Demystifying Spindle Sensors

After I converted the Orac and I was trying to use it with Mach3, I was checking the inverter frequency against the spindle RPM with a tachometer (a piece of reflective was already on the backplate when I got the lathe). Now with LinuxCNC I was just checking against the know VFD frequency and expecting the known RPM. For instance, I only get up to around 1500 RPM hen I max the VFD frequency, which should mean 2000 RPM.

Well, for someone coming from Mach3 your post is.. complex

All I did is follow the example here:
linuxcnc.org/docs/2.4/html/examples_spindle.html
for setting my VFD PWM 0-10V signal and the encoder at the back of the spindle.

I see no counter-mode related settings in that tutorial, so I definitely did not add such a thing. I will look into that.
Same for scaling, I don't even know what that is about at this moment.

One other thing, that panel displays random erroneous RPM even when idle. Like 0-500 random numbers, even if the spindle is still.

Should I take it the software thinks I am doing quadrature while the machine is A Z only?

i'd recon . this is mostly because of not selecting counter mode , so give it a go . and report back
see if you have this for counter-mode
setp encoder.0.counter-mode 1
( or in other words set encoder 0 counter-mode true)

i found setting the true spindle rpm equaling the shown spindle speed , and disregarding what the vfd was telling me , gave better results . and testing this at different speeds across the range .
so i.e s500 did give me 500 rpm both on my tacho and the shown spindle rpm

Tough one, I checked the HAL file and I have both settings there.
50.0000 for encoder scale and mode 1
They were set by the wizard, I remember looking at the file after that and seeing that very floating 50.00000.

I guess it is one of those points were I have to attach my HAL file, huh?
(that particular box is not networked yet)