Hi,
you say its a five axis board. Ten of the available 12 outputs would be required to drive five motors. One output for the relay so that only leaves one output
left over. If you can eliminate all the others your problem is solved.
You use this board to control your machine currently? If so you already know which pins are associated with each axis you have installed.
You could assign each unidentified pin in turn as Step for axis B for instance, I assume axis B is not currently equipped and assigned. Then you can MDI
commands that B axis rotate one way then the other and observe each pin on the board with a multimeter. Each time you identified a pin which changes state
on each axis reversal MDI command you will have identified Machs parallel port pin number for that board pin.
Craig