So I’ve taken a multimeter to the machine and I think I understand what’s going one now.
The stock tool-length probe is a Normally Closed probe. When I put a voltmeter across its two pins, I see a 5V reading, which vanishes when I press in the probe (the circuit is flipped open). One of the pins (the one on the right) is GND, the other I assume is the switch.
The probe I intend to use is also a normally closed probe.
On the v2.4e board that I have, the pins for the reserved header appear to be, from top to bottom:
So I think I need to install my new probe on pins 1 and 3, same as the other probe.
The question now is whether the reserved header is wired in series (good, multiple probes can work) or parallel (bad, only one can be connected and useful at a time). I’ll check tomorrow by depressing the tool length probe with the multimeter across the reserved pins.
@DanStory I think you got your terminology mixed up the same way I did. I heard “closed”, which in non-electrical contexts (e.g. a door) means “separating two things”, and thought “closed” meant the circuit wasn’t connected. In electrical contexts, “closed” means that the circuit forms an unbroken (“closed”) loop, i.e. works and electricity flows.
If you hooked up a bunch of Normally Open sensors in series, as long as one of them is open, no electricity would flow through the circuit, so it’d only function if all the probes were activated at once.
You want series for Normally Closed probes, so that if any one probe activates, the circuit breaks and the switch is “activated”. You want parallel for Normally Open probes, so that if any switch is closed, energy flows and the switch is “activated”.