7 Jan
2006
7 Jan
'06
9:34 a.m.
On Friday 06 January 2006 06:13 pm, Dave Dunfield wrote:
I forget where I ran across it on the 'net, but there was a design out there
where the tone would change somewhat for differing low values of resistance,
to the point where you could find that aspect of it real useful. I keep
thinking I'm going to get around to building one of those one of these days..
<...>
Got one here that was back when "Global Specialites" was still in their
earlier incarnation as "Continental Specialties", if I'm remembering right.
And another one I got at Yamaha that also gives an audible indication, which
I thought was pretty nifty when I saw it demonstrated, though I haven't
given it as much use as I thought I would.
<...>
I need to build one of these sometime. In the meantime, jamming a NOP on the
data bus when practical is often useful. I troubleshot a z80-based board
that way by removing one buffer chip and tack-soldering a bit of wire across
the bus pins, and watched the address lines cycle through the address space.
A bit more difficult with some other chips, but...
A Very useful
feature is a continuity buzzer, a beeper that sounds if the
resistance between the probes is less than a certain, fixed, value. But
make sure it repsonds quickly (and doesn't, for example, take the time
for the autoranging system to work and then for the instrument to take a
couple of samples). You will want, quite often, to clip one probe onto,
say, a wire at one end of a cable, and run the other probe down the pins
at the other end. You don't want to have to stop and wait on each pin.
#1 tool, and I agree completely on fluke/quality. Continuity buzzer is
essential, and I find a LOT of the cheaper meters (even the fixed range
ones) take too long detect continuity - basically, they do it by taking a
measurement and only after it has done it's "digital approximation" does
it
figure out the connection is below 'x' ohms. Thats a related problem - I
find the cheaper meters are fixed at 200-500 ohms as being "continuity".
Sometimes you need to know that it's zero (or almost so), and sometimes you
want to know that higher value paths exist - thats why I built a separate
continuity buzzer with very low in-circuit current, and an adjustable
threshold. 2) A logic
probe. HP have made some nice ones over the years (I've seen
them on E-overpay from time to time). Actually, a cheap one (Radio Shack
used to sell them) is all you need for most work. This is very useful for
fioding a signal that's stuck high, or something like that. If you are a
rich enthusiast, consider attempting to find an HP 'Advanced Logic Probe'
aka LogicDart. It's a handheld thing that acts as a digital voltmeter,
frequency meter, logic probe and 3-channel logic analyser. A word of
warning, if you ever use one of these you will be 'hooked'....
Disagree on the cheaper ones - Tried the RS one - it doesn't represent a
valid TTL input (it is supposedly switchable for TTL/CMOS) but don't
believe that you are actually close to representative of a real input. 5) An EPROM
Emulator. I put this under test equipment because it's very
useful to replace the ROMs in a system with an emulator containing a
little test program (even something as simple as a jump to itself), and
see what happens. I built my own, they are not complicated.
This one is pretty high on my list, perhaps #3 or 4 when fixing a computer.
Once you get the CPU and ROM to operate, you can start doing diagnostics
from "inside" with an eprom emulator - often in conjunction with the other
tools listed above (use the CPU to generate a repeating access and then
follow it through etc.) Also agree with the bench power-supply you posted in
another message.
I built one with a nice variable section, as well as fixed 5V and +/-12V
outputs. Panel volt/amp meters are essential.
Another tool I use a lot is a little analog amp meter - the old fashioned
"wedge" kind with two terminals on top - very handy from time to time.
I have one that I'm waiting to put into a box, 15-0-15A.
I also have a little "clock generator" I put
together which has DIP
switches to set the frequency and generates square waves - occationally it
has been very useful to clock a circult at a certain rate - often to slow
down "fast" stuff so I can see what it's doing.
What sort of rates do you find most useful? Sounds to me like this would be
easy enough to build.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
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Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin