Configuring Spindle Feedback with Proximity Sensor

rkappius wrote:

Seems to me that somehow the wiring from the scale.0 module doesn't connect to EMC proper. (sorry if this is messy nomenclature, I'm a hal newby.)

I am surprised to see that you are using a scale module, you really need to use an encoder module for G33 and G76.
For a single pulse channel you need to put the encoder in counter mode (ie setp encoder.0.counter-mode 1)
www.linuxcnc.org/docview/html/man/man9/encoder.9.html
You then need to link encoder position (or better, encoder position-interpolated to the motion.spindle-revs pin (this is very critical)
linuxcnc.org/docs/html/examples_spindle.html

Could someone help me get EMC to be aware of the current spindle speed so I can execute G33, G33.1, and G76 commands?

However, you still have one very important part of the puzzle missing. You also need a one pulse per rev index, as that is where EMC2 starts the thread. For this you will need to have another sensor working off a feature on the spindle or a deeper/longer slot on your existing feedback wheel (What are you using at the moment?)
It is probably possible to use a single pulse per rev as both index and counter (though I would not expect that to work especially perfectly) and it should also be possible to synthesise an index pulse from the encoder counts (but I can't see a HAL function to do that in the current version, you would have to write a custom component which checks if encoder counts is an integer multiple of the encoder scale, which is not exactly difficult, but needs some effort).
A real once-per-rev index is better.

And since I'm asking newby questions, can someone tell me what PWM rate and PDM mode means?

PWM generates variable mark-space ratio square waves. The PWM rate is the base frequency of those square waves.
Consider that the base thread on your machine runs at something around 20,000nS (or 20uS in sensible units). Every 20uS the PWM function runs, and decides whether to turn on or off the output. It does this by incrementing a counter, and seeing if the value of the counter is above a threshold.
This means that the resolution of the PWM generator is a function of PWM frequency.
Consider a 50kHz base thread frequency. This means that a 1kHz PWM can count from 0 to 50, switching at 25 for 50% PWM value, 10 for 20% PWM value, etc. This gives you a PWM with 50 possible mark-space ratios.
At 10kHz PWM frequency you only get 5 possible values.
PDM is a different way to achieve much the same effect, but it will run at a fixed frequency, equal to the base thread. Every time the function runs it decides if the time-averaged value of the output is above or below the target, and sets the output high or low accordingly.
en.wikipedia.org/wiki/Pulse-density_modulation

rkappius wrote:

As far as I can tell it just emits a pulse once every rotation.

Sounds very Mach3-ish

EMC2 works best with an index and multiple (hundreds) of pulses per rev, but you should be able to make it work with one PPR.

You need to connect the incoming pulse to both the encoder index and pulse channels (A and Z), connect encoder.0.position-interpolated to motion.spindle-revs and set the encoder.0.scale to 1.

Let us know how that goes.