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The CAN bus - basics of automotive bus systems

Why CAN bus?

Electronic systems in motor vehicles have increased rapidly over the past few decades. There is now a large proportion of electronics and less mechanics. This is also expressed by the car mechatronic profession that was created a few years ago and replaced the previous car mechanic. Constantly increasing demands on driving safety, exhaust gas behavior and fuel consumption require an increasing exchange of information between the control units and an ever more comprehensive central electronics. etoro copytrader experiences

Electronic control devices for window regulators, central locking systems and the like do their job as a matter of course. Engine and transmission controls, ABS, ESP, and airbags are essential for propulsion and safety. Multimedia components such as navigation systems, CD players or the Internet can no longer be found only in luxury-class models such as Audi, BMW or Mercedes. However, all of this has only become possible because modern electronics exchange information with one another.

CAN bus very easy!

The manual was developed for beginners in CAN bus technology. It describes in a simple way only the most necessary terms and test options on a CAN bus including a troubleshooting strategy. Ideally suited as a booklet for existing training courses.

Format: DIN A4, over 69 colored illustrations, approx. 52 pages Price: 17.80 euros + shipping / postage

for order

A small presentation of the book can be found here.

The CAN bus

Since a line is required for each piece of information in conventional cabling, the length and weight of the cable harness and the connections to the control units increase as the functionality of the vehicle electronics increases. A remedy here is the CAN bus, which transmits all information via just two lines. Data buses, also known as CAN (Controlled Area Network), connect up to 100 different control mechanisms that work together under the hood of a car.


The CAN bus is a serial data bus, which connects equal stations with each other. CAN is used to network control units.

In simple terms, the connected control units can "talk" to each other over the lines and exchange information with one another.

In the bus system, data transmission works in a similar way to a conference call. In the vehicle, the individual control units are networked with one another. In the telephone conference, one participant (control unit) "speaks" his information (data) into the line network, while the other participants "listen in" to this information.

Some participants find this information interesting and will use it. Other participants do not.

This enables complex system functions to be achieved, e.g. when shifting an automatic transmission. The CAN is a 2-wire bus system (twisted pair) and, despite its simple structure, can form very large networks with up to 100 control units. The data is transmitted serially in data packets, the structure of which is standardized. The processes involved in transmission, error protection, error correction and confirmation are precisely defined and described in the CAN specification (Bosch).

Advantages of CAN over conventional wiring

Bus systems enable a significant reduction in cables and connectors. This reduces price and weight. The result is an extended communication capability that would not be possible with simple cabling. A constant control is available through diagnostic components. A protocol detects transmission errors that can arise, for example, due to electromagnetic radiation, and corrects them automatically by repeating the transmission. Safety is therefore also given through redundancy. The modularization of e.g. control units also lowers the price, since control units often only have to be programmed accordingly.

Increasingly, sensors (e.g. steering angle sensors) and actuators (e.g. VW wiper motor) are now being equipped with processors to process the data. If the data of such intelligent components is sent directly to the bus system, they do not burden the control units with forwarding. In the diagnosis area, the CAN bus is used for the transmission of the states and error memory, as well as for the flash programming of the control units.

The advantages the linear CAN bus topology at a glance:

  • smaller wiring harness, the wiring effort is low

  • An inexpensive and easy-to-use twisted pair cable is used as the transmission medium.

  • Sensors can be used multiple times

  • CAN stations can subsequently be added to and removed from the existing CAN bus relatively easily. Only the connection to the bus line has to be established or disconnected. This aspect plays a particularly important role in troubleshooting and repair

  • The failure of a CAN station has no direct impact on the CAN bus. All other stations can continue to communicate without restriction.
     

The disadvantages of this bus topology affect the CAN bus as follows:

  • The bus line cannot be of any length, as the electrical properties (e.g. signal reflections) set physical limits in connection with the transmission speed.

  • The same applies to the stub lines to the control units in the vehicle. Depending on the transmission speed, they must not exceed a certain length.

  • In order to optimize the signal quality, the ends of the bus line must be "terminated" with terminating resistors. An incorrectly terminated cable end can render the entire bus inoperable, especially at high transmission speeds.

Historical development of the CAN

  • CAN was developed by Bosch as an automobile bus from 1983 onwards at the request of Daimler-Benz and BMW. The basis for this was the fact that a mid-range car has more than 600 different cable harness types with a cable length of more than 2000 meters and over
    Weighed 100 kg.

  • 1987 First CAN chip from INTEL

  • since 1989 series components for use in vehicles

  • CAN has been used in the Mercedes S-Class since 1992, and other automobile manufacturers followed suit later.

  • Over time, different bus protocols have emerged such as: B. CAN, VAN, J1850, ABUS. VAN and ABUS protocols have since been abandoned in favor of CAN.

  • Since 1994/95, CAN has been the most widely used protocol for automotive applications.

  • 2001 The CAN bus is also used in the drive train and in the body of small cars.
    2002 Optical buses are used in luxury vehicles. They are used to transmit control, video and audio data, and in the 7 Series BMW they are also used to trigger restraint systems. The signals are transmitted via plastic fiber optic cables.
    2003 Audi uses the Bluetooth interface known from GSM technology for wireless transmission between the telematics unit and the mobile handset in the new A8.
     

CAN classes

Current motor vehicles already network a large number of control units with one another, which entail different requirements. Therefore, several CAN bus systems are installed in the vehicle. These differ mainly in the transmission speed and are divided into three classes.

CAN A

<10 kBit / s

Diagnosis (conventional)

Vehicles with a CAN bus have a diagnostic system. Such systems read out fault memories and enable final control diagnosis. The data transfer speed is not so important as the data is only read out occasionally in the workshop for maintenance and diagnostic purposes. The diagnostic connection (also called K-line and L-line) must, however, be robust and fault-tolerant. In newer vehicles, the diagnosis is carried out directly on the actual bus line (CAN C).

CAN B

<125 kBit / s

Comfort, display, body

Control devices for lighting, air conditioning, locking and fittings, for example, communicate via this (low-speed CAN) bus. It is important to transfer important data when the speed is not as high (e.g. K-CAN, body CAN, convenience CAN). The bus still has to be fail-safe and robust. This is why he usually works in vehicles according to the fault-tolerant ISO 11989-3 standard.

CAN C

<1 Mbit / s

Engine, transmission, diagnosis (bus)

The control units for engine management, transmission, ESP, ASR and ABS, for example, are connected to this (high-speed CAN) bus. The bus must be real-time capable, i.e. the data transmission may only be delayed extremely briefly by the bus. Real-time diagnosis is now also possible via its own diagnosis bus. This bus has to be fast because large amounts of data have to be transferred in a short time. The ISO 11898-2 standard is mostly used in vehicles.
 

Different bus systems in comparison

The components used in modern motor vehicles place different demands on the bus system used. For example, its use in the area of ​​engine management requires fast data transmission, whereas an air conditioning system does not have to react to changes in the temperature in the passenger compartment within a split second. Significantly longer delay times can be accepted here.

Usually, different and differently fast bus systems are used in a motor vehicle. The drive bus (e.g. Powertrain-CAN) comprises the engine, transmission and brake control units as well as other directly related sensors / actuators. It is a high-speed CAN. Comfort systems such as window lifters, seat memory or tire pressure run via a convenience CAN or a body CAN. Simple applications such as air conditioning or wiper control often use a single-wire bus (LIN). In the infotainment area, MOST buses with fiber optic cables are used due to the large amounts of data. The information converges in one or more central modules (gateways) and is "coordinated" with the respective bus system. The data is also passed on from one CAN system to the other. Gateways can be queried for diagnostic purposes.

CAN bus

The CAN bus exists in different variants. Depending on the speed at which it can transfer data, it is used in the areas of drive, comfort and diagnosis. Low speed to high speed, 100 to 500 kBit / s including diagnosis CAN
LIN The LIN bus (Local Interconnect Network) is an inexpensive sub-bus system subordinate to the CAN bus; it is used to control certain local comfort components or for sensors. - max. 20 kBit / s
MOST

(Media Oriented Systems Transport) optical high-speed multimedia bus system, mostly ring-shaped, very high transmission rate (23 Mbit / s)
Bluetooth

Bluetooth radio link does not require cabling over short distances and is used for the secure transmission of voice signals.
The picture shows a simplified and abbreviated representation of the bus systems that are used in an Audi A8 (from model year 2003)

Other bus systems

FlexRayReal-time capable high-speed bus system (data rate 10 Mbit / s) for safety-critical applications,

Suitability for real-time systems. Use as backbone for other bus systems planned; in the future, if necessary, as a replacement for today's high-speed CAN, long-term: brake-by-wire, steer-by-wire - topology: line structure, star - application e.g. in the AdaptiveDrive special equipment of the BMW X5 (model 2007); Shock absorber valves, which adjust the chassis behavior, are controlled by Flexray.

byteflightHigh-quality, star-shaped bus system - very fast (10 Mbit / s), stable, but expensive. Transmission medium fiber optic cable. Currently used in the BMW upper class, preferably in the drive train and in safety systems, e.g. airbags.
  D2B Optical DaimlerChrysler 5.65 Mbit / s

Bus topology CAN and other bus systems

The participants are connected to a main line by short stub lines. All communication goes through this main line. If this is interrupted, two segments are created that mostly remain functional. This linear topology is also called "bus topology". CAN works with a linear topology as standard.

  • Tree-shaped (branched) topology

The tree topology is characterized by branches at any point. This enables flexible combinations of star and line topologies.

The point-to-point connections between the participants are characteristic. All connections are arranged in a closed chain. Communication can only be in one direction. A command from device A to device B usually has to go through another device. If a section fails, the entire system is usually inoperative. The error can be localized via a diagnosis line. e.g. with the MOST bus system

There is a central hub to which all participants are connected. Each participant has his own line. If the central point fails, all communication breaks down. e.g. at Byteflight

- Further details on CAN:

CAN drive

  • e.g. engine, transmission, ABS, ESP, airbag control unit
  • , Dash panel insert
  • ISO standard 11898-2 (high-speed CAN) for fast data transmission
  • Data rate 500kBit / s
  • 2-wire bus line, twisted, cross-section of 0.35 mm each2
  • at both ends with the wave impedance (120 Ω)
  • for VW: CAN high = orange / black, CAN low = orange / brown
  • The recessive level is 2.5 V on both lines, the dominant level 3.5 V for CAN H and 1.5 V for CAN L.
  • for VW: CAN H 3.75 V and CAN L 1.25 V.
  • PT-CAN (Powertrain-CAN) at BMW (H 4 V, L 1V),
    The bus structure of the PT-CAN differs in the third line. This only serves as a wake-up line that brings the control units from sleep mode (power-saving mode) to normal operating mode. The drive train CAN data bus (VW) is switched off with terminal 15 or after a short delay time.
     

Voltage changes on the CAN lines when changing between the dominant and recessive state using the drivetrain CAN data bus as an example:

In the idle state, both lines are at the same preset value of 2.5 V. This idle level is also referred to as the recessive state. In the dominant state, the voltage on the CAN high line increases by at least 1V. The voltage on the CAN low line drops by at least 1V. This means that with the drivetrain CAN data bus, the voltage on the CAN high line rises to at least 3.5V when it is active (2.5V + 1V = 3.5V). The voltage on the CAN low line then drops to a maximum of 1.5V (2.5V - 1V = 1.5V). Accordingly, the voltage difference between CAN high and CAN low is 0V in the recessive state and at least 2V in the dominant state.

Source: VW

Since the data bus lines are also laid in the engine compartment, they are also exposed to various interference influences. Short circuits to ground and battery voltage, flashovers from the ignition system and static discharges during maintenance are all conceivable. By evaluating the signals from CAN high and CAN low in the differential amplifier of the transceiver, the effects of interference are largely eliminated using what is known as differential transmission technology. Another advantage of the differential transmission technology is that fluctuations in the electrical system (e.g. when starting the engine) do not affect the data transmission to the individual control units (transmission reliability).

 

CAN convenience / K-CAN (body CAN)

  • ISO standard 11519 (earlier) or ISO 11989-3 (low-speed CAN)
  • Data rate 100kBit / s
  • 2-wire bus line, twisted, unshielded, cross-section of 0.35 mm each2
  • for VW: CAN high = orange / green, CAN low = orange / brown
  • The convenience CAN data bus is supplied with terminal 30 and must remain in standby. In order to load the on-board network as little as possible, the system goes into so-called “sleep mode” after “Terminal 15 off” if it is not required for the overall system.
  • The convenience CAN data bus can continue to be operated with the remaining line in the event of a short circuit on a data bus line or if a CAN line is interrupted. There is an automatic switchover to "single-wire operation".
  • In the recessive state (quiescent level) the CAN high signal is at 0V, in the dominant state a voltage of 3.6V is reached. With the CAN low signal, the recessive level is 5V, the dominant level is 1.4V. This means that the recessive level after the difference is formed in the differential amplifier is -5V and the dominant level is 2.2V. The voltage change between the recessive and the dominant level (voltage swing) was increased to 7.2V.
  • The low-speed CAN is less susceptible to interference because, on the one hand, the dependency of the two CAN signals on each other is eliminated by independent power amplifiers, and, in contrast to the drivetrain CAN data bus, the CAN high and CAN low lines are not connected to one another via resistors are. This means that CAN high and CAN low no longer influence each other, but work independently of each other as voltage sources.

Signal course CAN comfort:

Source: VW / Note: The time specification of 2 us is incorrect. The convenience CAN works at 100 kbit / s

 

Gateway / diagnostic bus

The individual bus systems send their data to the gateway. This ensures that the cross-system data, although they have different communication structures and speeds, are available in the entire system network. If a bus system fails or data is forwarded, it also fulfills the role of a filter. It is able to filter disturbances and amounts of data. Some vehicles have multiple gateways. There is often a gateway in the instrument cluster.

The gateway diagnosis CAN is used to exchange data between the diagnosis device and the control units installed in the vehicle. The K or L lines previously used are no longer required, with the exception of the exhaust-gas-related control units.

Due to the fast data transmission via CAN and through the gateway itself, the diagnostic device is able to display an overview of the installed components and their error status immediately after connection to the vehicle.

The diagnosis CAN uses an unshielded twisted pair cable with a cross-section of 0.35 mm each2. At VW, the CAN high line is orange / purple, the CAN low line is orange / brown. The data transfer takes place at a transfer rate of 500 kbit / s.

It is possible to access the CAN bus via the OBD connector.

 

Pin assignment on the diagnostic socket (SAE 1962)

Pin assignment: 2 + 10 data transmission according to SAE J 1850 (USA) ISO 9141-2 (Europe), 4 + 5 vehicle ground and signal ground, 6 + 14 data transmission CAN high and low, 7 + 15 data transmission according to ISO 9141-2 (Europe) too K and L output, 16 battery plus (terminal 15 or 30)

Sources: VW, BMW, Mercedes, Opel, multiplier course, BTZ Ingolstadt

Author: Johannes Wiesinger