The following is a post on the Mach group. Some tests as indicated may show your problem.
***************************** An LED array is handy to have, one sits on my desk since 2001 , and I use it ALLOT! , BUT you wont find it usefull in your case. This is because the pulse width is only 3us. The LED will show nothing unless your running at high speed, then it will glow at a % of average voltage. Single pulses cannot be seen, just a display of a varying brightness showing output speed. ( Still quite usefull in diagnostics.) Steve Stallings sells an excellent stepper test board for such things , it even has a small stepper on it to play with in motion, probably a little expensive for most for testing but invaluable for people who diagnose various computers as to driver function.
There is an easier way to know though, the machine coordinate DRO. Purposely, and mentioned many times in threads, that DRO was designed not to reflect where mach3 thinks it is, there are many variables internally for that, the DRO's programming is very simple, it counts output pulses. The code that actually changes a step pulse increments or decrements that counter. The DRO display is ( #ofsteps / steps/unit ).
So you can just reset the motor tuning to tell it that 1 step is one inch. Retune the speeds so it runs and watch the Machine coordinate DRO. It will advance 1 for each step taken. and decrement for each step in the opposite direction. If it counts 1, then you can be sure a step pulse was put out, no question about it. if it decrements 1, then a step was put out with a reversed DIR line, no question about it. Thinking about it, the DIR prechange time sounds like your trouble, picture what happens, the system tries to reverse, it changes the DIR line, waits 3us , then sets the step line, the driver hasnt yet seen the DIR line change so it outputs a reverse step. Why only in one direction? It may be case of the opto on the DIR line taking awhile ( longer than 3us) to turn on, but having a much faster turn-off time. Kinda like those annoying flourescent bulbs replacing incandecent ones, they take awhiel to get glowing very brightly, but when you flick them off, their instantly off, somewhet the reverse of the incandecent which is fast to glow, but fades out when switched off. Be pretty rare, but if your runnign on the edge of that opto's timing, it could in theory happen. Mach3 holds the DIR line on or off permanently until direction changes, it isnt a pulsed output, so this phenomenon can only happen at time of reversal, and is the only way a slow opto on the DIR input of the driver will be seen.
********************************** Richard