I just finished getting another function to work I couldn't figure out previously.  I wanted to be able to switch the spindle back & forth between manual (via potentiometer) and Mach speed control and one of the example schematics in the -240G manual showed clearly how to wire it up.  It works!

There is a big difference between the top speed in the 2 modes so I guess it'll require some juggling of the trim pots on the 3 boards to get it right but juggling I can handle....it's the smoke & flashing events I have trouble with.

ps: I got lucky & didn't have to buy a set of chips.