Can bus
What is the Can-Bus?
Can-Bus is a serial communication protocol developed by Bosch for the exchange of information between electronic control units of the car. Can
means Controller Area Network (Control Area Network) and bus, in computing, is understood as an element that allows to transport a large amount of information.
This system allows you to share a lot of information between control units credited to the system, causing a significant reduction in both the number of sensors used and the number of wires from the electrical installation.
This significantly increases the functions found in automotive systems where the Can-Bus used without increasing costs, and that these functions may be distributed among the units of control.
What are the main features of the CAN protocol?
• The information flowing between the control units via two cables (bus) are packets of 0 and 1 (bit) with a limited length and with a defined structure of fields up the message.
• One of these fields acting on the data type identifier that is transported, unity of command and passes it to transmit it priority over others. The message is not directed to any particular control unit, each recognized by the identifier if the message is interested or not.
• All control units can be disseminators and recipients, and the amount paid to the same system can be variable (within limits).
• If the situation calls for a unity of command may request certain information to another using one of the fields of the message (remote frame or RDR).
· Any controller introduces a message on the bus on the condition that is free, if someone tries the same time the conflict is resolved by the priority of the message indicated by the same identifier.
• The system is equipped with a number of mechanisms to ensure that the message is transmitted and good reception. When a message has an error is canceled and returned to transmit properly, just as a control unit to warn the other problems with the message itself, if the situation is irreversible, said control unit out of service but the system still works.
What elements make up the Can-Bus system?
Cables Information travels
two twisted wires that connect all control units that make up the system. This information is transmitted by voltage difference between the two cables, so that a high value of voltage represents a 1 and a low voltage represents a 0. The right combination of ones and zeros make up the message to convey.
In a cable the voltage range between 0V and 2.25V, so cable is called L (Low) and the other, the cable H (High) hit around 2.75V. and 5V. If the line is interrupted or derived H ground, the system will work with Low signal with respect to ground, in the case of an interruption to the line L, the opposite will happen. This allows the system to continue working with one of the wires cut or mass communicated, even with both statements would also be possible to operate, being out of service only when both cables are cut.
is important to note that the twisting between the two lines serves to cancel magnetic fields, so it should not be changed under any circumstances or the passage or the length of these cables. Element
are closing or terminating resistors connected to the ends of the wires H and L. Their values \u200b\u200bare obtained empirically and can adapt the operation of the system to different cable lengths and number of control units paid, since they prevent phenomena reflection that can disrupt the message.
These resistors are housed inside of some of the system control units for reasons of economy and reliability
is the essential driver for communication between the microprocessor control unit and transmitter-receiver. Work conditioning information in and out between the two components.
The controller is located in the control unit, so there are many such units are connected to the system. This item works with very low voltage levels and is what determines the speed of transmission of messages, which will be higher or lower depending on the commitment of the system. Thus, in the Can-Bus line of motor-brake-shift automatic is 500 K baud, and the comfort system of 62.5 K baud. This element is also involved in the necessary synchronization between different units of command for the correct transmission and reception of messages.
Transmitter / Receiver
The transmitter-receiver is the element that has the mission to receive and transmit data in addition to condition and prepare the information so it can be used by drivers. This preparation is to place stress levels as appropriate, amplifying the signal when the information is dumped on the line and reducing it when it is collected and supplied to the controller.
The transmitter-receiver is basically an integrated circuit is located in each of the control units credited to the system, works with intensities close to 0.5 A and in no way involved modifying the content of the message. Functionally located between the wires that form the line Can-Bus and driver.
How does the Can-Bus system?
control units that connect to the Can-Bus system are those that need to share information, whether or not a system. Automotive usually are connected to a line of engine control units, ABS and automatic transmission, and another line (low speed) control units associated with the system of comfort.
Can-Bus system is oriented toward the message and not the beneficiary. The information line is transmitted in the form of structured messages in which a part of it is an identifier that indicates the type of data it contains. All control units receive the message, filter and use only what they need this data. Naturally, all control units credited to the system are able both to enter and pick up line messages. When the bus is free any connected unit may start transmitting a new message.
In the event that one or more units intend to introduce a message at the same time, it will which has a higher priority. This priority is indicated by the identifier.
The data transmission process is developed following a series of several steps: Providing data
: A control unit receives information from its associated sensors (engine RPM, speed, engine temperature, door open, etc.).
The microprocessor passes the information to the controller where it is maintained and prepared for in turn be passed to the transmitter-receiver where it is transformed into electrical signals.
Data Transmission: The driver of the unit transfers the data and its identifier along with the request to start transmission, assuming responsibility for the message is successfully transmitted to all associated control units. To convey the message had to find the bus free, and in a collision with another control unit trying to transmit simultaneously, have a higher priority. From the moment this happens, the other control units become recipients.
reception of the message: When all the control units receive the message, check the ID to determine if the message is to be used by them. Control units that need the data processing of the message, if not need it, the message is ignored.
Can-Bus system has mechanisms to detect errors in the transmission of messages, so that all receivers perform a check of the message by analyzing a part of it, called CRC field. Other control mechanisms are applied in radio units that monitor the level of the bus, the presence of fixed-format fields in the message (check the plot), statistical analysis by the control units of their own errors etc.
These measures make the likelihood of error in transmission and reception of messages are very low, making it an extremely safe system.
The approach of the Can-Bus, as can be seen, greatly reduce the wiring in the car, because if a unit has a control information such as temperature motor, it can be used by other control units without requiring that each receives the information from that sensor.
Another obvious advantage is that functions can be shared among different control units, and increasing the capability of the same does not imply an excessive additional cost.
How is the message?
The message is a sequence of "0" and "1", which as explained in the beginning, are represented by different voltage levels in the Can-Bus cables that are called "bit."
The message has a number of areas of different size (number of bits) that enable to carry out the communication process between the control units according to the protocol defined by Bosch for the Can-Bus, which provided from identifying control unit, such as indicating the beginning and end of message, display data, allow different controls etc.
messages are introduced into the line at a rate ranging between 7 and 20 milliseconds depending on the speed of the area and the control unit that introduces them. Country
beginning of the message: The message begins with a bit dominant, whose falling edge is used by the control unit to synchronize with each other. Country
discretion: The 11 bit of this field is used as an identifier to allow units to recognize the priority of the message control. The lower the value of the identifier the higher the priority, and therefore determines the order in which they will be put messages on the line.
The RTR bit indicates whether the message contains data (RTR = 0) or if it is a remote frame with no data (RTR = 1). A data frame always has a higher priority than a remote frame.
The remote frame is used to request data from other control units or because they need or for a checkup.
control field: This field reports on the characteristics of the data field. The IDE bit indicates when a "0" is a standard frame and when a "1" is an extended frame. The four bits that make up DLC field indicate the number of bytes contained in data field.
The difference between a standard frame and extended frame is that the former has 11 bits and the second 29 bits. Both patterns can coexist eventually, and the reason for their presence is the existence of two versions of CAN.
Data Field: This field displays the message information with the data for the control unit enters the Can-Bus line. May contain between 0 and 8 bytes (0 to 64 bit). Country
insurance (CRC): This field has a length of 16 bits and is used for the detection of errors by the top 15, while the latter is always a recessive bit (1) which defines the CRC field. Confirmation Field
(ACK): The ACK field is composed of two bits are always transmitted as recessive (1). All units receive the same command that changes the first bit CRC field for a dominant ACK (0), so that the control unit recognizes that it is still transmitting at least one control unit has received a message correctly. Otherwise, the control unit interprets the broadcaster has an error message. Country
end of message (EOF): This field indicates the end of the message with a string of 7 recessive bits.
may be that in certain messages are produced long strings of zeros or ones, and this causes a loss of synchronization between control units. The protocol CAN solve this situation by inserting a bit of a different polarity every five bits equal: five "0" is inserted a "1" and vice versa. The control unit uses the message, ruling out a bit after five equal bits. These bits are called bit stuffing.
Example of a real message:
How is it diagnosed the Can-Bus? security systems featuring the Can-Bus allows the probabilities of failure in the communication process are very low, but it remains possible that cables, networking and control units themselves present some dysfunction.
For the analysis of a fault, it must be remembered that a faulty control unit paid to the Can-Bus in no way prevents the system works normally. Obviously it will not be possible to carry out functions involving the use of information provided by the failed drive, but all the others.
For example, if you stay out of the control panel of a door, would not work or lockout could trigger the other doors.
In the event that the fault is present on the bus lines would be electrically operated lock that door, but not the others. Remember that this can only happen if the two cables are cut or shorted to ground.
can also locate faults Can-Bus in referring to the self-diagnostic system of the vehicle, where you can find out from the operating state of the system to the control units associated with it, but necessarily requires the availability of appropriate screening equipment.
Another alternative is to use the software CANAlyzer (Vector Informatik GmbH) to the computer with a connection. This program displays the data traffic in the Can-Bus, indicating the content of the messages and performs the message statistics, performance and failure.
Probably the most appropriate and affordable tool is the digital oscilloscope with two channels, memory and a bandwidth of 20 MHz (FLUKE, ICAM etc.) With which can be displayed perfectly the messages using a time base of 100 microseconds and a base voltage of 5V. In this case, it should be noted that the bit stuff (which is added after five bits equal) should be eliminated.
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