Hi,
you can't drive a unipolar motor with a bipolar driver. its to do with the fact that current flows in both directions in the bipolar motors and only one direction in the unipolar. hence the names.
quote
"Unipolar steppers are easier to drive because the center tap supplies the +V, and the individual coils are energized by grounding one of the coil ends in a sequential fashion (single stepping). This can be done by using four MOSFETs or Darlington transistors.
With a bipolar stepper motor, you need to reverse the direction of current through the coil. Therefore, each coil must be controlled by an H-bridge configuration; you need two complete H-bridges to drive a bipolar stepper motor. "
understand you're not after redesigns, but some info below.
open source unipolar driver.
http://www.piclist.com/techref/io/st...step/index.htm
versatile bipolar microstepping driver : a4983 board from pololu. for $14, unbeatable value cause it does 1/16th microstepping, and just needs usual step, dir, enable, etc..
i'm experimenting with a chip from TI, the DRV8824. this does 1/32 microstepping and is pretty good.
thought it might help.