Thread: Reading 74HC165 through parallel port? (schematic included)
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Bart T.
 
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Default Reading 74HC165 through parallel port? (schematic included)
(David L. Jones) wrote in message . com...

Have you checked the input rise and fall times with a CRO?
74HC chips have a maximum rise/fall time spec, if you exceed it you
can get all sorts of WEIRD problems.
The capacitance of your connecting cable can cause problems like this.
Keep your cable as short as possible and check the lines with a CRO.

I don't have access to an oscope, unfortunately But I did make some
progress today:

I built a voltage divider out of 10K and 2.2K resistors I had handy to
bring the Vcc down to just under 5V. However, as soon as the parallel
port is connected, the voltage drops to 2.3V. I never experienced a
noticable voltage drop when just using the full potential of the
batteries.

But on the plus side: The chip works! I didn't even have to add RC
filters to the clock and parallel load lines, though future designs
will use them.

When I unplugged the batteries completely, the circuit still
functioned! The 2.3V Vcc seems to be coming from the parallel port
though I have no idea how. The outputs of the port are only connected
to the clock line and the data lines and the only connections to the
power rail are through data input lines (because I'm keeping them
high.) It's as if the Vcc is being supplied by 4 of the 165's data
inputs! I know the voltage divider idea isn't really suitable for a
final design but I'm still wondering why it isn't working as it
should. I'll try rebuilding the circuit from scratch tomorrow. I might
just have to buy a 5V battery or use the PC as a power source for my
prototypes.

Thanks for the help so far! I'll keep plugging away at it to see if I
can make it work the way it ought to with external power.
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