Originally Posted by
doorknob
What is unreliable about it? Are you sure that replacing the ULN2003 will cure the unreliability?
I have not done such a thing, and I do not know offhand of any off-the-shelf solution, but my guess is that a simple logic solution will not be enough - presumably you will need some kind of "memory element", because I suspect that figuring out the step and direction outputs will require determining the difference between (at least) the prior state of the 4 output pins and the new state of those pins, and then deducing the needed step and direction signals from the change between the patterns. You will also need to "poll" the output lines to determine when they change, or else you will need some way to derive a clock pulse from the changed output pattern.
A microcontroller should be able to do the trick, however. (I am assuming that you do not have access to the Z-80 source code)