 printable version
The digital CAN data bus
Communication via the CAN bus
 | B - Sensor 1
CAN - Bus Data
M - Actuators I - III (servos) | N - control units (controllers) I - V |
Basic information
By car tires used multiple network data exchange CAN (Controller Area Network) between the blocks (modules) control of various systems and controllers, actuators of the vehicle.
Individual control units are combined with each other in a common network, and can communicate.
The bus is bi-directional, i.e. any device connected to it can receive and send messages.
The signal from the sensing element (sensor) flows in the nearest control unit, which processes it, and passes on the data bus CAN.
Each control unit connected to the data bus CAN, can read this signal to calculate its value based on the manipulated and controlled by the executive servo.
Benefits
In normal cable connection of electrical and electronic devices are a direct connection to each of the control unit with all the sensors and actuators, from which he received the results of measurements or run.
The increasing complexity of the control system leads to excessive length or multiple cable lines.
Compared with the standard cabling data bus provides:
?� Reducing the number of cables. Wires from the sensors extend only to the nearest control unit, which converts the measured values in the data packet and sends it to the bus CAN.
?� Manage the actuator can be any control unit, which receives the corresponding CAN bus data packet, and on this basis calculates the control action on the servo.
?� Improved EMC.
?� Reducing the number of plug-in connections and reducing the number of pins on the control unit.
?� Weight loss.
?� Reduced number of sensors, as signals from one sensor (e.g., coolant temperature sensor) can be used in different systems.
?� Improved diagnostic capabilities. Since signals from one sensor (e.g., a speed signal) are used by different systems, in case the fault message issued using all the signal system, the fault is typically a sensor or a control unit processing its signals. If the error message goes only on one system, although the signal is used by other systems, the cause of the problem, most often lies in the manufacturing control unit or servomechanism.
?� High-speed data transfer - can be up to 1Mbit / s at a maximum cable length of 40 m. At present, the a / m Mercedes-Benz data rate of 83 kbit / s to 500 kbit / s.
?� Multiple messages can be serially transmitted on the same line.
CAN data bus consists of a twisted pair wire, made in the form of twisted pairs. This line is connected to all devices (control devices).
Data transfer is carried out in duplicate for both wires, with the logic levels of the data bus are a mirror image (that is, if one wire is transferred to a logic zero (0), then the other wire is passed level logical unit (1), and vice versa).
Two-wire transmission is used for two reasons: to identify errors and as the basis of reliability.
If the peak voltage occurs only on one wire (for example, due to problems with EMC (electromagnetic compatibility)), the blocks-receivers can identify this as an error and ignore this peak voltage.
If there will be a short circuit or an open one of two wires of the data bus CAN, thanks to the integrated hardware and software system changes will occur in the switching operation on a single circuit. Damaged transmission line will not be used.
The order and format of the data sent and received by users (subscribers) messages is defined in the protocol for data exchange.
A significant feature of the CAN data bus compared to other bus systems, based on the principle of addressing subscriber is The correlation with the message addressing.
This means that each message on the CAN data bus assigned to his permanent address (ID), marking the contents of the message (eg coolant temperature). Protocol CAN bus allows the transmission of up to 2048 different messages, with addresses 2033 2048 are permanently enshrined.
The volume of data in a single message on the CAN data bus is 8 bytes.
The block receiver processes only those messages (data packets), which are stored in its list received via the data bus CAN messages (control of acceptability).
The data packets may be transmitted only when the CAN bus is free (i.e., if after the last data packet followed by a 3-bit interval, and none of the control unit begins to transmit a message).
Thus the logic level of the data bus to be recessive (logical "1").
If several control units at the same time begin to transmit messages, it takes effect the principle of priority, according to which a message on the CAN data bus with the highest priority will be sent first without loss of time or bits (arbitration requests access to the shared data bus).
Each control unit loses arbitration rules, will automatically switch to receive and send retry your message as soon as the CAN data bus is free again.
In addition there is also a data packet request packet to a particular message via the data bus CAN.
In this case, the control unit that can provide the requested data packet responding to the request.
Data Packet Format
In normal mode, the transmission data packets have the following configuration blocks (frames):
??? Data Frame (message frame) to send messages via the data bus CAN (eg .: engine coolant temperature).
??? Remote Frame (frame request) to request messages to the CAN data bus from the other unit.
??? Error Frame (frame error) all connected control units are notified that an error has occurred and the last message on the CAN data bus is invalid.
CAN bus protocol supports two different frame format messages via the data bus CAN, which differ only in the length of the identifier:
- A standard format;
- Extended format.
Currently DaimlerChrysler uses only standard format.
A data packet for transmitting messages according to the CAN data bus comprises seven consecutive fields:
??? Start of Frame (start bit): Marks the beginning of the message and synchronizes all the modules.
??? Arbitration Field (identifier and the query): This field consists of an identifier (address) to 11 bits and 1 control bit (Remote Transmission Request-Bit). This control bits mark the package as Data Frame (message frame) or as Remote Frame (frame request) without data bytes.
??? Control Field (control bits): The control field (6 bits) contains IDE-bit (Identifier Extension Bit) for recognition of the standard and extended formats, reserve bits for future extensions and - in the last 4 bits - the number of bytes of data laid down in the Data Field (data field).
??? Data Field (data): data field may contain from 0 to 8 bytes of data. Communication on the CAN data bus length 0 bytes is used for synchronization of distributed processes.
??? CRC Field (control field) field CRC (Cyclic-Redundancy-Check Field) comprises 16 bits and is used to control recognition of transmission errors.
??? ACK Field (s acknowledgment): Field ACK (Acknowledgement Field) contains the acknowledgment signal reception of all blocks-receivers, received a message on the CAN bus without errors.
??? End of Frame (end of frame): Marks the end of a data packet.
??? Intermission (interval): The interval between two data packets. The interval should be at least 3 bits. Thereafter, any control unit can transmit the next data packet.
??? IDLE (idle mode): If none of the control unit does not transmit messages, CAN bus remains in sleep mode until the next data packet transmission.
Priorities
For processing data in real time should be capable of fast transmission.
This involves not only the presence of lines with high physical transmission rate, but also requires the operative to provide access to the shared bus CAN, if multiple control units must simultaneously transmit messages.
With a view to distinguishing transmitted via the data bus CAN messages according to the degree of urgency for the individual posts there are different priorities.
The ignition timing, for example, has the highest priority, values of slip - medium and the outside air temperature - the lowest priority.
Priority with which a message is transmitted via the bus CAN, determined identifier (address) of the corresponding message.
Identifier corresponding to a smaller binary number has a higher priority, and vice versa.
CAN bus protocol is based on two logical states: The bits are either "recessive" (logical "1"), or "dominant" (logical "0"). If a dominant bit transmitted by at least one module, the recessive bits sent by other modules are overwritten.
Example
If multiple control blocks simultaneously starting the transmission of data, the access conflict to a common data bus is enabled through the "bitwise arbitration requests share" by respective identifiers.
When transferring a block identifier field transmitter after each bit checks whether he has more right to transfer, or a different control unit transmits the CAN data bus for communication with a higher priority.
If passed the first block transmitter recessive bit is overwritten by a dominant bit of another block-transmitter, the first transmitter unit loses its right to (arbitration), and becomes a block-receiver.
The first control unit (NI) loses arbitration from 3rd bit.
The third control unit (N III) loses arbitration from the 7th bit.
The second control unit (N II) retains the right of access to the CAN data bus and can transmit its message.
Other control units try to convey their messages on the CAN data bus only after it is cleared again. At the same right to again be provided in accordance with the priority message via the data bus CAN.
Recognition errors
Interference can cause errors in data transmission. Such arising during transmission, error should be recognized and addressed.
Protocol CAN bus distinguishes between two levels of recognition errors:
?� Mechanisms at the level of Data Frame (message frame);
?� Mechanisms at the bit level.
The mechanisms at the level of Data Frame
Cyclic-Redundancy-Check
On the basis of the transmitted data bus CAN messages flow transmitter calculates the control bits that are transmitted along with the data packet in the ?�CRC Field?� (checksums). Block receiver recalculates the check bits based on the received data bus CAN messages and compares them with the control bits received along with this message.
Frame Check
This mechanism verifies the structure of the transmission unit (frame), that is cross-checked with the bit field defines a fixed format and length of the frame.
Recognized function Frame Check bugs marked as format errors.
Mechanisms at the bit level
Monitoring of the
Each module monitors the message transmission logic level CAN bus and determines if this difference between the transmitted and the received bit. This ensures reliable detection of emerging global and block-transmitter local bit error.
Bit Stuffing
Each packet of data between the field ?�Start of Frame?� and the end of the field ?�CRC Field?� should be no more than 5 consecutive bits with the same polarity.
After each sequence of five identical bits transmitter unit adds a bit stream of one bit with the opposite polarity.
Blocks-receivers, these bits are removed after receiving messages via the data bus CAN.
Troubleshooting
If a CAN bus module detects an error, it aborts the current process data, sending an error message. The error message consists of 6 dominant bits.
Due to the error message connected to the CAN data bus control units are notified of a local error occurred and accordingly transmitted to ignore this message.
After a short pause, all controllers will again be able to send messages via the data bus CAN, And again, the first message will be sent with the highest priority.
The control unit, whose message on the CAN data bus has caused an error occurred, is also beginning to retransmission of the message (function Automatic Repeat Request).
Types of CAN bus
For different areas of management used different tire CAN. They differ from each other by a data rate.
Transfer rate of the CAN data bus field "engine and chassis?� (CAN-C) is 125 kbit / s, and the data bus CAN ?�Salon?� (CAN-B) due to the smaller number of particularly urgent messages intended for the data rate is only 83 Kbps /from.
Communication between two bus systems via the so-called "gateways", ie control units connected to both data buses.
A fiber optic bus D2B (Digital Daten-Bus) data used for the field "Audio / communication / navigation." Fiber optic cable can transmit a much larger amount of information than a tire with a copper cable.
CAN C - bus' engine and chassis "
The terminal control unit mounted on each side of the so-called data bus terminating resistor with a resistance of 120 ohms that is connected between the two wires of the data bus.
CAN data bus engine compartment is activated only when the ignition.
By via CAN-C control units connected 7.
CAN B - bus "Salon"
Some control units connected to the CAN data bus interior, activated independently of the ignition is switched on (for example: single lock system).
Therefore, CAN data bus interior should be in operational readiness mode, even when the ignition is off, it means that the possibility of the transmission of data packets must be ensured even when the ignition is switched off.
With a view to the maximum possible reduction of current consumption rest, data bus CAN, in the absence of a required data packet enters the passive standby and activated again until the next time it is accessed.
If passive mode, waiting CAN bus interior of a control unit (for example, the control unit of the uniform lock) transmits a message on the CAN bus, it takes only the main system unit (electronic ignition, EZS). EZS unit retains the message in memory and sends the activation signal (Wake-up) to all control units connected to the CAN data bus interior.
When activated, EZS checks for all users of the data bus CAN, and then transmits a saved before the message in memory.
By via CAN-connected in 20 control units. Elements of a data network (CAN)
 | |
CAN B (Salon) | K1 | The front desk and management unit with fuse and relay box (SAM / SRB-V) | K2 | Rear registration unit and control box with fuses and relays (SAM / SRB-H) | K3 | Left seat control unit (SSG) | K4 | The control unit is right seat (SSG) | K5 | The control unit of the left front door (TSG) | K6 | The control unit is the right front door (TSG) | K7 | The control unit rear left door (TSG) | K8 | The control unit, rear right (TSG) | K9 | Roof control unit (DBE) | K10 | The upper control field (OBF) | K11 | Bottom Margin Management (UBF) | K12 | Electronic ignition starter switch (EZS) | K13 | Dashboard (KI) | K14 | System COMMAND / audio 10/30 Audio / Audio 30 APS | K15 | System Parktronic (PTS) | K16 | The unit trailer hitch (AAG) | K17 | Multifunction control unit for special models (MSS) | K18 | Stationary heating | K19 | Heater (KKLA / BKLA - SA) | K20 | Distributor CAN-B RBA Right | K21 | Distributor CAN-B RBA left | K22 | Distributor CAN-B Cockpit | K23 | Air bags with built-in call ARMINCA | CAN C (drive and chassis) | K12 | The ignition switch (EZS) | K13 | Dashboard (KI) | K24 | Electronic transmission control (EGS or KGS) | K25 | The engine control unit (MSG) | K26 | Electronic gear selector unit (EMW) | K27 | Distributor CAN Class-C RBA left | K28 | E protivozanosnaya system (ESP) | Not included in the network of SG | K29 | Automatic adjustment of range of light (ALWR) | K30 | TV-tuner |
Elements connected to the fiber-optic bus D2B
D2B (Audio / Communication / Navigation) | Fiber optic cable | K14 | COMMAND / audio 10/30 Audio / Audio 30 APS | K31 | Telephone system (MINNA, emergency call) | K32 | The device is voice control Linguatronic (SBS) | K33 | The controller is a mobile phone (interface) | K34 | Amplifier | K35 | CD-changer | Not all | A2 | Radio and cassette player | A2 / 6 | CD-changer | A40 / 3 | The display and control unit is functioning COMAND system | A2 / 13 | Amplifier | A35 / 11 | The control unit of the voice control | A59 / 1 | D2B interface for mobile / embedded telephone | A35 | Cell phone transceiver (CTEL) / emergency call system TELE AID | A35 / 8 | The control unit TELE AID | A, B, C | Compounds | M1 | Fiber Optic Cable 1 | M2 | Fibre optic cable 2 | M3 | Fiber-optic cable 3 | M4 | Fiber-optic cable 4 | M5 | Fiber optic cable 5 | M6 | Fiber optic cable 6 | M7 | Fiber Optic Cable 7 | ws | White Paste | rt | Red insert |
|
