Problem with Lindsay/Duncan DRO 5.1 program and IRQ interface

Forewarning: I'm not an electrical engineer (I'm a chem eng), so I might get
some of the terms wrong, please bear with me on this.



I'm using the DRO program and interface designed by Steve Lindsay and
updated by Robert Duncan, version 5.1 with the updated integrated circuit
with IRQ schematic (with one change, I used two 7486 chips (quad exclusive
or), in place of the two 4077 chips (quad exclusive nor), as I had them
on-hand, the wiring of the board was corrected to accommodate the change, as
functionally they should be identical in this application but the pinouts
are slightly different, according to my dad, an electrical engineer) and I'm
having a problem with the IRQ portion of the interface. The program is
running on a Pentium 133 laptop with 48 megs of ram using the IRQ mode of
operation, step size is 0.00039xxx"/step on X and Y, and 0.00069xxx"/step on
the Z (Encoder Tech 25cpmm linear strips on X and Y, US Digital 360cpi
linear strip on Z).



When two encoders transition simultaneously (for example, moving both X and
Y at the same time at a rapid pace), eventually the two axis will have a
transition extremely close together and the IRQ does not have time to reset
after the transition from one axis before the transition from the other axis
occurs. Thus, the IRQ 'misses' a transition, while the raw data catches it
and the program counts this as an error (as it saw a data transition, but no
IRQ transition) and loses the 'missed' movement.



This is giving me fits as my Enco Mill/Drill has enough vibration to trigger
transitions in the Z or Y axis (yes, I get lazy and don't always lock the
non-moving axis as I should, as I mostly use it to mill out areas wider than
the diameter of the end mill) when I am milling in the X direction, thus
losing count on one of the axis and screwing the whole thing up.



If I turn off the IRQ mode, I can move all three axis (with my dad lending a
hand to move the 3rd one) as fast as I can spin the handles with no errors,
but I'd prefer the certainty of the IRQ interface rather than relying on raw
computing power to not miss a transition.



Technical diagram and explanation:

If I'm a little unclear, download this interface schematic:
http://ns1.dicomm.net/~axtein/dro/irq.zip and examine the file irq.jpg
contained within: if pin 10 of U4 and pin 10 of U3 transition simultaneously
(as would happen if moving the X or Y axis while also moving the Z or A
axis)(not exactly simultaneously, just very, very close together with
regards to time), pin 11 of U3 will not transition as required and the
system will have an error. This also happens if pin 3 of U3 and pin 4 of U3
transition simultaneously, pin 10 of U3 will miss, or if pin 3 of U4 and pin
4 of U4 transition simultaneously pin 10 of U4 will miss, so all three gates
would need to be replaced.



So, in conclusion, I'll get to my questions:

Have other users of the IRQ interface mode of the DRO 5.1 software noticed
errors when two axis transition at the same time? Or is this a problem of
my own doing, due to the substituion of the 7486 chips for the 4077 chips?



Are there any logic chips (or other solutions) that can replace the
4077/7486 chips (rewiring the board would be acceptable, this is assuming
that the 4077 chips would have the same problem as my 7486 chips) that would
fix the problem of the missed transitions?



Thanks,

John