Other useful test equipment (was: RE: Scope use...)

From: Richard Erlacher <richard_at_idcomm.com>
Date: Fri Jul 21 18:57:51 2000

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_at_nswc.navy.mil>
To: <classiccmp_at_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_at_1bigred.com
>
>
>
> > -----Original Message-----
> > From: allisonp_at_world.std.com [mailto:allisonp_at_world.std.com]
> > Sent: Friday, July 21, 2000 1:17 PM
> > To: classiccmp_at_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_at_adrenaline.com is now my email address)
> > >
> >
>
>
Received on Fri Jul 21 2000 - 18:57:51 BST