As a matter of course, it is no good trying to test your LPT1 socket using the motor driver outputs. Since the outputs are pulsed, depending on speed, the voltage you see on the output pins will vary.
It is better to configure Mach 3 to put out M3, M4, M7 or M8 signals to pins. These are constant signals, either on or off, and are much easier and more reliable to test.