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ISO1050: Signal disappears when CANH is grounded externally

Henrik Giese
Prodigy 30 points
Part Number: ISO1050

Standard CAN bus setup. 

The majority of our units work correctly, but we've had a few units where the CAN bus signal sort of collapses when one of the outputs is connected to an external ground. Below is the output from one unit with the oscilloscope ground connected to CAN_H:

And here is the same unit with oscilloscope ground connected to CAN_L:

I can not find any fault with the passive components. Can this be caused by the ISO1050?

  • 0
    TI__Expert 6195 points

    Hi Henrick,

    Is your question about the behavior of the ISO1050 when CANH or CANL is connected to GND?

    Here is a scopeshot of the ISO1050 working normally. At the top you can see TX in Yellow. CANH (pink) and CANL (blue) are in the center. At the bottom we have RX (Green). I'm just inputing a 100kHz squre wave signal into TX. 

    If CANL is connected to GND, the ISO1050 should be able to continue to transmit and receive okay. See scopeshot below where a CANL to GND short event happens at -30us. 

    If CANH is connected to GND, you should not see any CAN communication. If CANH is shorted to GND, then there will be no positive differential voltage. The CAN bus requires CANH to be +900mV above CANL for logical zero state (CAN standard). If CANH is at 0V, CANL would need to be close to -1V to create a dominant bit and the driver will not be able to do this. Here is a scopeshot of CANH getting shorted to GND at -30us. 

    This behavior for short from CANH to GND is expected (and should be true of pretty much any CAN transceiver). 

    Does that answer your question? 

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Prodigy 30 points

    Dang, I wrote wrong in the header... It's CAN_L that's the problem when connected to ground.

    And unfortunately the problem is way stranger.

    The unit mounting the CAN circuit is powered from an isolated DC source (so its voltages are free-floating to the grid). When I connect my oscilloscope to the outputs with ground to CAN_L and the probe to CAN_H the signal disappears and I get evenly spaced negative pulses on the output. How can CAN_L even get lower than CAN_H?

    Note: there is no load other than the oscilloscopes on the CAN outputs.

  • 0 in reply to Henrik Giese
    Prodigy 30 points
    Ah, I think we found the cause. It looks like the CAN connector was attached reversed. That probably damaged the circuits.