BLTouch wiring on Duet3 power down SBC

  • Hi All,

    I can remember that I've spent a lot of time for wiring up the BLTouch to the Duet2 due to the different cable color nightmare. Now it's time for a Duet3 and unfortunately I'm running into problems again.

    BLTouch v3.1 (cable colors from left to right: white, black, yellow, blue, red).
    Duet3 wit latest firmware and SBC (RPI4) powered via the Duet3.

    Cabling was done as shown here

    White -> IO7_IN
    Black -> IO7_GND
    Yellow -> IO7_OUT
    Blue -> IO7_GND
    Red -> IO7_5V

    Now the interesting part 😉 As soon as I connect one of the BLTouch pins (Black or Blue or both) to IO7_GND the SBC is no longer working. It looks like it has power (the red LED is on) but it does not boot up and after some seconds it seems to power down.

    Don't want to break anything, so any good ideas are very appreciated.

    Thanks, Martin


  • thats odd; the problem is very unlikely to be the black or the blue wire - if you disconnect the others I'm pretty sure you'll be fine (but obviously it won't work).

    I'd start with blue+black and add red, yellow and white one by one in that order to see which one causes the issue, but admittedly its correctly wired according to the docs (preferably not making any connections live)

    bltouch looks genuine, but feel free to confirm

  • and maybe some more pictures of the wiring, i suspect there is at least one extension wire along there somewhere?

  • Just verified the cables from end to end with a multimeter and they are fine. Connected Black and Blue to IO7_GND and verified that the SBC is working when connecting the other single cables. It turned out that the "problem" is the Red VCC cable. As soon as this one is connected the SBC is no longer booting.

    Oh man....and yes, it's a genuine BLTouch. For pictures of the wiring it's too late. All cables are already nicely packaged 😉 Maybe that was not so wise but I'm sure that the cables are fine from end to end.

  • Hm, what sort of resistance do you get if you measure from the black/blue pin and to the red pin of the bltouch (when not connected to the Duet)?
    (mine has about 130k from red to both black/brown)

    unfortunately it seems like the sensor has an internal short in its supply

  • Red to Black

    Red to Blue

    I guess that does not look so good, right? The BLTouch worked fine a few days ago. I just moved it from an old to the new printer.

  • Hm, mega ohms shouldn't be a problem, but it is odd that its vastly different from mine but I suppose they could be different models. Mines been sat on a shelf for a bit, new in its box until today 😇 It says "Smart v3.0.

    Anyways, whats the value between the black and blue wire? Seems odd they should be so different unless there is a non 0 value there.

  • It's about 0.8 mega ohms.

  • That doesn't sound right, but can't say I'm too familiar with the internals and potential variation between versions.

    At the end of the day two options, its damaged or wired wrong, as you seem confident the wirng is ok, I'd maybe try it back on the old printer to verify it still works (or confirm its been damaged somehow)

  • I have one on my workhorse which is definitely working (at least 1 hour ago 😉 ). Will try this one.

  • In am not knowledgable on Duet3, but could this have anything to do with 5v / 3v logic settings? (M280 Px 140/150)

  • I would think they should still have a common ground between the two ground pins on the bltouch.

  • My other BLTouch is working os it seems to be broken. Better buying a new one than investigating again for days 😉

    Thanks for your help!

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