I have interfaced the parallel port of my pc to a high speed cmos logic chip
74hc164. It seems to work okay when about 2 or 3 volts are powering the
device but doesn't supply enough on the outputs to drive another device.
when I use a 5 volt supply it is very unstable (bits shift two extra places
etc). The parallel port outputs to +4 volts which seems to be within the
cmos threshold. I have even tried several pull-up resistor configurations.
This is very perplexing and outside of any of my experience. I am sure
there are some wise users on this group that would be kind enough to point
out anything I may be missing.