As far as we could tell it looks like someone put together an initial minimum design for using the Allegro 3967, and all subsequent versions have been small revisions of that design. The problem is that looking at the data sheet for the A3967 and the schematic for the EasyDriver there were a lot of places I could not figure out what the original engineer was thinking. We looked at making our own board around the A3967 – but instead chose to directly drive the stepper motors with darlington transistors and control the phase timing from a microcontroller.
Cassius V1.0 PCB
We spent about a month having problems with Sparkfun’s version of the Easy Driver boards before deciding to make our own driver board. When they worked – which was most of the time – they were awesome. We started off concerned, becuase disconnecting a motor while the board is powered can fry the chip responsible for microstepping the attached motor, the Allegro A3967. Now this is a problem because it means if a motor fails it also is likely to take the electronics with it. We still decided to try the Easy Driver bord out because we had heard good things about it.
Once we started hammering on them we found it was also possible to fry boards so they would only drive the motors in one direction. The plans are open so we could look over the datasheets and review the design. That made us go from concerned about using the Easy Driver to flat out running in the opposite direction.
[Update: So after talking to Brian Schmalz, the developer of the EasyDrivers he was unable to reproduce our error after hammering on a number of boards and motors. It looks like we were either having a problem with our stepper motors or - what I think to be the more likely case at this point - I just made a mistake in how I wired up the motors. My best guess is the trick I used to wire up a unipolar stepper motor so it could be driven by a bipolar stepper motor driver was at fault. Only problem is I cant figure out why it worked at all if that was the problem. The important update though is that you should be fine using the EasyDrivers with bipolar stepper motors. ]