ESP BMW 7 SERIES LONG 2004 E66 MOST Bus Diagnosis Workshop Manual

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

8
MOST Bus Diagnosis
Interfaces
The following contains a brief summary of the tasks of the connected control units in the
MOST framework:
Control Display
The Control Display is the system master of all MOST bus functions and serves as the
power master. It wakes up the bus and is responsible for switching it off (power down).
The DIS Plus and GT1 will show a Control Display and a Control Display (Gateway).
Audio System Controller (ASK)
The audio system controller has the following controller functions:
Network Master
The ASK performs the role of network master for the MOST bus. The functions of the net-
work master are the following:
 Wake-up, initialization, power-down
The network master wakes up the bus and has the task of achieving an orderly
initialization of the network. The ASK can operate with KLR off. To turn it on, push in
the volume/ON/OFF knob. Adjustments and control is carried out by using the
Controller and Control Display.
Another task of the network master is to control the power-down process. Each power-
down is initiated and started by the ASK.
 Configuration control
The network master detects the exact system configuration each time that the network
is started and compares it to the stored coded configuration.
 Control of the network operation
The network master controls the MOST transceiver of the slave equipment for correct
operation. The equipment which is not operating properly will be released by a reset or
switched to low power mode so that they do not affect bus communication.
 Fault code memory
The network master includes the fault code memory of the MOST network. It stores all
the faults occurring during the network operation as well as deviations from the
nominal configuration.

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

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.

18
MOST Bus Diagnosis
Status Wakeup
MOST control modules require high current during standby operation and must be discon-
nected or put in sleep mode to prevent the vehicle battery from being discharged. In case
of a fault on the the MOST network that continuously wakes up, the entire MOST bus will
be woken up. The Control Display will wake up the CAN Bus and all the vehicle busses will
be woken up. This will lead to battery discharge.
It is of great benefit to know which module initiated the wake up call. In order to find out
which MOST node woke up the MOST bus, the following procedure is performed:
In Control Unit Functions, press “STATUS WAKEUP”
Three different response are possible:
 Control Module woke up
 Control Module woken up
 Control Module not initialized
The Control Module with the status “Control Module woke up” is the module that woke up
the rest of the MOST bus.
This diagnosis only informs which control module woke, not the reason for the wake up,
diagnostic testing should be performed on the control module and related equipment.
Hints for Vehicle Equipment Identification
 
CDC - Look on passenger side of dashboard above glovebox.
 Logic 7 - Look for speaker grills on rear doors
Look on left side of trunk for
large amp.
 MMC - Look on left side of trunk.
Workshop Hint
Telephone modules may be swapped from vehicle to vehicle
for testing purposes, however if the original telephone mod-
ule is not returned to the original vehicle the Emergency func-
tions will not operated properly.
In case of an accident, information regarding location and
VIN will not be correct therfore help will not be dispatched.