Toyota Land Cruiser 100 / Amazon, Lexus LX 470

Since 1997 of release

Repair and car operation

The digital tyre of given CAN

The general data

Data exchange order on tyre CAN

On modern cars network tyres of data exchange CAN (Controller Area Network) between modules/blocks of management of various systems and controllers of actuation mechanisms of the car (address to an illustration above) are applied some.

The tyre is полнодуплексной (or simply duplex), i.e. any device connected to it can accept and transfer messages simultaneously.

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

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

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 1 Mbit/with at the maximum length of a line of 40 m.
• 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 (0), on other wire – level of logic unit (1) and on the contrary) is transferred.

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

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

In a case of short circuit or breakage of one of two wires of tyre CAN, thanks to the integrated hardware-software system of reliability switching in an operating mode under the single-wire scheme is carried out. The damaged transferring line ceases to 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.

The told means that to each message transferred on the tyre 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 2033 on 2048 are constantly fixed.

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

The block receiver processes only those messages (packages of the data) which are kept in its own identification list (the acceptability control).

Packages of the data can be transferred only in the event that the tyre of exchange 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 приоритетности according to which the message possessing 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 the data will be released again.

Except packages of the data packages of inquiry of the certain message on the tyre of data CAN are used also, that block of management which can give the required information reacts to similar inquiry.

Format of a package of the data

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

• Data Frame (a message shot) for message transfer on the tyre of data CAN (for example: temperature of a cooling liquid);
• Remote Frame (an inquiry shot) for inquiry of messages on the tyre of data CAN from other block of management;
• Error Frame (the error shot), 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 shots of the message on the tyre of data CAN which differ only on length of the identifier:

– A standard format;
– The expanded format.

Now in systems of data exchange of control systems of cars the standard format is used only.

Shot format

Each shot of messages transferred on tyre CAN consists of seven consecutive fields (address to an illustration above):

- 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 shot as Data Frame (a shot of the data) or as Remote Frame (a shot of remote inquiry) 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 – оличество 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 shot 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 the message with higher priority.

Management blocks

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 (address to an illustration above).

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 (a shot of the data);
• 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 (shot), that is bit fields with the set fixed format and length of a shot 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 shot 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.

CAN C – the tyre «the Engine and a running gear»

In the terminal block of management from each party 120 Ohm connected between both wires of the tyre of the data is established so-called согласующий the resistor of the tyre of the data with resistance.

The tyre of data CAN of an impellent compartment is activated only at the included ignition.
To tyre CAN-S can be connected more than 7 blocks of management.

CAN-B – Tyre "Salon"

Some blocks of management connected to the tyre of data CAN of salon, are activated irrespective of ignition inclusion (for example: system of the uniform lock).

Therefore the tyre of data CAN of salon should be in a mode of functional readiness even at the switched off ignition, it means that possibility of transfer of packages of the data should be provided even at the switched off ignition.

For the purpose of the greatest possible decrease in a consumed current of rest, the tyre of data CAN, in the absence of necessary to data transmission, passes in a mode of passive expectation, and is activated again only at the following reference to it.

If in a mode of passive expectation of the tyre of data CAN of salon any block of management (for example, the module of management of the uniform lock) transfers on it the message it is accepted only by the main system module (an ignition electronic lock, EZS/EIS). Module EZS keeps this message in memory and sends an activation signal (Wake-up) on all blocks of management connected to tyre CAN-V.

At activation, EZS checks presence of all users of the tyre of data CAN then transfers the message kept before in memory.

To tyre CAN-V can be connected more than 20 blocks of management.