Parallel Port 3 Axis CNC Driver, Opto-Isolated, Unipolar Steppers

Step 9 Limit Switches

While the driver does not use any external inputs from limit switches, they are easy to add without any more components other than the switch.

Take a Look at This PDF, page 6 shows schematics for both active low and active high method. 

The switch connections can be soldered from the bottom of the PCB to its input pin on the DB-25.