I have it working with G91. Unfortunately it's all coming back to me now. There is a reason I did it using jog buttons. The distances the motor has to turn change depending on if it's reversing direction or not. The load isn't equal either, and sometimes it loses steps. To make it repeatable i have to slam it into the index pin. Which isn't kind to things and doesn't sound very nice while the stepper is stalled. The only alternative is to go REALLY slow which makes tool changes annoying.

Is there a way to interrupt a G code command? If I could do that, I could really speed things up and avoid all the banging around. Maybe have an M00 in there after the conditions are met

M00 was the ticket. Everything is event driven now, with large enough margins on the incremental moves that it works repeatably. The good news is I was able to increase the speed significantly. The limiting factor is how fast the parallel port can read the strobe pulses and activate the lock solenoid. If I go too fast it misses the strobe....still a lot faster than before. I'll post the code tomorrow after I clean up the comments.

Now if the threading will just work!