[Techtoolslist] HP 10529A Logic Comparator Question

[Matt Rossiter]

I've been using this device for years.  Today I came across an idea that I 
hadn't thought about.  Normally I set the TTL outputs with the two 
dipswitches, put in a "known to be good" TTL chip and compare with the one 
on the board.  If any of the outputs are bad, the corresponding LED(s) 
will light up.

What about just setting TTL outputs, but not actually putting in a chip to 
compare with?  So, for example, I am looking at a 74LS32 chip on the 
board, the outputs are pins 3, 6, 8, 11.  Without a chip in the Logic 
Comparator, the 3, 6, 8, 11 LED's should all light up.  But today I 
noticed that only 3, 6, and 8 lit up.  I've been suspecting that the 
output on pin 11 was bad, but when comparing with a good 74LS32 it didn't 
light up either (indicating it was good).

So it turns out that the output on pin 11 *was* bad, but the comparator 
didn't catch it with the good chip inserted.  Without a chip inserted, it 
was the only output that *didn't* light up.  Has anyone else tried this 
"reverse" method before?  (and yes, all my LED's on the comparator are 
working fine.  I tested other chips just to make sure).


Matt