Cannot make PWM work on MESA 7i96S

Yes, the 7I96S analog output is a potentiometer replacement device
and supplies no power of its own. If the motor drive does not supply a
5V or 10V output for this purpose (most VFD drives do),  you must
supply an external 5 or 10V supply.
 

Just in case this will help another wayward newb, because this thread comes up when one is searching to make heads-or-tails of a PWM power supply with a potentiometer or pot style control, where we want to wire it to the Mesa 7i96s and control spindle speed using LinuxCNC.

In my case, a cheap PSU for a cheap spindle has a three wire connection for the potentiometer. If you put your voltmeter on those, you can determine which one is ground, which one is 10V (approximate) and which one is the variable "signal" output of 0-10V depending on how you crank your potentiometer. Hopefully there is also a on/off switch, which in my case (cheap PSU, remember) was a wire screwed between two terminals marked on/off (so, as shipped, always on).

If this is basically what you are looking at, then read on:

To wire it to the Mesa 7i96s:
For speed control:
On the TB2 Connector grouping, nearest to the "TB2" label, is the grouping of 4 analog outs for spindle control.
Wire the 0V to Terminal 22 = Spindle - , marked on the board as "SPIN-"
Wire the Signal to Terminal 23 = Spindle Out, marked on the board as "SPINOUT"
Wire the 10V to Terminal 24 = Spindle +, marked on the board as "SPIN+"

For on/off control: (which is a good safety feature, I recommend this)
On the TB3 Connector grouping, nearest to the "TB3" label, are outputs. Numbers 4A & 4B, as well as 5A & 5B are means to close a switch. Ie, when you turn these on, they close a switch, if you turn them off, the switch opens. However, you cannot run much current through them.  Yyou can substitute both 4A/4B below for 5A/5B.)
Wire one of the On/Off terminals through a 1/4 amp fuse to Terminal 23 = "OUT5A"
Wire the other one of the On/Off terminals to Terminal 24 = "OUT5B"

Then in the Pncconf wizard: (Note that this was on LinuxCNC ver 2.9.0~pre)
Once you've navigated through the wizard to the Mesa card page, there will be a page that has several tabs for the different connector groupings on the Mesa 7i96s card.

For speed control:
On the TB1/TB2 page, it can be confusing, but the option is the right-hand-lower Num0: change "Unused PWM Gen" to  "Spindle PWM" with the default option of "PWM GEN-P.

For On/Off control:
On the TB3 page, find the right-column "outputs" and change Num 05 from Not Used to Spindle > Spindle Enable.

The actual .hal code that does the linuxcnc signals to Mesa Wiring is:
(Note for newbs, this code is for understanding how what is written in the .hal file affects what is signalling to the hardware: this is not sufficient to get a working setup, because there are software internal connections that have to be made as well. The pncconf wizard is nearly essential for getting it all done.)

For speed control:

On/Off Control:


Hope that helps.