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Switch to disconnect CAN node during disconnect

Jared Becker
Genius 11665 points
Other Parts Discussed in Thread: SN74CBT3306, SN65HVD231, TCAN334

Hello,

Is there a switch like device to disconnect a faulty CAN node from pulling down the entire CAN bus communication? Or is the CAN transceiver tolerant of this?

Could I use a switch like SN74CBT3306, for this purpose across the CANH and CANL lines going to each interface, to control and disconnect that node during fault condition?

Thank you,

Jared

  • 0
    TI__Guru 62920 points
    Hi Jared,

    This is typically already handled by the CAN transceiver an controller. A node that detects it is transmitting errors will go into a passive state to avoid further disrupting communication on the bus. If there is a hardware malfunction that forces the CAN transceiver into a "dominant" state (preventing bus communication by other nodes), then most transceivers will detect it, override it, and switch into a recessive state to free up the bus. This feature is typically referred to as dominant time-out in transceiver datasheets.

    Let me know if you have any other questions or if you had any other fault conditions in mind that I didn't cover.

    Best regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 11665 points

    Hi Max,

    Thank you for the great information. Is that feature reference in the highlighted section below?

    Thank you!

    Jared

  • 0 in reply to Jared Becker
    TI__Guru 62920 points
    Jared,

    No, this is actually one of the transceivers that does not feature a dominant time-out circuit. If you are looking for a 3.3-V-powered transceiver with this feature, you may want to look at our newer TCAN33x series of devices.

    Best regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 11665 points
    Hi Max,

    Thank you for the clarification. If the engineer still really wants to use the SN65HVD231, would it still be acceptable to use the signal switch I referenced above to create the same behavior as the dominant time out feature?

    Thank you,
    Jared
  • 0 in reply to Jared Becker
    TI__Guru 62920 points
    Jared,

    The concern I would have with this particular bus switch is the series resistance introduced, which looks to be as high as 15 Ohms. The load impedance on a CAN bus is typically 60 Ohms between CANH and CANL, so introducing an additional 15 Ohms on each line would cause the output signals to be attenuated by ~33%. There isn't a large amount of amplitude margin inherent in CAN (the minimum driver dominant output differential level is 1.5 V, and the maximum dominant-level threshold at the receiver is 0.9 V), so this much attenuation could have an adverse affect on the signaling. If a lower-resistance option were available, though, I don't see why it would not work.

    Best regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 11665 points

    Hi Max,

    For the TCAN334, if the device is always in normal mode, will with Dominant Timeout Feature always work?

    Also, would this feature work when there is a short circuit on one of the cables, like the short circuit current limiting feature?

    Thank you,

    Jared

  • 0 in reply to Jared Becker
    TI__Guru 62920 points
    Hi Jared,

    I'm not sure I completely understand your question. There are dominant time-outs for both the transmit (TXD-to-CANH/L) and receive (CANH/L-to-RXD) directions that prevent the device outputs from remaining in the dominant state for longer than a certain duration. This feature is always in place in the device. If there were some external fault on the CAN bus that resulted in a stuck dominant condition (like a short on CANH to a supply voltage and short on CANL to ground) then it would prevent the RXD line from remaining low. The bus would still be unusable in this state, though.

    If instead there was some issue (a short, etc.) that pulled the TXD line low, the TX dominant time-out would prevent the node with the fault from keeping the bus continuously dominant. Without this feature, the entire CAN bus would be unusable rather than just the faulty node.

    I hope this makes sense; please let me know if you have other questions.

    Best regards,
    Max