Audi A4, S4

Since 2000 of release

Repair and car operation

The digital tyre of given CAN

The basic data

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

Separate blocks of management are united with each other in the general network and can exchange the data.

The tyre is two-forked, i.e. any device connected to it can accept and transfer messages.

The signal from a sensitive element (gauge) arrives in the nearest block of management which processes it and transfers to the tyre of data CAN.

Any block of management connected to the tyre of data CAN, can read out this signal, calculate on its basis value of operating influence and to operate an executive servomechanism.

Data exchange on tyre CAN

Advantages

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

Control system complication leads to excessive length or large number of cable lines.

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

• Reduction of quantity of cables. Wires from gauges last only to the nearest block of management which will transform the measured values to a package of the data and transfers it in tyre CAN.
• Operate the executive mechanism any block of management which on tyre CAN receives a corresponding package of the data can, and on its basis counts value of operating influence on a servomechanism.
• Improvement of electromagnetic compatibility.
• Quantity reduction штекерных connections and reduction of quantity of contact conclusions by management blocks.
• Weight reduction.
• Reduction of quantity of gauges since signals of one gauge (for example, from the gauge of temperature of a cooling liquid) can be used various systems.
• Improvement of possibilities of diagnosing. Since signals of one gauge (for example, the signal of speed) are used by various systems in case the message on malfunction is given out by all systems using a given signal, faulty is, as a rule, the gauge or the block of management processing its signals. If the message on malfunction arrives only from one system though the given signal is used also by other systems the cause of defect is concluded in the processing block of management or a servomechanism, more often.
• High speed of data transmission – is possible to 1Mbit/with at the maximum length of a line of 40 m. Now on а/м speed of data transmission makes from 83 Kbit/with to 500 Kbit/with.
• Some messages can serially be transferred on the same line.

The tyre of data CAN consists of the strong wire executed in the form of twisted pair. All devices are connected to this line (blocks of management by devices).

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

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

If the pressure peak arises only on one wire (for example, owing to problems about Ems (electromagnetic compatibility)) blocks-receivers can identify it as an error and to ignore this peak of pressure.

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

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

Essential distinctive sign of the tyre of data CAN in comparison with others шинными the systems which are based on a principle of user's addressing, is the addressing correlated with the message.

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

The volume of the data in one message on the tyre of data CAN makes 8 byte.

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

Packages of the data can be transferred only in the event that the tyre of data CAN is free (i.e. if after last package of the data the interval in 3 bits has followed, and any of blocks of management does not start to transfer the message).

Thus logic level of the tyre of the data should be рецессивным (logic «1»).

If some blocks of management simultaneously start to transfer messages the principle priority according to which the message on the tyre of data CAN with the highest priority will be transferred to the first without loss of time or bits (the arbitration of inquiries of access to the general tyre of the data) comes into force.

Each block of management forfeiting the right of arbitration, is automatically switched to reception and repeats attempt to send the message as soon as the tyre of data CAN will again be released.

Except packages of the data there is also a package of inquiry of the certain message on the tyre of data CAN.

In this case the block of management which can give a required package of the data, reacts to the given inquiry.

Format of a package of the data

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

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

• Remote Frame (the inquiry frame) for inquiry of messages on the tyre of data CAN from other block of management.

• Error Frame (the error frame) all connected blocks of management are notified that there was an error and last message on the tyre of data CAN is void.

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

• Standard format;
• The expanded format.

Now the standard format is used.

The package of the data for message transfer on the tyre of data CAN consists of seven consecutive fields:

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

• Arbitration Field (the 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 (the message frame) or as Remote Frame (the inquiry frame) without bytes of the data.

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

• Data Field (data): the Data field can contain from 0 to 8 byte of the data. The message on the tyre given CAN 0 byte is used in the length for synchronisation of the distributed processes.

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

• ACK Field (reception acknowledgement): Field ACK (Acknowledgement Field) contains a signal of acknowledgement of reception of all blocks-receivers which have received the message on tyre CAN without errors.

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

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

• IDLE (a rest mode): If any block of management does not transfer messages tyre CAN remains in a mode of rest before transfer of a following package of the data.

Priorities

For data processing in a mode of real time possibility of their fast transfer should be provided.

It assumes not only presence of a line with high physical speed of data transmission, but also demands also operative granting of access to the general tyre CAN if it is necessary for several blocks of management to transfer messages simultaneously.

For the purpose of differentiation transferred on the tyre given CAN messages on promptness degree, for separate messages various priorities are provided.

The corner of an advancing of ignition, for example, has the higher priority, values of proslipping - average, and temperature of external air - the lowest priority.

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

The identifier corresponding to smaller binary number, has higher priority, and on the contrary.

The report of the tyre of data CAN is based on two logic conditions: Bits are or «рецессивными» (logic «1»), or "prepotent" (logic «0»). If the prepotent bit is transferred at least by one module рецессивные the bits transferred by other modules, are rewritten.

Example

If some blocks of management simultaneously begin data transmission the access conflict to the general tyre of the data is authorised by means of «bit-by-bit arbitration of inquiries of the general resource» by means of corresponding identifiers.

By transfer of a field of the identifier the block transmitter after each bit checks, whether it possesses still right transfers, or already other block of management transfers on the tyre of data CAN the message with higher priority.

If transferred by the first block-transmitter рецессивный the bit is rewritten by prepotent bit of other block-transmitter the first block transmitter loses the right transfers (arbitration) and becomes the block-receiver.

The first block of management (N I) loses arbitration from 3rd bit.

The third block of management (N III) loses arbitration from 7th bit.

The second block of management (N II) keeps access right to the tyre of data CAN and can transfer the message.

Other blocks of management will try to transfer the messages on the tyre of data CAN only after it will again be released. Thus the transfer right will be given again according to приоритетностью messages on the tyre of data CAN.

Recognition of errors

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

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

Mechanisms at level Data Frame

Cyclic-Redundancy-Check:

On the basis of transferred on the tyre of data CAN of the message the block transmitter counts control bits which are transferred together with a package of the data in the field «CRC Field» (the control sums). The block receiver anew calculates these control bits on the basis of accepted on the tyre of data CAN of the message 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 length of the frame are rechecked.

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

Mechanisms at level of bits

Monitoring:

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

Bit Stuffing:

In each package of the data between a field «Start of Frame» and the field end «CRC Field» should be no more than 5 bits following one after another with identical polarity.

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

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

Elimination of errors

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

Thanks to an error message all connected to the tyre of data CAN management blocks are notified on the arisen local error and accordingly ignore the message transferred before.

After a short pause all blocks of management again can transfer messages on the tyre of data CAN, and the first will again send the message with the highest priority.

The management block, whose message on the tyre of data CAN was caused by error occurrence, also begins repeated transfer of the message (function Automatic Repeat Request).

Types of tyres CAN

Various tyres CAN are applied to different areas of management. They differ from each other in the speed of data transmission.

On the tyre of data CAN of area «the engine and a running gear» (CAN-C) makes speed of transfer 125 Kbit/with, and the tyre of data CAN "Salon" (CAN-B) owing to smaller quantity of especially urgent messages is calculated on speed of data transmission only 83 Kbit/with.

Data exchange between two шинными systems is carried out through so-called «gateway sluices», i.e. the blocks of management connected to both tyres of the data.

Fibre-optical tyre D2B (Digital Daten-Bus) the data is applied to area "Audio/communications/navigation". The information volume, than the tyre with a copper cable can transfer a fibre-optical cable essentially больший.