PET 4032 keyboard scanning problem / was Re:Anyone have an image

[Tony Duell]

> 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 
instead.

-tony