Mercedes C-Class

W203 since 2000 of release

Repair and operation of the car

Digital tire of data of CAN

Data exchange on CAN tire


B — the Sensor 1 CAN — the Tire of data M — the Executive I-III elements (servomechanisms)	N — Control units (controllers) the I-V

Main data

On the car several network tires of data exchange of CAN (Controller Area Network) between blocks (modules) of management of various systems and controllers of actuation mechanisms of the car are used.

Separate control units are united with each other in the general network and can exchange data.

The tire is bidirectional, i.e. any device connected to it can accept and transfer messages.

The signal from a sensitive element (sensor) comes to the next control unit which processes it and transfers to the tire of data of CAN.

Any control unit connected to the tire of data of CAN can read out this signal, calculate value of the managing director of influence on its basis and operate an executive servomechanism.

Advantages

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 1mbit/with 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 is transferred).

The two-wire scheme of transfer is used for two reasons: for identification of mistakes and as reliability basis.

If the peak of tension arises only on one wire (for example, owing to problems about EMS (electromagnetic compatibility)), then blocks receivers can identify it as a mistake and to ignore this peak of tension.

If there is a short circuit or break of one of two wires of the tire of data of CAN, then thanks to the integrated hardware-software system of reliability there will be a switching in an operating mode according to the single-wire scheme. The damaged transferring line will not 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.

It means that its permanent address (identifier) marking contents of this message is appropriated to each message on the tire of data of CAN (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 data of CAN makes 8 bytes.

The block receiver processes only those messages (packages of data) which are kept in its list of the data of CAN of messages (control of the acceptability) accepted on the tire.

Packages of data can be transferred only if the tire of data of CAN is free (i.e. if after the last package of data the interval of 3 bits followed, and any of control units does not begin to transfer the 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 on the tire of data of CAN with 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 of CAN again i