Steppers only move one way.

I've been building a mendel for sometime now, finally I scraped together the last bit of money and got all my parts. I've finished pretty much all of the mechanical and electronic construction. I decided to test the axes out and discovered that they will only move forward, when I click the button to move the other direction (both in the reprap software and in repsnapper) they don't move. The lights light up for a split second on the controller but then nothing. I can't figure it out, I've switched the wires around into all possible positions, I think, I've searched the forum and google trying to find someone else who's had this issue. I don't know what the deal is any help would be greatly appreciated.

Thanks

If you are testing it via a firmware and microcontroler for reprap.... understand that without limit switches on the home side, the motors will only turn in the + direction... you can compile the firmware to ignore limit switches.

Al...

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Ok, I was actually wondering if that had something to do with it as I have yet to install the optos.
Thank you so much for confirming that.

For anyone who stumbles on this thread, I had a similar issue where the motors would only move in one direction, but would make weird noises and not turn in the other direction. I am using ReplicatorG and I had incorrectly set the motherboard preferences to the wrong end-stop configuration. It was constantly thinking that it was already at zero.