Hi, Tweeter
Your printer port uses 5 volts & 0 volts to send or receive data, If your board inputs are floating at 5 volts, Then by grounding/pulsing them you'll get movement.
That's why Mach3 has Step/Direction "Low Active" settings in Config, Ports & Pins, Motor Outputs, You can select.
The only precaution is that your Printer port should have No More than 5 volts applied to it, Grounding or 5 volts applied to it is Normal.
Hope this Helps, Chip