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SN65HVD231: CAN-H wire latching low

John Ansell
Prodigy 40 points
Part Number: SN65HVD231

Hello,

A customer is having a problem with his CAN-bus installation at two separate sites in Holland. We have many installations working perfectly world-wide but this problem has me stumped.

The CAN bus runs at 50 kbit/s over a maximum distance of 500m. There are approximately 40 nodes distributed in clusters (cabinets) along the bus. No stubs are present. At the sites in question the CAN bus runs correctly for extended periods of time then will lock up. At this point the CAN-H wire is measuring approx 0V, with CAN-L approx 2.5V. If the engineer disconnects the CAN-H from the terminator the CAN-H reverts to 2,5V and the CAN bus continues to operate. 

My understanding of the CAN transceiver is that the CAN-H output driver cannot drive low, nor the CAN-L drive high. The data sheet block diagram implies this is correct, however the schematic of CAN-H and CAN-L outputs show in fact they are push-pull and conceivably the CAN-H could actually drive low in certain (fault) conditions.

Our schematic is fairly straight-forward, the CAN wires have a MOV on them to clamp spikes but nothing else. Terminations are correct on the site; 120R at each end of the bus and nothing at the intermediate nodes. Multimeter readings across the CAN pair confirm this, approx 60R. The RS pin connects to 0V via 47K to slew the edges. 15V and 0V accompany the CAN pair.

I have not yet been to site with a scope (although that's bound to happen soon), currently I'm just trying to gather ideas of what to look for when I go.

To me the problem smacks of a latch-up since reducing the current (by removing the CAN-H from the terminator) clears the fault.

Is there any mechanism whereby the transceiver can latch its CAN-H low? Are there any tricks to identify the offending node, if indeed that's the problem? If there were a volt drop in the 0V line causing common-mode issues, could these cause a latch-up like this?

I would greatly appreciate any thoughts you have. 

Thank you for your help.

Best regards

John Ansell

  • 0
    TI__Guru 62910 points
    Hi John,

    That is really strange - the HVD23x driver design wouldn't permit CANH to be driven low. I've seen overvoltage transients (like ESD) induce latch-up on CAN transceivers, though, and so that could be something that is going on here. (I've never seen it on this particular device and never resulting in CANH being pulled low, but I wouldn't rule it out entirely.)

    You mentioned that the bus was terminated via 60 Ohms, as is typical. Given that, though, the result you described where CANL remains at 2.5 V implies that CANH isn't actually sinking appreciable currents. The 2.5-V recessive level is generally very weakly biased by CAN receivers, and so any pull-down current on CANH should pull CANL down as well due to the coupling through the termination resistance. Is it possible, then, that CANH is simply disconnected somehow?

    In terms of isolating a problem node - rather than disconnecting the terminator, could you sequentially disconnect each node from the terminated bus? It would be good to monitor the CANH voltage at the node and the bus while doing this to see at which point one of them recovers. If a node remains "latched" after being disconnected, you could try pulling up the CANH line (i.e., connect through a series resistance to a positive supply voltage) to see how much current that pin is really sinking. If the voltage still remains low during this test, I'd touch the transceiver to see if it is heating up. If so, it's pretty likely the transceiver is latched up. If the voltage doesn't get pulled down or if it remains low but there is no additional power dissipation/heating in the transceiver chip, then the problem may be elsewhere.

    Max
  • 0 in reply to Max Robertson
    Prodigy 40 points

    Thanks Max, you raise some good points.

    Thanks for confirming that the driver itself cannot drive CAN-H low, it's a good starting point.

    Indeed, I don't understand why CAN-H could be low with CAN-L still at 2.5V. I don't think it's a simple disconnection since the bus springs back into operation again after disconnecting & reconnecting. But there's something going on that defies logic!

    I've been wondering about static discharges. The CAN bus is installed with screened CAT-5 cable but I suspect the screen is not earthed. With the static-induced latch ups you've seen, what is most likely to happen?

    I'll probably have to go to site soon, I'll follow up with a fix (or more problems!) when I know more.

    Thanks for your help.

    Best regards

    John

  • 0 in reply to John Ansell
    TI__Guru 62910 points
    John,

    When I've seen ESD-induced latch-up, the effect on the device was generally an increase in supply current. I don't remember seeing a noticeable effect on CANH/CANL - it seemed like an internal node was affected in a way that increased current without breaking functionality.

    The fact that disconnecting and reconnecting the node resolved the issue makes this feel like it may be cabling-related, but it's hard to know without being there. It would be good to get a sense for whether CANH is measuring 0 V because it is open or because it is being pulled low, though.

    Good luck with the debug, and please let us know what you find out.

    Max
  • 0 in reply to Max Robertson
    TI__Guru 62910 points
    Hi John,

    Just curious - did this issue ever get resolved?

    Max
  • 0 in reply to Max Robertson
    Prodigy 40 points

    Hi Max,

    Not as yet. I'm still waiting for clearance from Management before I can go to the site.

    I'll keep you informed as and when I get there!

    Cheers

    John

  • 0 in reply to John Ansell
    TI__Guru 62910 points
    Hi John,

    Just checking in on this again - did you have a chance to visit the site to debug this issue?

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 40 points

    Hi Max,

    Funny you should ask, I've been meaning to post about it for a couple of days  :)

    We had a message from the site guy who said he'd rewired one of the panels because the CAN voltages were not the same at  'in'  and 'out'.

    These have fixed his issues but I'm none the wiser really. I would say there was a manufacturing fault in the wiring of the panel (how else could the voltages be different), but the site guy doesn't care why, he just wants it to work - fair enough.

    So it's all working now and I guess we'll never know what the problem was...

    Thanks for your help in this, it's been a puzzle from the start.

    Cheers

    John

  • 0 in reply to John Ansell
    TI__Guru 62910 points
    John,

    Thanks for the update! It's too bad we'll never truly know the root cause (although I agree a wiring fault sounds plausible), but at least things are working now. If you encounter any issues in the future please don't hesitate to post again.

    Best regards,
    Max