Audi A4, S4

since 2000 release

Repair and operation of the car

Digital tire of data of CAN

Main data

On the car some network tires of data exchange of CAN (Controller Area Network) between blocks (modules) of management of various systems and controlers of actuation mechanisms of the car are used.

Separate control units are united with each other in the general network and can exchange data.

The tire is bidirectional, i.e. any device connected to it can accept and transfer messages.

The signal from a sensitive element (sensor) comes to the next control unit which processes it and transfers to the tire of data of CAN.

Any control unit connected to the tire of data of CAN can read out this signal, calculate value of the managing director of influence on its basis and operate an executive servomechanism.

Data exchange on the tire CAN

Advantages

At usual cable connection of electric and electronic devices direct connection of each control unit with all sensors and executive elements from which it receives results of measurements or which operates is carried out.

Complication of a control system leads to the excessive length or large number of cable lines.

In comparison with standard cable distributing the tire of data provides:

• Reduction of quantity of cables. Wires from sensors reach only for the next control unit which will transform the measured values to a package of data and transfers him to the tire CAN.
• Any control unit which on the tire CAN receives the corresponding package of data can operate the executive mechanism, and on its basis counts value of the operating impact on a servomechanism.
• Improvement of electromagnetic compatibility.
• Reduction of number of shtekerny connections and reduction of quantity of contact conclusions on control units.
• Weight reduction.
• Reduction of number of sensors since signals of one sensor (for example, from the sensor of temperature of cooling liquid) can be used by various systems.
• Improvement of opportunities of diagnosing. Since signals of one sensor (for example, a speed signal) are used by various systems in case the message on malfunction all give out the systems using this signal, the sensor or the control unit processing its signals is faulty, as a rule. If the message on malfunction arrives only from one system though this signal is used also by other systems, the cause of defect, most often, is concluded in the processing control unit or a servomechanism.
• High speed of data transmission – is possible to 1mbit/with with the maximum length of the line of 40 m. Now on and/m the speed of data transmission makes from 83 Kbps to 500 Kbps.
• Some messages can serially be transferred on the same line.

The tire of data of CAN consists of the strong wire executed in the form of twisted couple. All devices (control units of devices) are connected to this line.

Data transmission is carried out with duplication on both wires, and logical levels of the tire of data have mirror display (that is if on one wire the level of logical zero is transferred, on other wire the level of logical unit, and vice versa is transferred).

The two-wire scheme of transfer is used for two reasons: for identification of mistakes and as reliability basis.

If the peak of tension arises only on one wire (for example, owing to problems about EMS (electromagnetic compatibility)), blocks receivers can identify it as a mistake and to ignore this peak of tension.

If there is a short circuit or break of one of two wires of the tire of data of CAN, thanks to the integrated hardware-software system of reliability there will be a switching in an operating mode according to the single-wire scheme. The damaged transferring line will not be used.

The order and format of the messages transferred and accepted by users (subscribers) is defined in the protocol of data exchange.

Essential distinctive sign of the tire of data of CAN in comparison with other tire systems which are based on the principle of subscriber addressing is the addressing correlated to the message.

It means that on the tire of data of CAN its permanent address (identifier) marking contents of this message (for example is appropriated to each message: temperature of cooling liquid). The protocol of the tire of data of CAN allows transfer to 2048 various messages, and addresses with 3 on 2048 are constantly fixed.

The volume of data in one message on the tire of data of CAN makes 8 bytes.

The block receiver processes only those messages (packages of data) which are kept in its list of the given CAN of messages (control of the acceptability) accepted on the tire.

Packages of data can be transferred only if the tire of data of CAN is free (i.e. if after the last package of data the interval of 3 bits and any of control units followed does not start transferring the message).

Thus the logical level of the tire of data has to be recessive (logical "1").

If some control units at the same time start transferring messages, the principle of priority according to which the message on the tire of data of CAN with the top priority will be transferred to the first without loss of time or bits (arbitration of inquiries of access to the general tire of data) comes into force.

Each control unit losing the right of arbitration automatically switches to reception and repeats attempt to send the message as soon as the tire of data of CAN again is released.

Except packages of data there is also a package of request of a certain message on the tire of data of CAN.

In this case the control unit which can provide a required package of data, reacts to this inquiry.

Format of a package of data

In the usual mode of transfer packages of data have the following configurations of blocks (frames):

• Data Frame (a message frame) for transmission of messages on the tire of data of CAN (for example: temperature of cooling liquid).

• Remote Frame (an inquiry frame) for request of messages on the tire of data of CAN from other control unit.

• Error Frame (a mistake frame) all connected control units are notified that there was a mistake and the last message on the tire of data of CAN is invalid.

The protocol of the tire of data of CAN supports two various formats of frames of the message on the tire of data of CAN which differ only on identifier length:

• standard format;
• expanded format.

Now the standard format is used.

The package of data for transmission of messages on the tire of data of CAN consists of seven consecutive fields:

• Start of Frame (starting bit): Marks the beginning of the message and synchronizes all modules.

• Arbitration Field (identifier and inquiry): This field consists of the identifier (address) in 11 bits and 1 control bit (Remote Transmission Request-Bit). This control bit marks a package as Data Frame (a message frame) or as Remote Frame (an inquiry frame) without bytes of data.

• Control Field (the operating bits): The field of management (6 bits) contains IDE bats (Identifier Extension Bit) for recognition of a standard and expanded format, reserve bit for the subsequent expansions and - in the last 4 bits - number of bytes of the data put in Data Field (data field).

• Data Field (this): The data field may contain from 0 to 8 bytes of data. The message on the tire of data of CAN 0 bytes long is used for synchronization of the distributed processes.

• CRC Field (control field): The field CRC (Cyclic-Redundancy-Check Field) contains 16 bits and serves for control recognition of mistakes by transfer.

• ACK Field (reception confirmation): The field ACK (Acknowledgement Field) contains a signal of confirmation of reception of all blocks receivers which received the message on the tire CAN without mistakes.

• End of Frame (end of a frame): Marks the end of a package of data.

• Intermission (interval): Interval between two packages of data. The interval has to make not less than 3 bits. After that any control unit can transfer the following package of data.

• IDLE (rest mode): If any control unit does not transfer messages, the tire CAN remains in the rest mode before transfer of the following package of data.

Priorities

For data processing possibility of their fast transfer has to be in real time provided.

It assumes not only existence of the line with a high physical speed of data transmission, but also demands also expeditious providing access to the general tire CAN if several control units need to transfer messages at the same time.

For the purpose of differentiation of the data of CAN of messages on urgency degree transferred on the tire, for separate messages various priorities are provided.

The ignition advancing corner, for example, has the highest priority, values of pro-slipping - average, and temperature of external air - the lowest priority.

The priority with which the message is transferred on the tire CAN, is defined by the identifier (address) of the corresponding message.

The identifier corresponding to smaller binary number has higher priority, and vice versa.

The protocol of the tire of data of CAN is based on two logical states: Bits are or "recessive" (logical "1"), or "prepotent" (logical "0"). If the prepotent bit is transferred by at least one module, the recessive bits transferred by other modules are rewritten.

Example

If some control units at the same time begin data transmission, the conflict of access to the general tire of data is resolved by means of "bit-by-bit arbitration of inquiries of the general resource" by means of the corresponding identifiers.

By transfer of a field of the identifier the block transmitter after each bit checks, whether he has still rights of transfer, or already other control unit transfers on the tire of data of CAN the message with higher priority.

If the recessive bit transferred by the first block transmitter is rewritten by prepotent bit of other block transmitter, the first block transmitter loses the right of transfer (arbitration) and becomes the block receiver.

The first control unit (N I) loses arbitration from the 3rd bit.

The third control unit (N III) loses arbitration from the 7th bit.

The second control unit (N II) keeps right of access to the tire of data of CAN and can transfer the message.

Other control units will try to transfer the messages on the tire of data of CAN only after it again is released. Thus the right of transfer will be granted again according to priority of the message on the tire of data of CAN.

Recognition of mistakes

Hindrances can lead to mistakes in data transmission. Such, arising by transfer, mistakes should be distinguished and eliminated. The protocol of the tire of data of CAN distinguishes two levels of recognition of mistakes:

• mechanisms at the level of Data Frame (a message frame);
• mechanisms at the level of bits.

Mechanisms at the level of Data Frame

Cyclic-Redundancy-Check:

On the basis of the message CAN transferred on the tire of data the block transmitter counts control bits which are transferred together with a package of data in the field of "CRC Field" (checksums). The block receiver anew calculates these control bits on the basis of CAN of the message accepted on the tire of data and compares them to the control bits received together with this message.

Frame Check:

This mechanism checks structure of the transferred block (frame), that is bit fields with the set fixed format and frame length are rechecked.

The mistakes distinguished by the Frame Check function are marked as a format error.

Mechanisms at the level of bits

Monitoring:

Each module by transfer of the message traces the logical level of the tire of data of CAN and defines thus distinctions between the transferred and accepted bit. Thanks to it reliable recognition of the global and arising in the block transmitter local mistakes on bits is provided.

Bit Stuffing:

In each package of data between the field "Start of Frame" and the end of the field "CRC Field" there has to be no more than 5 following one after another of bits with identical polarity.

After each sequence from 5 identical bits the block transmitter adds to a stream of bits one bit with opposite polarity.

Blocks receivers delete these bits after reception of the message on the tire of data of CAN.

Elimination of mistakes

If any module of the tire of data of CAN distinguishes a mistake, it interrupts the current process of data transmission, sending an error message. The error message consists of 6 prepotent bits.

Thanks to an error message all CAN control units connected to the tire of data are notified on the arisen local mistake and respectively ignore the message transferred before.

After a short pause all control units again will be able to transfer messages on the tire of data of CAN, and the first will again send the message with the top priority.

The control unit, whose message on the tire of data of CAN were caused by emergence of a mistake, also begins repeated transfer of the message (Automatic Repeat Request function).

Types of tires CAN

Various tires CAN are used to different areas of management. They differ from each other in data transmission speed.

Transfer speed on the tire of data of CAN "engine and running gear" areas (CAN-C) makes 125 Kbps, and the tire of the given CAN "Salon" (CAN-B) owing to smaller number of especially urgent messages is calculated on data transmission speed only of 83 Kbps.

Data exchange between two tire systems is carried out through so-called "gateway locks", i.e. the control units connected to both tires of data.

The fiber-optical tire D2B (Digital Daten-Bus) of data is used to the Audio/communication/navigation area. The bigger volume of information, than the tire with a copper cable can significantly transfer a fiber-optical cable.