I would like to universal stepper motor driver circuit. Is it possible for one circuit to be able to drive unipolar and bipolar motors with only a firmware change or do they require much different circuits? I know that bipolar motors use 4 wires and unipolar uses 6 so if i use a connector with 6 pins would it be possible?
You can drive a unipolar stepper with a bipolar stepper circuit-- just ignore the center taps. In fact, it should be more efficient to drive a unipolar stepper this way. The only reason (that I know of) to drive a unipolar stepper in the traditional way is to simplify the hardware; if you've already got a pair of H-bridges, you may as well use them.
There are actually three stepper motor topologies. "Bipolar", "Unipolar", and "Universal" (the parlance I have always heard, anyways).
Bipolar motors are 4-leaded, and have two field separate field coils.
Unipolar motors are 5 leaded, and effectively have 4 field coils, with one end of each connected together (The "Common" lead) See Here.
Universal motors have 6 leads, and are effectively a center-tapped bipolar motor.
A 5-leaded unipolar motor cannot be driven with a bipolar stepper motor driver. A bipolar driver can only drive a universal or bipolar motor.