be caused by:
 Defective optical wave guide, Harness twisted too tightly (Min. bend radius 50mm.)
 Light output or reception sensitivity of a diode is too low
 Connector not installed correctly
 Voltage fluctuation while powering up a control module
If the fault is stored, the system triggers a reset and starts up again. The music is switched
off briefly and the display screen of the Control Display continues to operate.
To find the module responsible for the fault, the fault memory of the modules must be read
in MOST ring order.
Fault lies between the module with the fault code (B) and the preceding module (A).
If the voltage has dipped below 9v, the fault may be incorrectly stored. If the voltage is low
perform the following test after connecting a battery charger.
1.Clear the fault memory in control module B.
2.Lower the light output in control module A.
3.Read out the fault memory in the MOST ring in order.
4.If control module B is again the first to store the fault, it can be assumed the fault lies
between control modules A and B.
Then, check control modules A and B for loose connections and check the optical wave
guide for kinks. If the visual inspection is OK, the fault can be located using the OPPS tester
or optionally performing the following tests.
 Remove the input optical wave guide from control module B and confirm the presence
of light.
If light is present, install by-pass optical wave guide in place of control module A, clear
fault codes in module B and perform ring break test. If MOST network operates prop-
erly, then control module A is at fault and must be replaced.
If MOST network still has a fault, put module A back in the network and by-pass mod-
ule B. Clear faults and again perform ring break test. If MOST network operates now
problem is with control module B and it must be replaced.
 If light is not present at input of module B, perform by-pass of module A as above.
Note: AMP Butt connector # 1355734-1

14
MOST Bus Diagnosis
The possible fault scenarios are:
 Transmit diode in module A bad
 Receive diode in module B bad
 Optical wave guide fault between modules A and B
 Software error or fault in module A or B
Control Module Does Not Switch Off Light
When the MOST network is requested to sleep, the Control Display switches off the light in
the MOST ring. The lack of light input is a signal to the individual control modules to switch
off their light output and enter sleep mode.
If a control module does not switch off its light, all down stream control modules register
the fault “A Control Module is not switching light off.”
Important:
Failure of a control module to turn its light off, will cause the MOST network NOTto enter
sleep mode. If the MOST network fails to sleep, the rest of the car will not be able to enter
sleep mode. This will lead to battery discharge.
To diagnose:
 Read out fault memory in MOST ring order
The fault lies in the control module that precedes the module where the fault is
first stored.
Always confirm the problem by first clearing the fault and performing the diagnosis a sec-
ond time. If the same results occur, replace the defective control module.
Network Wakeup Unsuccessful
This fault indicates a problem with the optical transmission. An insufficient volume of light is
coming through one position of the ring and may be caused by:
 Control Module is receiving no voltage
 Optical Wave Guide harness defective
 Optical Element in a control module defective (transmit or receive)
 Connector not installed correctly
A distinction must be made as to whether the fault is currently present or sporadic.
For faults currently present, run the Ring Break DiagnosisTest Plan.
For sporadic faults perform the Luminous Power ReductionTest Plan.

15
MOST Bus Diagnosis
Ring Break Diagnosis Carried Out (FC E190)
Reading out the fault memory of the Control Display (Gateway) after performing the Ring
Fault Diagnostic, results in a fault of Ring Fault Diagnosis Carried Out being stored.
This fault memory is not a true fault memory entry, but only an output of additional infor-
mation for relative node position.
Testing
Light Output Reduction
Reducing the light output of individual control modules is a convenient method of deter-
mining the area of a defect.
Switch on the radio.
In Control Module functions, begin to activate luminous power reduction in the individ-
ual control module. (In this test the light output of the selected control module is
reduced for 5 seconds and then automatically reset to normal output)
If the optical transmission for control module Ato the next control module in the ring
(control module B) is OK, a slight noise may occur when the light output is reduced
however the radio will continue to play.
If the radio goes off and comes back on again(radio volume may be reduced) in 5 to 10
seconds, the optical transmission between control modules Aand Bis defective.
If the visual inspection is OK, the fault can be located using the OPPS tester or optionally
performing the following tests.
 Remove the input optical wave guide from control module Band confirm the presence
of light.
If light is present, install by-pass optical wave guide in place of control module A, clear
fault codes in module B. If MOST network operates properly, then control module A
is at fault and must be replaced.
If MOST network still has a fault, put module Aback in the network and by-pass
module B. Clear faults.
If MOST network operates now problem is with
control module Band it must be replaced.
 If light is not present at input of module B,
perform ring break diagnostics.
12

16
MOST Bus Diagnosis
Ring Break Test
If there is a break in the ring (a defect between two control modules) the following fault pat-
terns may occur:
 Transmit diode of the transmitting control module defective
 Power supply of the transmitting control module defective
 Internal control module fault of the transmitting control module
 Receiver diode of the receiving control module defective
 Power supply of the receiving control module defective
 Internal control module fault of the receiving control module
 Optical wave guide between transmitting and receiving control module defective
These faults may occur alone or in combination. To diagnose a ring break, the first step is
to locate the two control modules between which the transmission failure has occurred.
This is accomplished with the ring break diagnostic function. Once the two control modules
have been identified and the diagnostics have been performed, remember to check the
power supply and ground circuit of both modules before condemning a module.
Testing of the transmit/receive diodes will be possible using the OPPS tester.
Perform Ring Break Test
The ring break test mode is entered automatically when the power to all the modules in the
MOST network is switched off and then switched back on. The most effective method of
switching the power off and on is to disconnect the battery negative terminal for 45 sec-
onds. This time will allow the capacitors of all the control modules to dissipate.
When the battery is reconnected the control modules wake up and in MOST network order
transmit a light signal to the next module. Each module checks to see if it has received a
light signal from the previous module. If the control module does NOT receive a light input
signal it still transmits a signal to the next module. A relative node number of 0 is stored in
the control module that did not receive a signal but that transmitted one.
The Control Display receives the light signal back and identifies which modules responded.
Go to “Control Unit Functions” Control Display Gateway and read fault memory.