For posterity's sake, I ended up buying an SI-5 isolator board for my KBIC-240ds. I bought two PWM-to-analog converters, and both featured no optical isolation between all grounds. If you want true isolation (which you need for the KBIC-240ds), it seems that you need to buy this separate component or some sort of dedicated device that gives you galvanic isolation. My spindle seems to be working quite well currently.
