21 Jul
2000
21 Jul
'00
11:57 a.m.
You'll get no flame from me, since I bought one (TEK 1240) about ten years
back, and have never regretted it, though I've seldom had to use it.
It's quite straightworward to make your own PC-based logic analyzer. I even
have a 20-year-old PCB for a multiplexer that displays each of eight traces
in sequence, with a fixed voltage offset between them. This was publushed
in "PopTronics" back in the '70's. These are pretty handy, but with
srams
as cheap as they are today, an all-digital version would be quite easy to
build.
What's tricky, however, is knowing when you're done with the trigger
circuit. Triggering is a major issue with logic analyzers, particularly
with wide and not-so deep sample buffers. It may take a week to specify the
trigger spec correctly, yet it takes only a second to use it, followed by a
week of poring over the resulting data. Building all the desired features
into a single circuit is not an easy trick, however.
Back in '80 or so I needed a very wide LA, wider than what I could afford at
the time, so I built one with the 384-bit width that was required. It was
no picnic! Today's ultra-fast CPLD's lend themselves to building the fast
and wide samplers that are such a pain to build with TTL. These can capture
data at 200+ MHz and feed the 8-bit assembled bytes into an sram with little
or no pain.
It might be interesting to revisit such a problem. An old '486 PC dedicated
to that function would almost be worthwhile . . .
Dick
Dick
----- Original Message -----
From: Corda Albert J DLVA <CordaAJ(a)nswc.navy.mil>
To: <classiccmp(a)classiccmp.org>
Sent: Friday, July 21, 2000 12:30 PM
Subject: Other useful test equipment (was: RE: Scope use...)
Another useful piece of equipment is a multi-channel
logic analyzer. Most of these units can read out
pulse-timing directly, and will provide an invaluable
way to display TTL signal patterns and relationships.
Now, before anyone flames me, I will admit that
new versions of these can cost multiple thousands
of dollars, but I've noticed that a significant
number of older units are showing up at hamfests
for << $200. (I ran across a Tek. DAS analyzer at
the Trenton Computerfest for <$25.00)
These older units are becoming useless for
new and/or cutting-edge hardware development,
and are being dumped by a number of hardware
development firms, since most of them can't
sample systems with clocks > ~10-20 Mhz reliably,
but for the kind of antique-system tinkering most
of us do, these can be quite a help! The one thing
you should look out for when buying one is to
make _certain_ it comes with a set of test
pods/cables/probes. A podset can be impossible to
locate after the fact, and may cost you a horrendous
amount of cash if you try to order them from the
original manufacturer.
Also, I suggest that you stick to analyzers made by
larger companies such as HP and Textronix. You'll
have better luck finding docs and getting help
from other hobbyists that way.
-al-
-acorda(a)1bigred.com
-----Original Message-----
From: allisonp(a)world.std.com [mailto:allisonp@world.std.com]
Sent: Friday, July 21, 2000 1:17 PM
To: classiccmp(a)classiccmp.org
Subject: Re: Scope use...
hi,
think of an oscope as a graphic voltmeter that plota against time.
the X axis is volts and Y axis is time.
That is along way from troubleshooting with one. To trouble
shoot with one
you really need to have some idea of waht you can expect to
see vs what
you actually saw. Most prints do not automatically give you that.
While a scope is handy, for fixing machines that were
formerly working a
DMM and logic probe tend to be more useful. Exceptions exist
like setting
hammer flight time on charaband printers or slice levels and
timing for
older core stacks. The latter being adjustments rather than
fixed by rules
of logic.
Allison
On Fri, 21 Jul 2000, Bill Sudbrink wrote:
I was going to post this request sometime, John
Fousts
post has inspired me to do it now. Anyway, I have also
recently acquired an oscilloscope (Tektronix TDS 3012)
and, while I have all the documentation and some notion
of what an oscilloscope does, I really don't know how to
bring it to bare against the several dead/flakey systems
currently in my possession. I hope this message will
start a "how to use an oscilloscope to diagnose vintage
hardware problems" thread.
Thanks,
Bill
(by the way... bills(a)adrenaline.com is now my email address)