13.1.6. Digital tire of data of CAN
On modern cars some network tires of data exchange of CAN (Controller Area Network) between modules/control units of various systems and controlers of actuation mechanisms of the car are applied.
Data exchange order on the tire CAN
The tire is polnodupleksny (or simply duplex), i.e. any device connected to it can accept and transfer messages at the same time.
The signal from a sensitive element of corresponding information (sensor) arrives in the next control unit which processes it and transfers to the tire of data exchange of CAN.
Any control unit connected to the tire of data of CAN, can read out this signal, calculate on its basis parameters of the managing director of influence and supervise functioning of the corresponding executive servomechanism.
At usual cable connection of electric and electronic devices direct connection of each control unit with all sensors and executive elements from which he receives results of measurements or which operates is carried out.
Complication of a control system brings to 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 last only to the next control unit which will transform the measured values to a package of data and transfers the last in the tire CAN;
- Operate the executive mechanism any control unit which on the tire CAN receives the corresponding package of data can, and on its basis counts value of operating influence;
- 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 possibilities of diagnosing. Since signals of one sensor (for example, the signal of speed) are used by various systems in case the message on malfunction is issued by all systems using these signal, faulty is, as a rule, the sensor or the control unit processing its signals. If the message on malfunction arrives only from one system though this signal is used also by other systems, the cause of defect is concluded in the processing control unit or a servomechanism, more often;
- High speed of data transmission - is possible to 1 Mbps at the maximum length of the line of 40 m. Now on and/m speed of data transmission makes Mercedes-Benz 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 pair. All devices are connected to this line (control units by devices).
Data transmission is carried out with duplication on both wires, and logic levels of the tire of data have mirror display (that is if on one wire level of logic zero (0) is transferred, on other wire - level of logic unit (1), and on the contrary).
The two-wire scheme of transfer is used for two reasons: for control of mistakes and as reliability basis.
If the peak of tension arises only on one wire, - for example, owing to the problems connected with electromagnetic compatibility (EMS), - that blocks receivers can identify it as a mistake and to ignore this peak.
In case of short circuit or break of one of two wires of the tire CAN, thanks to the integrated hardware-software system of reliability switching in an operating mode according to 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 protocol.
Essential distinctive sign of the tire of data of CAN in comparison with other tire systems which are based on a principle of user's addressing, is the addressing correlated with the message.
The told means that its permanent address (identifier) marking the contents of this message (for example is appropriated to each message transferred on the tire: temperature of cooling liquid). The protocol of the tire of data of CAN allows transfer to 2048 various messages, and addresses with 2033 on 2048 are constantly fixed.
The volume of data in one message on the tire CAN makes 8 bytes.
The block receiver processes only those messages which are kept in its own identification list (acceptability control).
Packages of data can be transferred only if the tire of an exchange of CAN is free (i.e. if after the last package the interval in 3 bits followed, and any of control units does not start to transfer the next message). Thus logic level of the tire of data should be retsessivny (logic «1»).
If some control units at the same time start to transfer messages, the priority principle according to which the message possessing the highest 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 forfeiting the right of arbitration, is automatically switched to reception and repeats attempt to send the message as soon as the tire of data will be released again.
Except packages of data at exchange of information packages of requests of certain messages on the tire of data of CAN are used also, - that control unit which in a condition to provide required information reacts to similar inquiry.
Data transmission format
In a usual mode of transfer packages of the following configurations are used:
- Data Frame (message shot) for transmission of messages on the tire of data of CAN (for example: temperature of cooling liquid);
- Remote Frame (inquiry shot) for request of messages on the tire of data of CAN from other control unit;
- Error Frame (mistake shot), - all connected control units are notified that there was a mistake and the last message on the tire of data of CAN is void.
The protocol of the tire of data of CAN supports two various formats of frames of the message which differ only on length of the identifier: Standard and Expanded.
Now in systems of data exchange of control systems of cars of the Daimler Chrysler company only the standard format is used.
Each shot of messages transferred on the tire CAN consists of seven consecutive fields:
Start of Frame (starting bit): Marks the beginning of the message and synchronizes all modules;
Arbitration Field (arbitration field): This field consists of the 11-bit identifier (address) in and one control bit (Remote Transmission Request-Bit) marking a shot as Data Frame (a shot of data) or as Remote Frame (a shot of remote inquiry);
Control Field (operating bits): the 6-bit field of management contains the defining IDE bit (Identifier Extension Bit) used for recognition such as a used format (standard or expanded), reserve bit for the subsequent expansions and, - in the last 4 bits, - information on number of bytes of the data put in Data Field (see further);
Data Field (this): The data field can contain from 0 to 8 bytes of data, - transfer on the tire CAN of messages in length of 0 bytes 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 have received the message on the tire CAN without mistakes;
End of Frame (shot end): Marks the shot end;
Intermission (interval): An interval between two next frames of data. The length of an interval should make not less than 3 bits after which identification of any of control units can start transfer of the next package;
IDLE (rest mode): If any control unit does not transfer messages, the tire CAN remains in a mode of rest (expectation) till the beginning of transfer of the following package.
For data processing possibility of their fast transfer should be in real time provided.
It assumes not only line existence with high physical speed of data transmission, but also demands also operative providing access to the general tire CAN if it is necessary for several control units to transfer messages at the same time.
For the purpose of differentiation of data of CAN of messages transferred on the tire on urgency degree, for separate messages various priorities are provided.
The corner of an advancing of ignition, for example, has the highest priority, values of pro-slipping - average, and temperature of external air - the lowest.
The priority with which the message is transferred on the tire CAN, is defined by the identifier (an address field) corresponding message.
The identifier corresponding to smaller binary number, has higher priority, and on the contrary.
The protocol of the tire of data of CAN is based on two logic conditions: Bits are or "retsessivny" (logic «1»), or "prepotent" (logic «0»). If the prepotent bit is transferred by at least one module, the retsessivny bits transferred by other modules, are rewritten.
Example of the organization of arbitration
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
If some control units at the same time begin data transmission, the conflict of access to the general tire of data is authorized 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 it possesses still the right of transfer, or already other control unit transfers on the tire the message with higher priority.
If the retsessivny bit transferred by the first block transmitter is rewritten by prepotent bit of other block transmitter, the first block transmitter loses the right of a broadcast (arbitration) and becomes the block receiver.
Other control units will try to transfer the messages on the tire of data of CAN only after it will again be released. Thus the right of transfer will be provided 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, it is necessary to distinguish and eliminate mistakes. The protocol of the tire of data of CAN distinguishes two levels of recognition of mistakes:
- Mechanisms at Data Frame level (a shot of data);
- Mechanisms at level of bits.
Mechanisms at Data Frame level
On the basis of transferred on the tire of data of CAN of the message the block transmitter counts control bits which are transferred together with a package given in the field «CRC Field» (checksums). The block receiver anew calculates these control bits on the basis of accepted on the tire of data of CAN of the message and compares them to the control bits received together with this message.
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 mistakes distinguished by the Frame Check function are marked as an error of a format.
Mechanisms at level of bits
Each module by transfer of the message traces logic level of the tire of data of CAN and defines thus distinctions between the transferred and accepted bit. Thanks to it reliable recognition of local mistakes global and arising in the block transmitter on bits is provided.
In each shot of data between the field "Start of Frame" and the end of the field "CRC Field" there should be no more than 5 following one after another bits 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 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 connected to the tire of data CAN control units are notified on the arisen local mistake and, respectively, ignore the message transferred before.
After a short pause all control units again can transfer messages on the tire of data of CAN, and the message with the highest priority will be again sent the first.
The control unit, whose message on the tire of data of CAN were caused by mistake emergence, also there begins repeated transfer of the message (Automatic Repeat Request function).
Types of tires CAN
Various tires CAN are applied to different areas of management. They differ from each other in the speed of data transmission.
Speed of transfer on the tire of data of CAN area «the engine and a running gear» (CAN-C) makes 125 Kbps, and the tire of these CAN "Salon" (CAN-B) owing to smaller number of especially urgent messages is calculated on speed of data transmission only 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 applied to the Audio/communication/navigation area. Essentially bigger volume of information, than the tire with a copper cable can transfer a fiber-optical cable.
CAN-C - the tire «The engine and a running gear»
In the terminal control unit from each party the so-called soglasuyushchy resistor of the tire of data with resistance of 120 Ohm connected between both wires of the tire of data is established.
The tire of data of CAN of an impellent compartment is activated only at the included ignition.
More than 7 control units can be connected to the tire CAN With.
CAN-B - the tire "Salon"
Some control units connected to the tire of data of CAN salon, are activated irrespective of ignition inclusion (for example: system of the uniform lock).
Therefore the tire of data of CAN salon should be in a mode of functional readiness even at the switched-off ignition, it means that possibility of transfer of packages of 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 tire of data of CAN, in the absence of necessary to data transmission, passes to a mode of passive expectation, and is activated again only at the following address to her.
If in a mode of passive expectation of the tire of data of CAN salon any control unit (for example, the module of management of the uniform lock) transfers on it the message, it is accepted by only the main system module (the electronic lock of ignition, EZS/EIS). The EZS module keeps this message in memory and sends a signal of activation (Wake-up) on all control units connected to the tire of CAN Century.
At activation, EZS checks presence of all users of the tire of data of CAN then transfers the message kept before in memory.
More than 20 control units can be connected to the tire CAN In.