has anyone interfaced linuxcnc with an abb NextStep?

not sure about your microflex have or not profibus or canbus line ... if yes you can symply connect a profibus line using pyprofibus and run ... if no really precision/velocity/perrformance you need ... and for sure you can use the first close loop in drive (all modern drive It have these close loop inside it electronics) ... so it is an quite good option use parallel port and step/dir command ( if you not would use too much money, is the best solution ... only you make a long latency test on your pc to detect the right base period to use).

For precision I have not suggest mesa card type ... because I'm not read microflex datasheet (your is microflex150? or older one? witch power? etc etc).

regards
giorgio