I just saw this thread and am posting even though it is a late bump.
From my experience with wire edm, I don't believe that it is just an issue of backing up. Just backing up would only require keeping an array of the last motions in memory (or a file) and reading it in reverse.
It is also an issue of sensing the spark and moving away from it, creating a new local tool path. Sometimes the wire needs to back up because of a flushing problem and would short if returning along exactly the same path. Of course then the power also needs to be reduced to prevent overburning. It's sort-of-like adaptive control, but with intelligence based on burning. It also needs to make decisions if it "sees" it has moved more than a minimum distance with the spark reduced, which could mean a sliver has shorted the wire and it hasn't cut for a while.
Other times material can spring or a slug can twist at the end of a cut and the wire needs to back up. But if it repeats the same path, again it can ruin the part.
I'll look around here a little more on the subject, but meanwhile it seems to me that you'll definitely need a PLC to handle the spark control with IO lines between the PLC(PMC) and CNC portions. Some PLC memory would also needed to hold the local tool path in order for the adaptive path to be acted upon and sent back to the CNC.
4 axis wire would not be so difficult, but to implement 2 axis with angle and wire offset simultaneously would also need PLC intervention, I think.
If I were to start the project, I would see if I could pick up an old wire edm where the iron and wire adaptive feedback electronics wire still good. Then I would hook up the axis motors, get the wire mechanism controllable, and then work on interfacing linuxcnc and the feedback box. This assumes that the feedback box is separate or can be addressed without affecting any other electronics in the unit.
