UPDATE: I discovered that the configuration v0.9 wiki page was outdated, and that a pull-down resistor SHOULD NOT BE USED. It was a vestige of simpler times and the code has since changed to no longer require it. I made the changes to Grbl’s wiki page for configuring v0.9 and removed the offending mention of a pull-down resistor being needed if the probe pin is inverted. This could lead to someone crashing their machine if they try to use an always-on/digitizing touch probe with $6=1… Beware! Here’s the issue I opened on Github explaining the situation with the code: https://github.com/grbl/grbl/issues/1329
I have been successfully probing by bringing the A5 pin to ground (hooking GND up to the cutter and A5 to the material surface), albeit after adding a capacitor to eliminate false positives, for two years now. In order to graduate from probing off of conductive materials only in order to detect the topography of the surface and autolevel my programs I have just fabricated a low-profile touch probe of the always-on variety which breaks a circuit upon contact with the surface being probed (i.e. digitizing probe), but being more of a software guy I’m unsure about this pulldown resistor stuff that the Grbl github page mentions when inverting the probe pin via $6=1 command. Clearly Grbl supports breaking a circuit with A5 as opposed to forming one, but I am simply unsure of how to wire it all up.
To my mind: the A5 pin is already pulled high by an internal resistor. A pulldown resistor on there will have it wherever, and then ontop of that the touch probe will have it almost always grounded completely. When the probe registers contact it breaks the circuit and leaves the parallel pulldown resistor to be pulling the A5 pin down. Is this how it’s supposed to work? Seems kinda funky. Why doesn’t it need a resistor when probing the conventional hobby way, completing a circuit between A5/GND? Shouldn’t that potentially overload it like the Github page says?
EDIT: On Grbl’s Github page it says “NOTE: If you invert your probe pin, you will need an external pull-down resistor wired in to the probe pin to prevent overloading it with current and frying it.” So you can understand my apprehension about just cowboying this and instead seeking out a second opinion online.
EDIT: The probe operates just like all of these, which people have apparently been using with Arduino/Grbl, but nobody discloses the exact wiring or info about the resistor that the Grbl Github page mentions: