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  1. #1
    Join Date
    Dec 2019

    Question Probe input & ground (??)

    I don't understand how to hook up the probe circuit.
    My understanding is that the power supply neutral (24V0) is supposed to be isolated from machine ground. Is this correct?
    If so, how do I wire up the probe circuit while keeping 24V0 isolated from ground?
    The tool is grounded thru the spindle to the machine frame. So ground definitely needs to be factored in, right?
    I think I understand I need to apply +24VDC to the touch plate thru a resistor (2.4k = 10mA?) to prevent a short circuit when being grounded.
    Then when the touch plate touches the tool the voltage is pulled low. +24V is shorted to ground but thru a resistor so it's ok (?)
    Then the CNC controller monitors voltage of the plate using an isolated digital input. So it's an active-low input and triggers when pulled low.
    But this means I need the negative side of this input to be connected to both ground and 24V0, right?
    Won't this will tie 24V0 directly to ground?
    The schematic I'm working from (AXBB-E wiring example) ignores machine ground.
    The question is so dumb the Balazs CNCDrive support guy apparently won't answer the question.
    Can someone point out a circuit diagram or instructions (that factors in ground) for how to hook this up?
    Sorry I'm confused. I'm obviously not an electronics guy.
    Thanks for any help.

  2. #2
    Join Date
    Aug 2007

    Re: Probe input & ground (??)

    Quote Originally Posted by dwg123 View Post
    I don't understand how to hook up the probe circuit.
    My understanding is that the power supply neutral (24V0) is supposed to be isolated from machine ground. Is this correct?
    The only clarification you need is that the 24v0 does not need to be isolated from machine ground, there isn't a reason you can't tie the 0V side of the control reference voltage to the frame of the machine. you'll need that for the touch plate to work, as you have figured out.

    If you want to be extra sure your machine frame and the 24v0 are not floating at different potentials, touch a volt meter between the frame and the 24v0 terminal. You should read no volts, and then its perfectly safe to hook up a wire to tie the control 0v to the frame.

    The way you wrote up everything is correct in your understanding of how the signal works, you're just over-thinking the 'ground' and 0v reference needing to be separated.


  3. #3
    Join Date
    Dec 2019

    Re: Probe input & ground (??)


    One thing I should've mentioned is that I've had electrical noise issues with this machine, especially on the probe line since it floats. This has been creating false probe touch events in Mach-3. I tried shielding the cable but it had no effect. I'm wondering if I have a weak earth ground. (?) It checks out OK using a line tester but today I called an electrician to come check it out and potentially install a new earth ground rod.

    I checked voltage between 24V0 & earth ground, such as it is. DC voltage fluctuated wildly between 0V-0.5V but using the amperage meter (10A first, then mA meter input) I see 0A flowing between the circuits. So I think I'm good to go to ground 24V0. I was hoping to avoid this to prevent introducing noise one way or the other, but I think you're right it's just unavoidable in this case.

    A few follow-up questions:
    1. Would it be better to tie 24V0 to GND thru a capacitor or resistor or both? Something that allows DC to flow but filters noise. Can you recommend whether to do this and/or what values to use? I want to be careful not to slow down probe detection (slow voltage rise due to an overly large capacitor?) though, I'm hoping to get better z-height calibration from this retrofit than I had before.
    2. The machine also has a separate 5V power supply. Should I tie 5V0 to ground as well?

  4. #4
    Join Date
    Dec 2019

    Re: Probe input & ground (??)

    [Quick update, after some experimentation I've found that a 2k pull-up resistor seems to be a good value to use. It pulls about 5mA open-circuit and about 12mA when the probe touches. This appears to be enough current to reliably trip the CNC controller input without wasting power. Not that a 10-20mA is anything to be concerned about.]

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