Hi,
couple of things you can try...
If you are using a 540 try unplugging the x axis motor and plugging in the y axis motor instead- should prove either the motor
or the port is faulty.
Next idea requires a multimeter. Identify which pin in your printer port cable is the x axis direction pin. Test the pin, it should be logic
high in one direction and logic low in the the direction. The direction pins are persistent so they will remain the same until you change it.
To set the pin; from the MDI line;
G1 X5 F0.001 <enter>, the X dro should very very slowly start increasing allowing enuf time for you to probe the pin.
To reverse direction; G1 X0 F0.001, the DRO should now decrease and allow you to probe the pin.
Let us know how you get on.
Craig
You should double check that the pin you assigned for X DIR in Mach3 is in fact the same as the G540 expects the signal to be on.