Parallel Port Woes

Having transitioned to using a Windows based controller program on my
Engraving Machine and the parallel port instead of a proprietary
driver card, I started to feel the need for more computer
horsepower. I found a deal on a motherboard and chip (900Mhz AMD for
89.00) and added a few dollars for 128M more RAM. After the upgrade
my machine just sit there never moving a step while the program
cranked away. I chased all the wrong things like checking the
cables, the grounds and the +5 power supply for the optoisolators.
How hard could this be?! I finally disconnected it and moved the
power supply with motor drive cards into the lab and applied the
scope. The PC signal looked good enough but when I manually applied
a ground to the step input with the PC cable disconnected I got a
twitch from the motor...ah, a clue. Seems that the new PC's parallel
port was a bit weak and could not sink the 30ma the driver cards
needed. I searched through my goode box and fished out a 74HT07 open
collector hex non-inverting driver chip. I ran the 6 step and
direction lines though that little jewel and PRESTO, nice smooth
moves. I could have used any open collector hex driver but the non-
inverting one kept me from having to redefine all my output
polarities.

Lesson learned: New LSI printer port chips may not have enough pookey
to drive a TTL load or an optoisolator input. If you are having funky
things happening with the parallel port, maybe a buffer chip is in
order!