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The digital CAN data bus

Overview

On modern cars use multiple network data exchange bus CAN (Controller Area Network) between the modules / control units of various systems and controllers, actuators of the vehicle.

The procedure for the exchange of data via the CAN bus

B - Sensor 1 CAN - bus data exchange M - Actuators I - III (servos)	N - Control units / Controllers I - V

The bus is full duplex (or duplex), ie any devices connected to it can simultaneously receive and send messages.

The signal from the sensing element corresponding to the information (sensor) flows in the nearest control unit, which processes it and transmits CAN bus of data exchange.

Each control unit connected to the data bus CAN, can read the signal, calculated on the basis of its impact and the control parameters to control the operation of the corresponding actuator 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. The wires from the sensors extend only to the nearest control unit which converts the measured values in the data packet and transmits the last in the CAN bus;
• Manage actuator can be any control unit, which receives the corresponding CAN bus data packet, and on this basis calculates the control action;
• Improved electromagnetic compatibility;
• Reducing the number of plug-in connections and reducing the number of pins of the control unit;
• Weight loss;
• Reducing the number of sensors, as signals from one sensor (e.g., coolant temperature sensor) can be used by 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 1 Mbit / 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 is transmitted in duplicate through both wires, and the logic levels of the data bus are a mirror image (i.e., when one wire is passed a logic zero (0), then the other wire - logic level one (1), and vice versa).

Two-wire transmission is used for two reasons: to control error and as a basis of reliability.

If the peak voltage occurs only on one wire - for example, due to problems related to the electromagnetic compatibility (EMC) - the blocks-receivers can identify it as an error and ignore the rush.

In the case of a short circuit or an open one of two wires of the bus CAN, thanks to the integrated software and hardware system reliability and to switch to the mode of operation on a single circuit. Damaged transmission line is no longer 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 correlated with a message addressing.

This means that each transmitted message on the bus is assigned its permanent address (ID), marking the contents of the message (eg coolant temperature). Protocol CAN bus allows the transmission of up to 2048 different messages, the addresses 2033 2048 are permanently fixed.

The amount of data in a message on the CAN bus is 8 bytes.

The block receiver processes only those messages that are stored in its own identification list (acceptability of control).

The data packets may be transmitted only if the exchange CAN bus is free (i.e., if the last packet followed after an interval of 3 bits, and none of the control unit begins to transmit the next 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 has 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 the right to arbitration, automatically switches to the reception and continues to try to send your message as soon as the data bus is again free.

In addition to the data packets in the exchange of information request packets are also certain messages via the data bus CAN, - to such a request responds to the control unit, which is able to provide the requested information.

Format data

In normal mode transmission packets used the following configurations:

• Data Frame (message frame) for transmitting data messages to CAN bus (for example, the coolant temperature);
• Remote Frame (request frame) to request messages to the CAN data bus from the other control 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 that differ only in the length of the identifier: Standard and Advanced.

Currently, the systems for the exchange of data management systems vehicles Daimler Chrysler company uses only the standard format.

Aspect Ratio

Each frame is transmitted via the CAN bus message consists of seven consecutive fields:

• Start of Frame (start bit): Marks the beginning of the message and synchronizes all the modules;
• Arbitration Field (arbitration field): This field consists of a 11-bit identifier (address) to and one control bit (Remote Transmission Request-Bit), marking the frame as Data Frame (frame data) or as Remote Frame (remote frame);
• Control Field (control bits): 6-bit control field comprises determining IDE-bit (Identifier Extension Bit), is used to detect the type of format (standard or extended), reserve bits for future extensions and - in the last 4 bits - information the number of bytes of data embedded in a Data Field (see., etc.);
• Data Field (data): data field may contain from 0 to 8 bytes of data - transmission on the CAN bus messages of byte 0 is used to synchronize 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 (acknowledgment): Field ACK (Acknowledgement Field) contains the acknowledgment signal blocks all receivers that have received a message on the CAN bus without errors;
• End of Frame (end of frame): Marks the end of the frame;
• Intermission (interval): Interval between two adjacent data frames. The length of the interval should be at least 3 bits after detecting that any of the control units can begin to transmit the next packet;
• IDLE (idle mode): If none of the control unit does not transmit messages, the CAN bus is in the quiescent (idle) before the start of transmission of the next packet.

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.

The priority with which the message is sent on the bus CAN, determined identifier (address field) 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 is sent by at least one module, the recessive bits sent by other modules, are overwritten.

Example

Example of arbitration

The first control unit (NI) loses arbitration from the 3rd bit The third control unit (N III) loses arbitration from 7th bit The second control unit (N II) retains the right of access to the CAN data bus and can transmit your message

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 it has more transmission right, or the other control unit already transmitting on the bus a message with 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 receiver unit.

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:

• The mechanisms at the level of Data Frame (data 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 the 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 the frame length.

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 global and emerging in the block of local transmitter-bit error.

Bit Stuffing

In each frame 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 the message receiving CAN bus data.

Troubleshooting

If any of the 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 an error message, all connected to the CAN data bus control units are notified of a local error occurred and, consequently, transferred 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 a control unit 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 connected between the two wires of the data bus.

CAN data bus engine compartment is activated only when the ignition is switched on.

K CAN-bus C can be connected to a control unit 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 the necessary data for transmission, switches to a passive standby and activated again until the next invocation.

If passive mode, waiting CAN bus interior of a control unit (for example, the control of the uniform lock) transmits a message on it, it takes only the main system module (electronic ignition, EZS / EIS). EZS module stores this message in memory and sends an activation signal (Wake-up) at all control units connected to the CAN-Bus In.

When activated, EZS checks for all users of the data bus CAN, and then transmits previously stored in the memory of a message.

K CAN-bus B may be connected to more than 20 control blocks.