More Bringing up a CPM - test-drb@ccmp.vtda.org

2 Jun 1999

-----Original Message-----
From: Dwight Elvey &lt;elvey(a)hal.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Wednesday, June 02, 1999 4:24 PM
Subject: Re: More Bringing up a CPM

...
 ard(a)p850ug1.demon.co.uk (Tony Duell) wrote:
--snip--
  Then I got a small (small enough to sit on top of
any part of a machine),
 simple (but expensive!) logic analyzer. 
 I still don't like LA's because they can often hide real
signals. You see this nice squared wave that has been sampled
by the LA's input. The real live circuit may see something
else entirely. 

The logic analyzer won't hide significant information about the logic and
timing.  If you sample at twice the frequency of the fastest harmonic you
want to observe, you won't miss a thing.  My ten-year-old TEK1240 only
samples at 100 MHz on 9 of its 72 inputs.  The remainder can sample at
maximally 50 MHz.  It is also capable of catching glitches.  This is not
much by today's standards, but that sample rate will certainly answer
questions about the S-100 bus without fear of ambiguity.

...
   What it
hasn't told you is that one input does nothing,
 and the gate is a simple inverter on the other input.
 Yes, I've seen exactly that fault. 
This is why I use my oscilloscope instead of a logic probe.
I can use more than one channel ( I don't consider single
channel 'scope to be useful for much more than patterns
in Sci-Fi movies ). Two channels is a minimum. While it
is true that non-repetitive patterns are hard to deal with,
in a computer I can often find a way to make the signal of
interest repetitive. In the rare case that I can't ( only
twice in 20 years of working with these things ), I rent
a logic analyzer.
IMHO
Dwight