I searched through all the threads I could find....  couldn't see what I'm looking for.
Maybe I'm looking for the wrong answer?

I have a G0704 mill.  It's got a factory installed tach. It uses a 16 slot timing wheel that has an opto that would normally feed the tach.
I stole the signal from that, and fed it to my C3 board. Output of the C3 goes into pin 11 and ground.  So far so good!
I've got mach configured to see that via the Index under ports&pins.

Combo problem, part of which has nothing to do with mach, but I'll tell you anyways because my wife just sighs and rolls her eyes when I tell her. 

I tell mach to run the spindle at 300rpm (M3 S300).  Spins up. Hook my scope to the output of the C3, and I'm getting about 144hz.
And unless I'm doing it wrong...  144hz / 16(slots) * 60 = 540 actual RPM.  Ok, I can adjust my C6 board to fix that.
Big problem is, Mach thinks it's turning about 2900 or something!  The indicated speed flops around a lot.
I *did have* closed loop turned on. Boy did that confuse mach when it couldn't pull the speed down low enough!  I shut it off.

Isn't there a spot I can tell Mach that I have 16 pulses per rev?  I hope I don't have to block off 15 of the holes.

Ideas?

Thanks!

You would be much better just having a single slot as the parallel port may struggle to see the RPM at higher revs.
Regarding the 16 pulses per rev, you could enter a ratio in Spindle pulleys (from Config menu) for that so that Mach divided the index pulses seen by 16 to display the correct speed.
Hood

There we go - 16 holes was just too high a pulse rate for it to keep up with.

Through a complex process of algorithms and trig, detailed machining process's and... 
... oh who are we kidding, I used to black tape and an exacto knife, and blocked off 15 of the holes in the stock timing disc.


Got my timing feedback into Mach. Ran the spindle calibration routine. Turned on closed loop.

It's all happy now!     Thanks