System Control Module J519.
This is in part because the climate control system is now
also a function of J519.
This means that a separate air conditioning control module
is no longer installed in the Audi A8.

The topology diagram cannot provide a realistic representation of the actual networking situation, especially in the
area of the FlexRay bus. In the case of the LIN bus users,
multiple LIN slaves are grouped together in the diagram in
order to provide some measure of clarity.
In the area of the AC servomotors or the interior light
modules, for example, only one LIN user is shown symbolically, although the LIN slaves are actually daisy-chained.
The following table provides an overview of the bus
systems used in the Audi A8.

Address Word 08 is no longer used for the climate control
system. All climate control diagnostic functions are implemented via Address Word 09.

33

Bus systems used in the Audi A8
Bus system

Cable
color

Configuration

Data transfer rate

Comfort/convenience systems CAN

Electrical bus system

500 kbit/s

Comfort/convenience systems CAN 2

Electrical bus system

500 kbit/s

Extended CAN

Electrical bus system

500 kbit/s

Infotainment CAN

Electrical bus system

500 kbit/s

Modular Infotainment Matrix (MIB)
CAN

Electrical bus system

500 kbit/s

Diagnostics CAN

Electrical bus system

500 kbit/s

Instrument cluster CAN

Electrical bus system

500 kbit/s

Hybrid CAN

Electrical bus system

500 kbit/s

FlexRay

Electrical bus system

10 Mbit/s

MOST bus

Fiber-optic bus system

150 Mbit/s

LIN bus

Electrical single-wire bus system

20 kbit/s

Sub-bus system

Electrical bus system

500 kbit/s
1 Mbit/s

LVDS1)

Electrical bus system

200 Mbit/s

Ethernet

Electrical bus system

100 Mbit/s

LVDS
This data transfer system is notable for its relatively low
voltage. As is the case with high-speed CAN bus systems,
the differences in voltage are evaluated. In the case of
LVDS, the voltage is approximately 0.3 V. A typical application of LVDS is the activation of liquid crystal monitors. The
LVDS cables in the Audi A8 are used, for example, to transfer image data between Information Electronics Control
Module 1 J794, Instrument Cluster Control Module J285
and both MMI touch screens.

1)

34

LVDS = Low Voltage Differential Signaling

The LVDS lines are sheathed, much like the FlexRay lines.
Unlike the FlexRay bus, the sheathing of the LVDS lines not
only protects against mechanical stresses and moisture,
but also shields against electromagnetic interference.
In the event of damage, LVDS lines must always be
replaced rather than repaired.

Ethernet
Ethernet is a technology developed and specified for data
networks (LANs2)). This technology is commonly referred to
as LAN technology. It provides data transfer rates ranging
from 10 Mbit/s to 100 Gbit/s, depending on configuration
level. "Fast Ethernet" system with a data transfer rate of
100 Mbit/s is used in Audi models. This form of data transfer is used in the Audi A8 for communication between the
VAS Scan Tool and the gateway as well as between the
Information Electronics Control Module 1 J794 and the
Driver Assistance Systems Control Module J1121.

The high-speed transfer of data between the Scan Tool and
the gateway significantly reduces the time required for
parameterizing and updating control modules. The Ethernet
connection between Information Electronics Control Module
1 J794 and the Driver Assistance Systems Control Module
J1121 is used for fast updating of J1121. For this purpose,
an SD card containing the required data is inserted into J794,
and then the data is transfered across the Ethernet line. This
process is much faster than transferring data first to the
gateway and then via FlexRay to the J1121.

FlexRay
The aim of using the FlexRay bus is to meet the rising
demands on in-car connectivity in future.

It is particularly worth noting that the FlexRay bus offers
much higher data transfer rates and a higher level of fail
safety than the CAN bus. These are indispensable features
given the many complex dynamic control systems and
driver assistance systems.

The FlexRay bus has the following features:
›› Electrical two-wire bus system.
1 bit

›› Data transfer rate: maximum 10 Mbit/s.

›› "Data 0" – the bus positive wire has a low voltage level
and the bus negative wire has a high voltage level.

Voltage [V]

›› "Idle" – both bus wires have a voltage of 2.5 V.

Differential voltage

›› Data transfer with three signal states:

›› "Data 1" – the bus positive wire has a high voltage
level and the bus negative wire has a low voltage
level.
›› "Active" star topology (gateway).
›› Real-time capability.

Time [ns]

664_096

›› Enables distributed control and the use of safety-related
systems.

Repairing a FlexRay line
The FlexRay wires are twisted in the same way as the CAN
wires. Depending on model and year of manufacture, these
wires may be sheathed or non-sheathed.
In principle, sections of FlexRay lines can be replaced. Note
the untwisted length (1) and the unsheathed length (2).
Always refer to the electronic service and repair literature
for specifics.

2)

664_097

LAN = Local Area Network
35

FlexRay
Given that the overall topology diagram cannot provide a
realistic representation of the layout of the FlexRay control
modules, the following diagrams show how the modules
are distributed to the individual FlexRay branches. The
control modules of the 2019 A8 are shown here with a full
range of features. As always with the FlexRay bus, the
modules connected to the end of a branch have a resistance
of 94 ohms. The intermediate control modules have a
resistance of 2.6 kohms.

The FlexRay technology allows the use of two channels per
branch - an option not utilized in previous Audi models.
These channels are designated channel "A" and channel "B".
The 2nd channel generally offers two options:
›› Sending data redundantly for higher fail safety.
›› Doubling the volume of transferable data.

J104

J500

J527

J234

94 Ω

94 Ω

94 Ω

94 Ω

J623

J217

J792

2.6 kΩ

2.6 kΩ

2.6 kΩ

J1121
2.6 kΩ

J775
2.6 kΩ

Branch 1A
94 Ω

Branch 2A
94 Ω

Branch 3A

Branch 4A

94 Ω

94 Ω
94 Ω

J533

Branch 8A

2.6 kΩ

J187

94 Ω

J1019
664_094

Reference
For further information about the design and working principle of the FlexRay bus, refer to eSelf-Study Program
970103 The 2011 Audi A8 Convenience Electronics and Networking Systems.
36

The "B" channel in the Audi A8 is used to increase the
volume of transferable data. If the same control module is
connected to both channels, the diagnostic data of this
module is transferred via channel "A". If a branch channel
fails, for example, due to a short circuit in the FlexRay lines,
the channel affected by the fault is also indicated in the
diagnostic tester. This allows the relevant control modules
and/or lines to be systematically tested.

As data transfer across the FlexRay bus is time-controlled,
the network may only be started up by "cold start" control
modules.
These are the following control units in the Audi A8:
›› Data Bus on Board Diagnostic Interface J533.
›› ABS Control Module J104.
›› Airbag Control Module J234.

J104

J1121

J234

94 Ω

94 Ω

94 Ω

J428

J769

J770

2.6 kΩ

2.6 kΩ

2.6 kΩ

J1089

J1088

2.6 kΩ

2.6 kΩ

2.6 kΩ

Branch 1B

Branch 2B

Branch 3B

94 Ω

94 Ω

94 Ω

J1122

J533

664_095

Key:
J104
J187
J217
J234
J428
J500
J527
J533
J623
J769

ABS Control Module
Differential Lock Control Module
Transmission Control Module
Airbag Control Module
Control Module for Adaptive Cruise Control
Power Steering Control Module
Steering Column Electronics Control Module
Data Bus on Board Diagnostic Interface (gateway)
Engine Control Module
Lane Change Assistance Control Module

J770
J775
J792
J1019
J1088

Lane Change Assistance Control Module 2
Drivetrain Control Module
Active Steering Control Module
Rear Axle Steering Control Module
Control Module for Left Front Object Detection
Radar Sensor
J1089 Control Module for Right Front Object Detection
Radar Sensor
J1121 Driver Assistance Systems Control Module
J1122 Laser Distance Regulation Control Module

37

Optical data bus system MOST150

664_098

History
This data bus system is named after the "Media Oriented
Systems Transport (MOST) Cooperation". This organization
is made up of various automobile manufacturers, their
components suppliers and software companies which
joined forces to create a standardised high-speed data
transfer system.

The term "Media Oriented Systems Transport" means that
address-oriented messages are sent to a specific recipient.
This technology is used in Audi cars to transfer data within
the infotainment system.

MOST150
The MOST150 was first used by Audi in the A3. The new
Audi A8 also benefits from this advancement in MOST
technology. The data transfer rate is six times that of the
MOST25.

The development process necessitated making various
modifications to the components of the MOST bus. For
instance, the transmitter and receiver units – Fiber Optical
Transmitters (FOT) – had to be adapted.
Other components such as the optical connectors, the fiber
optics and the electrical connectors of the control units are
identical to those of the MOST25.

Systems manager
In the Audi A8, up to six control modules, including, for the
first time, the windshield projection control module (headup display), can be integrated into the MOST ring in the
following order.
›› Information Electronics Control Module 1 J794.
›› Instrument Cluster Control Module J285.
›› Windshield Projection Head Up Display Control
Module J898.
›› DVD Changer R161.
›› Digital Sound System Control Module J525.

38

In the Audi A8, Information Electronics Control Module 1
J794 acts not only as the system manager for the MOST bus
but also as the diagnostic manager - a function previously
assigned to theData Bus on Board Diagnostic Interface
J533 in the Audi A8. As always with a MOST bus in
Audi vehicles, the control modules are interconnected by an
electronic ring break diagnostics line.
This line is only required for electronic ring break diagnostics in the event of a fault.

Ring break diagnostics

The ring break diagnosis line is, as the name suggests, only
required for diagnostics after loss of MOST communication.

A ring brake diagnosis line is not relevant for data transfer
between the control modules.

Digital Sound System Control
Module
J525

DVD Changer
R161

Information Electronics Control
Module 1
J794

Windshield Projection Head Up
Display Control Module
J898

664_099
Instrument Cluster Control Module
J285

Key:
MOST
Ring break diagnosis line

Optical replacement control unit VAS 6778
Diagnostics
The ring break diagnosis procedure is identical to that of
the previous MOST bus system. Diagnostic information is
retrieved via Address Word 5F using the VAS Scan Tool.
Even though the ring break diagnosis procedure remains
unchanged, a modified tool – the optical replacement
control unit VAS 6778 – must be used in the event of an
optical fault in the MOST150 bus, due to the modified
transmitter and receiver units in the control modules.

664_100

Reference
For further information on the MOST bus, refer to eSelf-Study Program 971303 Audi New Data Bus System – LIN, MOST,
BluetoothTM.
39

Control modules
Data Bus on Board Diagnostic Interface J533
Brief description
It is located at the center of the new Audi A8 beneath the
rear bench seat. It can still be accessed with the via Address
Word 19 using the VAS Scan Tool.
The gateway performs the following functions:
››
››
››
››

Network system gateway.
Controller for FlexRay bus.
Diagnostics master.
Energy manager for low-voltage electrical system
(12 volts).
›› Energy manager for medium-voltage electrical system
(48 volts).
›› Interface for various connect services.
Special feature:
›› The gateway manages the diagnostic firewall featured in
the new Audi A8 for the first time.

It is a user of the following data bus systems:
›› Hybrid CAN.
›› Comfort/convenience systems CAN.
›› Comfort/convenience systems CAN 2.
›› Infotainment CAN.
›› Instrument panel insert CAN.
›› Extended CAN.
›› FlexRay.
›› Diagnostics CAN.
›› Ethernet.
It is not a user of:
›› Modular Infotainment Matrix (MIB) CAN.
›› MOST bus.
It is the LIN master for:
›› Battery Monitoring Control Module J367.
›› for alternator C (with 12-volt generator).
›› for Multifunction Steering Wheel Control Module J453.

664_040

Data Bus on Board Diagnostic
Interface
J533

40

Diagnostic firewall
The diagnostic firewall is a new safety function of the Data