[Cruiser]

QUOTE
"I suppose IH supplied the non-isolated board as a cost-cutting measure but I think it was a very poor decision - I don't see how any Mach3 mill could run reliably without one."

Think about it a little differant ! The opto isolator will isolate ! It will not cure the noise or better the stray influence of voltages where they don't belong. The opto is a good thing, but It does not get rid of the potential for causing problems. The best way of dealing with it is to ground out the negitive potential so it can not influence anything. I may not be explaining it correctly, and I'm sure someone with more knowlege will chime in. I didn't realize how significant the grounds were till I started setting them to the chassis. The cost cutting board you mention is just a board used for making of connections to the parrallel ports. It is not any more than what is neaded. The grounds are not any part of the circuitry proper. They are only a way of grounding the stray potential which is percieved and described as noise. It is the true fix !