Jonathan,
I am using USBCNC which does have a pwm output as well but at the moment I prefer to have a manual adjustment via a pot. Maybe later on when I get more used to USBCNC I will connect it to the pwm output. Looking at the manual I have 4 or 5 settings to change to get the vfd to work on an external input. I might get a chance to give this a try tomorrow night.

Ian