OK, couple more questions. Can you give us a picture of the output pins on the board so that we can read the labelling - if there is any. Or anything that suggests something like "analogue output" or similar. Just looking for clues here!

Also, in Mach3, one of the drop-down menus - "Plugin Control" - might have an entry which gives a clue about the type of USB board you are using. This is generally where any board-specific parameters are set up, but just knowing what the board is might help find some documentation on it.

Funny that the appropriate wires to the old VFD have disappeared, though.