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.

have you got a capacitor across the power to the chip? 100nF or so.. you may
be getting "parasitic oscillations" induced onto your circuit, CMOS is very
high impedance (read low current) and so is particularly susceptable to
electrical noise.



"John Smith" wrote in message
news:UL1ac.128147$Cb.1459710@attbi_s51...
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.