The CAN transceiver is a CAN bus communication signal transmission channel, transforms the TTL level of single chip microcomputer for CAN bus level, is CAN standards - the most basic physical layer chip ISO11898 series. If the CAN transceiver is abnormal or failure, this node will be affected by the deadly of communication.
In the operation of electric buses, the engineer is the most afraid of the scene in "downs", if not thoroughly CAN node communication, is a good chance that transceiver is damaged, replace it; If fault occurs when the vehicle running, but when the fault node down test, the but again can work normally, this is very headache.
Electric bus and conventional bus is different, it is the use of batteries, capacitors to store energy, and then through the inverter into communication, drive motor drive the vehicle. When coach acceleration or deceleration, inverter produces huge current changes, to form a strong magnetic field interference, through the power cable coupling to the nearby CAN bus, CAN make the CAN transceiver damage rate increased. And in some cases, the CAN transceiver is working in the damaged condition, is what we call "downs" phenomenon. According to the general test standard is hard to find and calibration failure, can't timely troubleshooting node in vehicle maintenance. Recently some electric bus is encountered a case of damage
CANScope eye diagram analysis method
Eye diagram is logic pulse overlap, used to measure the signal quality. Popular point, that is, all the "0" and "1" superposition together, observation, signal distortion degree of a statistical method.
Regulations such as the CAN - bus ISO11898-1 dominant logic of differential input voltage is greater than 0.9 V. As shown, if you want to make a CAN bus communication, normal eye diagram grey area the level of the minimum not less than 0.9 V. Figure used in CANScope hardware the eyes of the eye diagram function to measure high of 1.75 V, is to meet the requirements of communication.
eAN anser with normal waveform eye diagram
For electric passenger cars on normal doing eye CAN node figure, there is no obvious distortion.
Visible to fault doing eye diagram, CAN node fault node though the CAN communication, but the waveform distortion has already happened. More than dominant threshold has amounted to 4.3 us the width of 0.9 V, it indicates that the fault node at this time the dominant level (logic 0) the width of 4.3 us, and standard deviation 0.3 4 us have us.
We can see that in the case of temperature change, the bit width will increase, resulting in abnormal bus baud rate, all nodes will be its interference, because the program made since the restore function, so the fault node unable to exit the bus, bus has been interference, eventually lead to paralysis of the vehicle.