With the rapid development of automotive technology, as well as electronic technology and control technology in the car on a large number of applications, the car used more and more electronic control module. From the original few pieces of development to the present dozens of pieces, it is clear that the traditional data transmission has been unable to meet the requirements of data transmission between modules. The new vehicle control system uses a new type of data transmission network, abbreviated as CAN (ControllerAreaNetwork), its purpose is to make the car control system data transmission to achieve high speed, and to simplify the car control system. The new public POLO passenger car that uses this city network control system.
    First, CAN data transmission system composition and working principle
    CAN data transmission system will be the traditional multi-line transmission system into a two-lane (bus) transmission system (shown in Figure 1). Such a car no matter how many control modules, and no matter how much their information capacity, each control module only leads to two lines connected to the two nodes, the two wires called the data bus. Data bus is like an information highway, the information through the expressway on the BUS to pass, so CAN data transmission system, also known as CAN-BUS.
    1.CAN The composition of the data transmission system
    Each control unit in the CAN data transfer system contains a CAN controller and a CAN transceiver. Each control unit is connected via two data buses. Data transmission terminals are installed at both ends of the data bus. It is not difficult to see that CAN data transmission system consists of the following four parts:
    ⑴CAN controller
    The role of the CAN controller is to receive data from the microprocessor in the control unit, process the data and pass it to the CAN transceiver. At the same time, the CAN controller also receives the data received by the CAN transceiver, processes the data and passes it to the microprocessor.
    ⑵CAN transceiver
    The CAN transceiver is a combination of a transmitter and a receiver that converts the data provided by the CAN controller into an electrical signal and sends it over the data bus. It also receives CAN bus data and transfers the data to the CAN controller.
    ⑶ data transmission terminal
    The data transmission terminal is actually a resistor, its role is to protect the data, to avoid the data transmission to the terminal is reflected back to produce reflected waves.
    ⑷CAN data bus
    CAN data bus is the transmission of data bi-directional data lines, divided into high data lines and low data lines. In order to prevent external electromagnetic interference and outward radiation, CAN data bus is usually wrapped together. The potential on these two lines is constant, and if the voltage on one line is 5V, the voltage on the other line is zero.
    2.CAN data transmission system works
    The control unit provides the CAN controller with the data to be sent. This data is made up of binary numbers, ie "0" or "1", "1" indicates that the circuit is on and "0" is off. That is, a single digit can represent two states, two digits can represent four states; three digits can represent eight states, and so on, the largest data is 64, it can represent the amount of information is 2 Of 64 power, equal to 1.8 multiplied by 10 of the 19 th power. Examples of temperature information are shown in Table 1.
    In order to avoid collision of multiple information in the transmission, CAN data bus at the same time only to pass a data. The order of data transfer is determined by the priority of the data, the data with higher priority is sent first, and the priority of the data is represented by the binary 11-bit value. When multiple control units transmit data at the same time, compare the 11-bit numbers representing the priority level from left to right on the data transmission line. If a control unit sends a low potential (indicated by "1") and detects a high potential to be received (indicated by "0"), then the control unit stops sending and becomes the reception state; if a control (Indicated by "0"), while the other control unit sends a low potential (indicated by "1"), the data transmission line will represent a high potential (indicated by "0"). For example, the data to be transmitted by the engine control unit is "00101000000"; and the data transmitted by the automatic transmission control unit is "01000100000"; the data to be transmitted by the ABS control unit is "00011010000".
    So how will the data transmission line deliver this data? First, the first bit is "0", the data transmission line is also reflected as "0"; the third digit of the three data, the automatic transmission control unit is ready to send out "1", and the engine control unit and ABS control unit ("1"), and receive a high potential (indicated by "0"), then the automatic transmission control unit will be lost, and the automatic transmission control unit will send a low level (indicated by "1"), and the automatic transmission control unit will lose The engine control unit is ready to send "1" outwards, and the ABS control unit is ready to send "0" outwards, and the engine, the engine control unit is ready to send out "1", and the ABS control unit is ready to send "0" The control unit will lose priority and go to the receive state, the data transmission line transmission "0".
    By comparing the state fields of the three data, it is possible to determine that the ABS control unit has the highest priority so that it can take over the control of the data bus, which ensures that it continuously sends data until the end of the transmission. When the ABS control unit finishes transmitting the data, since the priority of the engine control unit is higher than that of the automatic transmission control unit, the data bus transmission order is that the ABS control unit data is transmitted first, and then the engine control unit data is transmitted, and finally, the automatic transmission control unit data.
    Second, the public POLO passenger car CAN data transmission system maintenance
    1. The composition of the public POLO passenger car CAN data transmission system
    Volkswagen POLO passenger car CAN data transmission system by the comfort control CAN bus, power control CAN bus, vehicle network control and gateway and other components.
    ⑴ comfort control CAN bus
    Comfort control CAN bus system by the vehicle network control unit J559, data bus diagnostic interface J553, electronic control system control unit J255, air conditioning control unit J301, comfort system central control unit J393, driving side door control unit J386, former passenger Side door control unit J387, left rear right rear door control unit J388, J389, radio and navigation control unit. Its data transfer rate is lower than the power control system, to 100kB / s rate of data transmission.
    ⑵ power CAN data bus
    CAN data transmission system consists of vehicle network control unit J559, data bus diagnostic interface J533, dashboard control unit J285 and engine control unit, automatic transmission control unit, ABS control unit, airbag control unit, steering auxiliary control unit and so on.
    ⑶ vehicle network control system
    The vehicle network system monitors the various signals of the vehicle through the control unit, and controls the execution elements such as the fuel pump relay and the interior lighting according to the set procedure.
    ⑷ gateway
    Gateway is the same time to connect a variety of different data transfer rate of CAN data bus computer, in the transfer of data from the role of translation. In other words, the gateway will be different rates of data format conversion, to meet the CAN network requirements of the data, on the CAN network. Volkswagen POLO passenger car gateway computer and instrumentation computer installed together.
    2. Volkswagen POLO passenger car CAN bus system maintenance
    Volkswagen POLO passenger car CAN system failure can use the latest version of VAG1551, VAG1552 detector for detection and diagnosis, is to power CAN data bus system as an example to illustrate.
    Volkswagen POLO Passenger Car Power CAN Data Bus System Connects each control unit one by one. There are 10 groups of data to be transmitted by the bus system, including data group 5 of the engine control unit, 2 sets of data of the automatic transmission control unit and 3 groups of data of the ABS control unit.
    Fault check: use VAG1551, VAG1552 type detector, respectively, enter the 01,02,03 address of the engine, ABS and automatic transmission control unit for self-diagnosis, and then enter the function code 02 query three electronic control unit whether to store CAN data transmission system Fault code, power data bus fault code see Table 2.
    Data bus system failures are mostly due to short circuit, open circuit or CAN high data lines and CAN low data line caused by mixing, you can use the oscilloscope to check the exclusion. Check the data transmission terminal resistance, you should turn off the ignition switch, unplug the engine control unit plug, use a multimeter to measure the resistance between 58 and 60 pins, which is the resistance value of the data transmission terminal, the value of 60 ~ 72Ω, if not The engine control unit should be replaced.