PET 4032 keyboard scanning problem / was Re:Anyone have an image
ard at p850ug1.demon.co.uk
Wed Apr 7 14:10:51 CDT 2010
> I had scoped all 4 lines and determined the count sequence from the time
> relationship of the pulses on those lines. While it is possible that on one of
> those 4 lines I was somehow blind to the distinction between a pulse and a
> constant high, it is highly unlikely. In this instance, the unlikely did not occur.
A trick which I am sure you know, but it's worth mentioning in case
others don't is to apply a 'weak' 2.5V source to the scope probe (say
100k to +5V, 100k to ground. Any logic driver will override that and pull
high or low, but it will tell you the difference between high/low/floating.
If you are working on old TTL that often only went up to 2.5V or so on
the otuputs, then pick the resistors to get 1.5V or so on a floating signal
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