Page 1
Service.
Self-Study Programme 238
Data Exchange On The CAN Bus II
Basics
II
Page 2
The CAN bus system in a car interlinks the control units to form a network.
This produces new functions in the car and in
diagnostics which span across control units.
SSP 186 "The CAN Databus" gave an initial overview of the technology. SSP 238 will now
describe the basic functions of the current CAN
Bus system.
SSSSSSSSPPPP 222233338888::
•
Deals with basic functions of the current CAN
system, e.g. the data exchange process
SSSSSSSSPPPP 222266669999::
•
Deals with special bus systems such as
the Drive Train CAN bus and the Convenience
CAN bus as used by VOLKSWAGEN
and AUDI.
::
::
Self-Study Programmes present the design and
function of new developments.
The contents will not be updated.
2
238_001
new Important
Note
Please always refer to the relevant Service Literature
for all inspection, adjustment and repair instructions.
Page 3
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
What is a bus system for?. . . . . . . . . . . . . . . . . . . . . . . . 4
Design, main features . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Development stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Handling the CAN bus . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Basic system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Networking principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Information exchange . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Functional units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Control unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
CAN module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Transceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Data transfer process . . . . . . . . . . . . . . . . . . . . . . . . . 18
Send process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Receive process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Simultaneous send attempt by several control units . 22
Transmission protection, interference response . . . . 24
Internal error management . . . . . . . . . . . . . . . . . . . . . 24
Diagnostic information . . . . . . . . . . . . . . . . . . . . . . . . . 26
Test your knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3
Page 4
Introduction
What is a bus system for?
The use of a CAN bus system in a car makes it possible to network electronic modules such as control
units or intelligent sensors such as the wheel angle sensor.
The abbreviation "CAN" means Controller Area Network.
advantages for the car as an overall system:
Data exchange between control units take place on a uniform platform.
•
This platform is call a protocol. The CAN bus acts as a so-called data highway.
Systems involving several control units, e.g. ESP, can be implemented efficiently.
•
System expansions are easier to implement in the form of optional extras.
•
The CAN bus is an open system which permits adaptation to various transmission media
•
such as copper or optical fibre cables.
Control units are diagnosed via the K-wire. Inside the car, diagnosis already takes place
•
via the CAN bus in some cases (for example the airbag and the door control unit).
In this context, this is called a "virtual K-wire" (see page 7). In future cars, there will be no K-wire.
A cross-system diagnosis is possible across several control units.
•
The CAN bus system provides the following
From a central control unit to a networked system
Car with central control unit
4
238_002
Page 5
Introduction
Car with 3 control units
Car with 3 control units and bus system
238_003
238_004
Sensor
ABScontrol unit
Actuator
Drive train CAN network with 3 control units
CAN-bus
Engine
control unit
Dash panel
insert
238_005
5
Page 6
Introduction
Design, main features
Many individual modules are connected in parallel to the CAN bus system.
This results in the following requirements for the design of the overall system:
High level of error protection: transmission interference caused by internal or external sources must
•
be detected with a high degree of certainty.
High availability: if a control unit fails, the rest of the system must continue to be functional as far as
•
possible in order to exchange information.
High data density: all control units have the same information status at all times.
•
This means there is no difference in data between the control units.
In case of faults anywhere in the system, all the connected users can be informed
with equal certainty.
High data transmission rate: data exchange between networked users must be very fast
•
in order to meet real time requirements.
Signals are sent over the CAN bus system digitally, at present over copper wires.
Secure transmission is possible at a maximum rate of 1000 kbps (1 Mbps).
The maximum data rate at VOLKSWAGEN and AUDI has been fixed at 500 kbps.
The CAN bus system is divided into 3 special systems due to the different requirements regarding signal
repetition rate and the large data volume:
Drive train CAN bus (high-speed) at 500 kbps with almost real time requirements
•
Convenience CAN bus
•
Infotainment CAN bus
•
(low-speed) at 100 kbps with low time requirements
(low-speed) at 100 kbps with low time requirements
11
11
Data transmission rates on the CAN bus system
6
22
22
238_051
1111
= 500 kbps = Drive train CAN bus
22
22
= 100 kbps = Convenience CAN bus
33
33
= 100 kbps = Infotainment CAN bus
4444
= 1000 kbps = Maximum data transmission rate
33
33
44
44
Page 7
Introduction
Diagnosis
plug
Engine
control unit
only Highline
combi
Combi-
instruments
Radio
Gearbox
control unit
Radio
Navigation
Gear selec-
Airbag
control unit
DSP ...
tor lever
Phone inter-
face box
control unit
Steering
angle sensor
Brake
...
DDDDrrrriiiivvvveeee ttttrrrraaaaiiiinnnn
Parking
aid
CCCCAAAANNNN bbbbuuuuss
CCCCoooonnnnvvvveeeennnniiiieeeennnncccceeee CCCCAAAANNNN bbbbuuuuss
((((IIIInnnnffffoooottttaaaaiiiinnnnmmmmeeeennnntttt CCCCAAAANNNN bbbbuuuussss))
ss
ss
))
Convenience
control unit
Gateway control unit
Car diagnosis plug
Real K-wire
Virtual communications line
Climate
control unit
Driver door
control unit
Parking
aid
Tyre pressure
Front pass.
door control
unit
...
Driver seat
check
RL door con-
trol unit
memory
RR door con-
Other control unit planned
trol unit
...
CCCCoooonnnnvvvveeeennnniiiieeeennnncccceeee
CCCCAAAANNNN bbbbuuuuss
238_006
238_006b
ss
CAN bus system (example: Polo MY 2002)
7
Page 8
Introduction
Production launch and development statuses
The first production launch at Volkswagen took place in MY 97
the Passat.
Other development stages include:
MY 98 Drive train CAN bus in Golf and Passat, 500 kbps
MY 00 Gateway K-wire on CAN in Golf and Passat
with the 62.5 kbps convenience system in
238_007
238_008
MY 00 Convenience CAN bus 100 kbps standard in Group, e.g. in SKÔDA Fabia
Gateway drive train CAN bus / convenience CAN bus in SKÔDA Fabia
MY 01 Convenience CAN bus 100 kbps standard in Group, for example in Passat
238_009
238_010
238_011
8
Page 9
Introduction
Handling the CAN bus
The CAN bus is an independent system with the car's electronics systems and acts as a data line to
exchange information between control units.
Due to its design and construction, the system works with a high degree of intrinsic safety.
If faults still occur, they are mainly stored in the fault memory of the related control unit and are accessible by the Diagnostic Testing and Information System.
The control units contain self-diagnosis functions from which the system can detect CAN-related
•
faults.
After reading out the CAN fault entries with the Diagnostic Testing and Information System
•
(for example VAS 5051, 5052), this information is available for specific fault-finding processes.
The entries in the fault memory of the control units are suitable for initial fault detection.
•
Beyond this, it provides you with confirmation that there are no more faults present after fault
remedial action. The engine must be restarted to update the fault memory.
A key requirement for a car with the status "CAN bus OK" is that there should be
•
entry in any vehicle operating state.
To start an analysis which may lead to fault detection or fault remedy, a basic knowledge is required
about data exchange on the CAN bus.
no CAN fault
9
Page 10
Notes
10
Page 11
Basic system
The networking principle
The basic system consists of several control units. They are connected in parallel to the bus line by transceivers. This means that the same conditions apply to all stations. In other words, all the control units are
handled equally, none has any preference. In this context, this is called a multimaster architecture.
Information is exchanged serially (in series).
Basically, the CAN bus is already fully functional with a single line!
The system can also be equipped with a second bus line.
The second line is used for signals travelling in the reverse order.
It is possible to suppress external interference more effectively by reversing the signals.
To explain the basic principle of data transmission in a simpler way, we will
assume a single bus line in the following examples.
CCCCoooonnnnttttrrrroooollll uuuunnnniiiitttt AA
RX TX RX TX RX TX
AA
CCCCoooonnnnttttrrrroooollll uuuunnnniiiitttt BB
BB
CCCCoooonnnnttttrrrroooollll uuuunnnniiiitttt CC
CC
Transceiver
The networking principle
CAN-bus
238_012
11
Page 12
Basic system
Information exchange
Exchange information is referred to as messages. Any control unit can send or receive messages.
A message contains physical values such as the engine speed (rpm).
The engine speed In this case, is represented as a binary value (a string of ones and zeroes).
For example: (The engine speed of 1800 rpm is represented as 00010101 in binary notation.)
Before sending, the binary value is converted into a serial bit stream.
The bit stream is sent over the TX line (transmit line) to the transceiver (amplifier).
The transceiver converts the bit stream into voltage values which are then sent over the bus line one by
one.
In the reception process, voltage values are converted back into a bit stream by the transceiver and sent
over the RX line (receive line) to the control units.
The control units then convert the serial binary values back into messages.
For example: (the value 00010101 is converted back to the engine speed 1800 rpm)
A message sent can be received by any control unit.
This principle is also called a broadcast message. The idea is derived from a transmitter which broadcasts a programme which any tuner (receiver) can receive.
The broadcasting process ensures that all control units connected to the bus have the same information
status.
The broadcasting principle: one sends, everyone receives.
12
238_013
Page 13
Basic system
Control unit A Control unit B Control unit C
Message
parallel
Engine speed Engine speed Engine speed
0001 0101 0001 0101 0001 0101
Bit stream
serial
0001 0101
1010 1000
RX TX RX TX RX TX
Electrical
signal transmission
One sends, all receive
Information exchange of a message on the CAN bus (broadcast principle)
1010 1000
Transceiver
238_014
Signal level
5V
0V
Electrical signal transmission in chronological sequence
238_015
t (time)
13
Page 14
Functional units
K wire
The K-wire is provided for connection to a VAS tester for vehicle diagnosis when servicing.
Control unit
The control unit receives signals from the sensors, processes them and passes them on to the actuators.
The main components of a control unit are: a microcontroller with input and output memories and a program memory.
The sensor values received by the control, e.g. engine temperature or engine speed, are interrogated at
regular intervals and stored in the input memory in their order of occurrence.
This process corresponds to the principle of a mechanical step-by-step system with a rotating input selector switch (see figure).
The microcontroller links the input values based on the program configuration. The results of this process
are stored in each output memory and from there, they are sent to each of the actuators.
In order to process CAN messages, each control unit has an additional CAN memory area for received
and sent messages.
CAN module
The CAN module controls the data transfer process for CAN messages.
It is divided into two sections, the receive section and the send section.
The CAN module is connected to the control unit via the receive mailbox or the send mailbox. It is normally integrated in the chip of the control unit microcontroller.
Transceiver
The transceiver is a transmitter and receiver amplifier. It converts the serial bit stream (logic level) of the
CAN module into electrical voltage values (line level) and vice versa.
The electrical voltage values are designed for sending over copper wires.
The transceiver is connected to the CAN module via the TX line (transmit line) or via the RX line
(receive line).
The RX line is directly connected to the CAN bus and permits continuous monitoring of bus signals.
14
Page 15
KKKK----wwwwiiiirrrree
Functional units
ee
CCCCoooonnnnttttrrrroooollll uuuunnnniiiitt
Input selector switch
Sensors include:
• Engine speed sensor
• Temperature sensor
• Oil pressure sensor
• etc. ...
CCCCAAAANNNN mmmmoooodddduuuullllee
tt
ee
Input
memory
Fault message
Micro-
processor
CAN area with time
monitor
Output
memory
Actuators include:
• Engine throttle valve
• Solenoid valve
•LED
• etc. ...
Receive mailbox Transmit mailbox
Receive section Transmit section
XX
RRRRXX
TTTTrrrraaaannnnsssscccceeeeiiiivvvveeeerr
CAN bus
Functional unit: control unit, CAN module and transceiver
rr
TTTTXX
XX
Logic level: 0 or 1
Signal level: 0V or 5V
238_016
15
Page 16
Functional units
Special features of transceiver
RX TX
5V
Transceiver with connection to TX line
+ 5V
Block diagram with one switch
A special feature is the connection of the TX line to the bus. It is normally connected via
an open collector.
This results in two different states on the bus line.
State 1: inhibited state, transistor inhibited, (switch open)
Passive: bus level=1, high-resistant via resistor
State 0: switch-through state, transistor switched through (switch closed)
Active: bus level=0, low-resistant without resistor
Bus line
238_018238_017
Three transceivers connected to a bus line
Transceiver A Transceiver B Transceiver C
Connection of 3 transceivers to bus line (principle), transceiver C active
Switch open means 1 (passive) Switch closed means 0 (active)
16
+ 5 V+ 5 V+ 5 V
Bus line (0V)
238_019
Page 17
Functional units
The previous example (three transceivers connected to bus line) results in the following
switch positions:
TTTTrrrraaaannnnsssscccceeeeiiiivvvveeeerrrr AA
00
00
00
00
00
00
00
00
Possible switch positions with 3 transceivers connected to a bus line, transceiver C active
AA
11
11
11
11
11
11
11
11
TTTTrrrraaaannnnsssscccceeeeiiiivvvveeeerrrr BB
BB
11
11
11
11
00
00
00
00
11
11
11
11
00
00
00
00
TTTTrrrraaaannnnsssscccceeeeiiiivvvveeeerrrr CC
11
11
00
00
11
11
00
00
11
11
00
00
11
11
00
00
CC
BBBBuuuussss----LLLLeeeeiiiittttuuuunnnngg
1111 ((((5555VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
0000 ((((0000VVVV))
gg
))
))
))
))
))
))
))
))
Response:
If any switch is closed,
•
current flows across the resistors.
A voltage of 0V is generated
on the bus line.
If all switches are open, no current flows.
•
No voltage drops across the resistor.
A voltage of 5V is generated
on the bus line.
This achieves the following:
If the bus is in state 1 (passive), any other station can overwrite this state with state 0 (active).
The passive bus level is called recessive. The active bus level is called dominant.
This relationship is important in the following situations:
a) For signalling transmission faults (fault messages about error frames).
b) Collision detection (if several stations want to send simultaneously).
17
Page 18
Data transmission process
Data transmission using the example of engine speed detection > transmission > display
The following example describes the complete process for exchanging engine speed information from
detection through to display in the rev counter. It explains the chronological sequence of the data transmission process and the interaction between the CAN modules and the control units.
First the engine control unit sensor detects the engine speed value.
This value is stored in the microcontroller input memory at regular intervals (cyclically).
Since the present engine speed value is also required for other control unit, e.g. the dash panel insert, it
has to be sent over the CAN bus.
The engine speed value is first copied to the transmit memory of the engine control unit.
From there the information goes to the transmit mailbox of the CAN module.
If a current value is located in the transmit mailbox, it is indicated by the transmit flag (the flag is raised).
Once the message is sent to the CAN module, the engine control unit has completed its task for this process.
The engine speed value is first converted into an engine message with a CAN-specific form in
accordance with the protocol. The main components of a protocol are:
IIIIddddeeeennnnttttiiiiffffiiiiccccaaaattttiiiioooonnnn:::: ((((IIIIddddeeeennnnttttiiiiffffiiiieeeerrrr 11111111 bbbbiiiittttssss))
acts as message identifier
MMMMeeeessssssssaaaaggggeeee ccccoooonnnntttteeeennnntttt:::: ((((mmmmaaaaxxxx.... ddddaaaattttaaaa ffffiiiieeeelllldddd 8888xxxx8888 bbbbiiiittttssss))
contains the message information
((((11116666----bbbbiiiitttt CCCCRRRRCCCC cccchhhheeeecccckkkk))))::
Checksum for error protection
))
In the following diagrams, the CAN
message is represented as a letter
::
icon.
))
::
Basic format of a CAN message
AAAAcccckkkknnnnoooowwwwlllleeeeddddggggeeeemmmmeeeennnntttt ((((2222----bbbbiiiitttt AAAAcccckkkk))))::
Acknowledge
The components of an engine message would therefore include: identifier=engine_1, content= rpm.
The engine message also contains other values, e.g. idling speed, torque etc.
18
238_020
Page 19
Data transmission process
The CAN module then checks via the RX line whether the bus is active (whether information is in the
process of being exchanged). If necessary, it waits until the bus is free.
(Level 1 (passive) for a specific period).... If the bus is free, the engine message is sent.
Transmit process
EEEEnnnnggggiiiinnnneeee
ssssppppeeeeeeeedddd
sssseeeennnnssssoooorr
rr
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
EEEEnnnnggggiiiinnnneeee
tt
SS
AAAABBBBSS
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
DDDDaaaasssshhhh ppppaaaannnneeeell
tt
iiiinnnnsssseeeerrrrtt
ll
tt
OOOOuuuuttttppppuuuutt
ccccoooouuuunnnntttteeeerr
RRRReeeevv
tt
vv
rr
RRRRaaaaiiiisssseeee tttthhhheeee
ffffllllaaaagg
TTTTrrrraaaannnnssssmmmmiiiitttt
bb
jjjjoooobb
IIIIssss bbbbuuuussss
ffffrrrreeeeeeee??
CCCCAAAANNNN bbbbuuuuss
Start of a transmit process
gg
??
RX
ss
TX
WWWWaaaaiiiitt
RX
tt
TX
RX
TX
238_021
IIIIssss bbbbuuuussss ffffrrrreeeeeeee??
RRRRXXXX----LLLLeeeeiiiittttuuuunnnngg
Detail: Interrogation format for 'Is bus free?'
??
gg
?
YYYYeeeess
NNNNoo
ss
oo
238_022
19
Page 20
Data transmission process
Receive process
The receive process consists of two steps:
Step 1 = check message for errors (at monitor level)
•
Step 2 = check message for usability (at acceptance level)
•
--
TTTTeeeemmmmppppeeee--
ee
rrrraaaattttuuuurrrree
sssseeeennnnssssoooorr
rr
EEEEnnnnggggiiiinnnneeee
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
T
RX
CCCCAAAANNNN bbbbuuuuss
Receive process
ss
tt
tt
TX
SS
AAAABBBBSS
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
RX
DDDDaaaasssshhhh ppppaaaannnneeeell
tt
TX
iiiinnnnsssseeeerrrrtt
RX
ll
tt
TX
OOOOuuuuttttppppuuuutt
RRRReeeevv
ccccoooouuuunnnntttteeeerr
238_023
vv
rr
All connected stations receive the message sent by the engine control unit.
It travels over the RX lines to the receive areas of the CAN modules.
Ye s
Ye s
No
No
Acceptance level
Monitoring level
238_024
Detail: receive area, monitoring and acceptance levels
20
Ye s
Ye s
No
No
238_025
Page 21
Data transmission process
The receivers have all received the engine message and have checked them for correctnesss at the associated monitoring level. This helps to detect local faults which may occur only in one control unit under
certain circumstances. This results in the high data density mentioned before (also refer to the sections on
"Transmission protection, fault response“).
All connected stations receive the message sent by the engine control unit (broadcast). Using the CRC
checksum, they can detect whether any errors have occurred in transmission. CRC is an acronym for
Cyclic Redundancy Check. When a message is sent, a 16-bit checksum is generated from all the bits and
included in the transmission.
The receivers calculate the checksum from all the bits received using the same protocol.
Then the received checksum is compared with the calculated checksum.
If no error is found, all the stations send an acknowledgement to the transmitter
(called the Acknowledge) confirming correct reception.
AAAAcccckkkknnnnoooowwwwlllleeeeddddggggeeeemmmmeeeennnntttt ((((2222----bbbbiiiitttt AAAAcccckkkk))))::
Acknowledge
Information flow, acknowledgement, date
::
Finally the correctly received message goes to the acceptance section of the associated CAN modules.
There a decision is made whether the message is necessary for the function of the related control unit.
•
If not the message is discarded.
•
If so, the message is placed in the receive mailbox.
•
When the "receive flag" is raised, the connected combi-instrument knows that a current message, e.g.
engine speed, has arrived for processing.
The combi-instrument calls the message and copies the value to its input memory.
This concludes the sending and receiving of a message via the CAN modules.
238_026
After the microcontroller in the dash panel insert processes the engine speed value,
•
the value is sent to the actuator and then to the rev counter.
Data exchange of a message is repeated depending on the cycle time setting
•
(for example, every 10 ms).
21
Page 22
Data transmission process
Simultaneous send attempt by several control units
If several control units attempt to send at the same time, there would be a data collision on the bus line.
To avoid this, the CAN system uses the following strategy:
every active control unit starts its transmit process by sending an identifier.
All the control units monitor the bus traffic by monitoring the bus on their RX line.
Every transmitter compares the state of the TX line bit-by-bit with the state of the RX line.
The comparison may show differences.
The CAN strategy regulates this situation in the following way: the control unit whose TX signal was overwritten by a zero must withdraw from the bus.
Message weighting is controlled by the number of leading zeroes in the identifier.
This ensures that messages are sent in the order of their priority.
Rule: the lowers the number in the identifier, the more important the message.
This procedure is called arbitration. Association: arbiter = referee or judge
XX
ee
EEEEnnnnggggiiiinnnnee
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
SS
AAAABBBBSS
ccccoooonnnnttttrrrroooollll uuuunnnniiiitt
DDDDaaaasssshhhh ppppaaaannnneeeellll
tt
iiiinnnnsssseeeerrrrtt
TTTTXX
tt
tt
RRRRXX
TTTTXX
RRRRXX
TTTTXX
RRRRXX
XX
XX
XX
XX
XX
retains allocation and remains
in transmit mode
loses allocation and goes to
receive mode
loses allocation and goes to
receive mode
Arbitration process to avoid collisions
22
DDDDaaaattttaaaa bbbbuuuussss lllliiiinnnnee
ee
238_027
Page 23
Data transmission process
The next example shows that the wheel angle sensor has the highest priority when several control units
attempt to transmit simultaneously. The wheel angle sensor's message is therefore sent first.
Explanation: the wheel angle sensor with the smallest number (mainly leading zeroes) asserts itself.
Engine_1
Brake_1
Combi_1
SSSStttteeeeeeeerrrriiiinnnnggggaaaannnngggglllleeee____11
Gearbox_1
Possible identifiers in drive train CAN
11
238_027b
Conclusion when transmitting sensor values (e.g. engine speed)
Due to the high transmission protection in CAN, all errors are detected reliably, e.g. electrical faults or
interruptions in the CAN system.
The engine speed of 1800rpm is correctly sent or not at all if a fault occurs
•
(no display, rev counter shows "0").
For example, if implausible engine speeds occur, the cause may not lie with the transmission
•
(CAN) but with a defective sensor, display instrument or the supply line.
23
Page 24
Transmission protection, interference response
Internal error management
To ensure high data protection, the CAN has an extensive integrated error management system.
This is capable of detecting any transmission errors occurring with a high level of certainty. Corrective
action can then be initiated.
The rate of undetected errors, what is known as residual error probability, is about < 10
This value is equivalent to 4 errors over the lifespan of the car.
Using the broadcast process (one sends, all receive and evaluate), any network user detecting an error
immediately notifies all other users by sending an error message called an error frame.
The current message is then rejected by all users.
This is followed by an automatic transmission repetition. This process is completely normal and may be
caused by major voltage fluctuations in the onboard power supply, e.g. on engine start or strong external interference.
-12
.
What is more critical is if transmission repetitions become more frequent due to continuously detected
errors.
In this case, every station has an integrated error counter which increments detected errors and decrements once the transmission repetition has been sent.
Control unit
switched off
Cannot send
any more
PPPPaaaassssssssiiiivvvvee
eeeerrrrrrrroooorr
BBBBuuuuss
ooooffffff
ss
ff
RX-Fehlerzähler
00
1111222200
ee
rr
TX-Fehlerzähler
66
2222555566
55
2222555555
77
1111222277
00
00
55
2222555555
77
1111222277
System time
Interner Fehlerzähler
24
Normal
state
AAAAccccttttiiiivvvvee
rr
eeeerrrrrrrroooorr
ee
00
00
Errors occurring,
error counter
increments
No errors,
error counter
decrements
Massive occurrence of errors,
error counter threshold value
exceeded
System time
238_028
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Transmission protection, interference response
The internal error counter is responsible for internal error management and cannot be read out.
If the preset threshold value is exceeded (equivalent to max. 32 transmission repetitions), the affected
control unit is informed and is switched off by the CAN bus.
After the bus goes off-state twice (without any intermediate communication), an entry is made in the
fault memory.
After a fixed waiting time (approx. 0.2s) the control unit attempts to access the bus again.
Message traffic is normally cyclical with prescribed cycle times.
This ensures that the messages are transmitted in good time.
If there are delays, it means that at least ten messages are not received and this triggers the message
timeout.
This causes a entry in the fault memory of the receiving control unit.
This is the second element of the error management system. The following error messages are available
for in-service diagnosis:
1. Data bus defective
Fatal errors were detected in the affected control unit.
The control unit disconnected at least twice from the bus (bus off).
2. Missing messages from .... or no communication with the affected control unit.
Messages are not received in good time. Timeout monitor responded.
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Transmission protection, interference response
Diagnosis information using defective engine speed transmission as an example
The engine speed is transmitted correctly or not at all due to a fault (value not displayed).
•
In this case, the Vehicle Diagnostic Testing and Information System VAS 5051 sends notification that
there is a fault in the CAN system:
238_029a
VAS 5051 display
For example, if implausible engine speeds occur, the cause may not lie with the
•
CAN transmission) but with a sensor or actuator (display instrument or rev counter).
238_029b
238_029c
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Transmission protection, interference response
If there is a fault in the CAN system, the Vehicle Diagnostic Testing and Information System VAS 5051
indicates a general fault message.
This message indicates which component is defective in the CAN system.
To localise errors, data blocks 125, 126 can be read out from the active state gateways of the control units
connected to the CAN bus (1=active, 0=passive).
If necessary, further electrical measurements, for example, signal testing using the oscilloscope) may be
required.
Outlook
This SSP 238 explains the basic functions of the CAN system.
SSP 269 "Data Exchange on CAN Bus II, Drive Train CAN Bus/Convenience CAN Bus“ describes the
CAN bus system specially implemented in Volkswagen and Audi vehicles.
It describes in detail the special features with the Drive Train CAN bus and Convenience CAN bus with
respect to function and diagnosis.
Finally, the entire system combining the Drive Train CAN bus and Convenience CAN bus via the
Gateway is explained. The fault-finding procedure is also part of this SSP.
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Test your knowledge
1. Why are bus systems used in cars?
A Increasing complexity in motor vehicle electronics
B System expansions in the form of optional extras are easily possible
C Prescribed by law
2. What is the data transmission rate on the Drive Train CAN bus?
A 10 kbps
B 100 kbps
C 500 kbps
3. The Diagnostic Testing and Information System VAS 5051 helps to detect ... ?
A CAN line errors
B CAN hardware errors
C Displays CAN messages
4. What messages are received and tested by control units?
A Only the messages meant for a particular control unit
B All messages sent
C Messages with the highest priority
28
5. 3 control units wait until the bus is free and then they attempt to send messages ...
A ... All can send messages immediately
B ... A data collision occurs
C ... Arbitration controls the order in which messages are sent
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Test your knowledge
6. What does Bus OFF mean?
A All bus users switch off
B A bus user withdraws from bus traffic temporarily
C The bus is totally switched off
7. What is the function of the internal error counter?
A To count CAN messages
B To count errors in order to switch the control unit Bus OFF
C For statistical purposes
8. What does "high transmission protection" means on the bus?
A Almost no transmission errors occur
B Transmission errors are detected with certainty
C All bus users are informed when errors are detected
9. The identifier of a CAN message ...
A ... identifies the name and priority of a message
B ... indicates the destination address
C ... helps to control access rights
10. The function of the protocol is ...
A ... to protect data
B ... detect errors
C ... control access rights
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Glossary
ACK:
Acknowledge, receive confirmation of a correct message. Occurs
by setting a dominant bit from all bus users.
Actuators
Drive elements and displays in the vehicle.
Acceptance range
Filtering received messages which are relevant for the affected
control unit.
Arbitration
Mechanism for avoiding collisions when several users want to send
at the same time.
Arbitration makes sure that messages are sent in the order of their
importance.
Message
A message is a data packet which is sent by a control unit.
Message timeout
Time monitor that monitors transmitted messages on receiver side.
Broadcast
Transmitting principle - one sends - all receive.
BUS line
Electrical connection made of copper in car, twisted wire pair. The
bus line connects control unit together.
Bus OFF
Control unit is switched off from the bus when the internal error
counter is exceeded.
Error frame
Error message (>6 dominant bits) to indicate transmission errors on
the bus.
Error memory
Memory area in control unit; readable by VAS Tester.
Identifier
Start of a message; used for identifying and distinguishing between
message priorities.
K-wire
Customer Service line; connecting line between control unit and
diagnostic plug in vehicle for connecting to the VAS Tester.
Logic level:
State 0 or 1 at a connection point in the system.
Microcontroller:
1-chip computer system, comprises CPU, memory and
input/output modules
RX line
Receive-side connecting line between CAN module and transceiver
Transmit mailbox
Memory in the CAN module storing messages sent by control units.
Sensors
Electronic sensors in the car, for capturing operating states
Signal level
Electric voltage state on a wire
Bus transceiver
Electronic transmitter-receiver amplifier to connect acontrol unit to
the bus.
CAN
Controller Area Network, bus system to network control units.
Drive Train CAN bus
Subsystem for control units in drive train.
Convenience CAN bus
Subsystem for control units in the convenience system.
Infotainment CAN bus
Subsystem for control units in the radio and information system.
CAN module
Process data exchange for CAN messages.
CRC
Cyclic Redundancy Check, checksum (16 bits) for error detection.
Receive mailbox
Memory which stores messages received from the CAN module.
Transceiver
Electronic transmitter/receiver amplifier, for coupling the CAN
module to the bus line.
TX line
Transmit-side connecting line between CAN module and
transceiver
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Notes
Answers to test questions:
1: AB / 2: C / 3: AB / 4: B / 5: C
6: B / 7: B / 8: BC / 9: AC / 10: ABC
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238
For internal use only. © VOLKSWAGEN AG, Wolfsburg
All rights reserved. Technical specifications subject to change without notice.
140.2810.57.20 Technical status: 10/01
❀ This paper is produced from
non-chlorine-bleached pulp.
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