I've been through all of the documentation that I can find, and cannot fully sort out the spindle speed issue. 

Right now I'm getting very strange results - sometimes get 16.66 Hz operation and sometimes less.  The highest voltage I've measured on the PWM output from my CNC4PC C11 breakout board is 2.7 volts (which corresponds to the 16.7 Hz). 

I THINK I have the pulley settings correct (Pulley 1 is set up for 420 - 4200 RPM, and I can input "S" codes within that range and not receive an error message.  However the speed only changes at the very low end of that range, and it does so in the opposite direction expected (for instance, speed will be at the minimum of 10 Hz. if I set the spindle speed for 3000 RPM, but will speed up to 16 Hz. if I set it for something like 500 RPM).

Some of the references in the manual are to (I presume) an older version of Mach, and are difficult to fully understand.  Is there a complete description of the setup for the spindle speed control somewhere?  Does anyone recognize these symptoms?

Thx
Scott Kelley

Try to change the ouput high/low in the step pin of the spindle (this can be your problem).
Forget the pulleys and so on for now. Bassically you need to set the speed and direction for the spindle axis, so that at max speed you would get 10VDC.

The best description is the cnc4pc c11 board.

That didn't make any difference.

I would like to narrow this down to either an board issue, or a Mach issue. 

Is there a detailed specification somewhere of the outputs from Mach that will tell me what pin carries the PWM signal, and what the expected charicteristics of that signal are?

I would like to narrow this down to either an board issue, or a Mach issue. 

Is there a detailed specification somewhere of the outputs from Mach that will tell me what pin carries the PWM signal, and what the expected charicteristics of that signal are?

The C11 doesn't use PWM - perhaps that's your problem?  It's step/direction interface.  I had a helluva time getting mine to work (on several C11s), and finally determined, among other things, the F/V chip on the C11 was bad.  I replaced it, which got it working, sorta.  It's still never been terribly accurate or reliable, especially first thing in the morning.  It's about to be replaced with a DigiSpeed....

Regards,
Ray L.