14.6. Digital tire of data of CAN
On modern cars several network tires of data exchange of CAN (Controller Area Network) between modules/control units of various systems and controllers of actuation mechanisms of the car are used.
Data exchange on CAN tire
B — the Sensor 1
M — the Executive I-III elements (servomechanisms)
The tire is full-duplex (or just 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) comes to the next control unit which processes it and transfers to CAN data exchange tire.
Any control unit connected to the tire of data of CAN can read out this signal, calculate parameters of the managing director of influence on its basis and operate an 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 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 CAN tire;
- Any control unit which on the tire of 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 is given by all 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, then the cause of defect, most often, is concluded in the processing control unit or a servomechanism;
- High speed of data transmission - is possible to 1 Mbps with the maximum length of the line of 40 m. Now on the Mercedes-Benz car the speed of data transmission makes from 83 Kbps to 500 Kbps;
- Several 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 (0) is transferred, then on other wire - the level of logical unit (1) and vice versa).
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 of 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 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.
Told means that its permanent address (identifier) marking contents of this message is appropriated to each message transferred on the tire (for example: temperature of cooling liquid). The protocol of the tire of data of CAN allows transfer to 2048 various messages, and addresses from 2033 to 2048 are constantly fixed.
The volume of data in one message on the tire of CAN makes 8 bytes.
The block receiver processes only those messages which are kept in its own identification list (control of the acceptability).
Packages of data can be transferred only if the tire of exchange of CAN is free (i.e. if after transfer of the last package the interval of 3 bits followed, and any of control units does not begin to transfer the next message). At the same time the logical level of the tire of data has to be recessive (logical "1").
If several control units at the same time begin to transfer messages, then the principle of priority according to which the message possessing 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 is released again.
Except packages of data also packages of request of a certain message on the tire of data of CAN are used, - that control unit which can provide required information reacts to similar inquiry.
Data transmission format
In the 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 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 in systems of data exchange of control systems of cars of the Daimler Chrysler company only the standard format is used.
Each shot of CAN of messages transferred on the tire 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 shot as Data Frame (a shot of data) or as Remote Frame (a shot of remote inquiry) without bytes of data;
- Control Field (the operating bits): The management field (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 - the 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): Cyclic-Redundancy-Check Field contains 16 bits and serves for control recognition of mistakes by transfer;
- ACK Field (reception confirmation): Acknowledgement Field contains a signal of confirmation of reception of all blocks receivers which received the message on CAN tire without mistakes;
- End of Frame (end of a shot): Marks the end of a shot;
- Intermission (interval): Interval between two frames of data. The interval has to make not less than 3 bits. After that any control unit can begin transfer of the next package;
- IDLE (rest mode): If any control unit does not transfer messages, then the tire of CAN remains in the rest mode prior to transfer of the following package.
For data processing the possibility of their bystry 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 of 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 CAN tire, 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, then the recessive bits transferred by other modules are rewritten.
If several control units at the same time begin data transmission, then 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 the 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 the message with higher priority.
If the recessive bit transferred by the first block transmitter is rewritten by prepotent bit of other block transmitter, then the first block transmitter loses the right of transfer (arbitration) and becomes the block receiver.
Example of the organization of arbitration
The first control unit (N I) loses arbitration from the 3rd bit.
The second control unit (N II) keeps right of access to the tire of data of CAN and
Other control units will try to transfer the messages on the tire of data of CAN only after it again is released. At the same time the right of transfer will be granted according to priority of the message on the tire of data of CAN again.
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 shot of data);
- Mechanisms at the level of bits.
Mechanisms at the level of Data Frame
On the basis of CAN of the message 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.
This mechanism checks structure of the transferred block (shot), that is bit fields with the set fixed format and shot length are rechecked.
The mistakes distinguished by the Frame Check function are marked as format errors.
Mechanisms at the level of bits
Each module by transfer of the message traces the logical level of the tire of data of CAN and defines at the same time 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.
In each shot of data between the field "Start of Frame" and the end of the field "CRC Field" there have 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, then it interrupts the current process of data transmission, sending the error message. The error message consists of 6 prepotent bits.
Thanks to the 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 will be able to transfer messages on the tire of data of CAN again, and the first will send the message with the top priority again.
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 of CAN
Various tires of CAN are used to the different fields of management. They differ from each other in data transmission speed.
Transfer speed on the tire of data of CAN of the "engine and running gear" area (CAN-C) makes 125 Kbps, and the tire of data of CAN "Salon" owing to the 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 of 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.
CAN C - the tire "Engine and Running Gear"
The so-called coordinating resistor of the tire of data with resistance of 120 Ohms connected between both wires of the tire of data is installed in the terminal control unit from each party.
The tire of data of CAN of a motive compartment is activated only at the included ignition.
More than 7 control units can be connected to the tire CAN-C.
CAN-B - the tire "Salon"
Some control units connected to the tire of data of CAN salon are activated irrespective of inclusion of ignition (for example: system of the uniform lock).
Therefore the tire of data of CAN salon has to be in the mode of functional readiness even at the switched-off ignition, it means that the possibility of transfer of packages of data has to be provided even at the switched-off ignition.
For the purpose of the greatest possible decrease in the consumed rest current, the tire of data of CAN, in the absence of necessary to data transmission, passes into the mode of passive expectation, and is activated again only at the following address to her.
If any control unit (for example, the module of management of the uniform lock) transfers in the mode of passive expectation of the tire of data of CAN salon on it the message, then 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) to all control units connected to the tire CAN-B.
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-B.