• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 32 subscribers
  • Views 660 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN65HVD234: Transceiver output not as expected -- appears like high capacitance on a non-high-capacitance bus

Eric Ye37
Prodigy 30 points
Part Number: SN65HVD234

I am having issues with the CAN transceiver in the title. I have attached a screenshot of the input to the D line and the output of the CAN_L line. We have checked that the chip is soldered the right way and that the EN, Vcc are connected directly to our 3.3V rail and the Rs and ground are connected directly to ground. The output of the chip is connected to a bus with 60 ohms differential impedance and low capacitance (1m cable). We have tried multiple chips.

This image is the input to the D pin of the chip,. This is the output. The rate should be 500kb/s

This is the output at CAN_L with a ~50kb/s signal. Even at these speeds it still exhibits slow transition times.

Thank you.

  • 0
    TI__Guru 62920 points
    Hi Eric,

    Do you have termination resistances present on your CAN bus? Typically CAN buses contain two 120-Ohm resistors, one placed at each end of the bus between the CANH and CANL signals. This helps signal integrity by matching the impedance of the twisted-pair cabling that is typically used (thus preventing end-of-line reflections), but more importantly it is needed in order to pull the CANH and CANL voltages together during the recessive periods. Without this resistance, there is nothing to strongly drive the dominant-to-recessive edge (it is only weakly biased by the CAN transceivers on the bus) and so transition times can become very long.

    Please let me know if this does not resolve the issue you are seeing.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 30 points
    Thank you, it turns out we had a very hard to find break in our trace from the transceiver to the termination resistor so that measuring across our termination resistor, we had 60 ohms but not at the transceiver.