You will have to find where the command signals are sent to the drive. It may be step and direction or it may be "wave drive" which means there is one signal for each coil. If you can get the machine running, you can poke around with a logic probe or multimeter to find the wires. Then its just a matter of connecting those wires you find to a Breakout Board. That breakout board cna provide isolation between the computer and machine and will connect to the computer using a parallel port.
Matt |