7 Jan
2006
7 Jan
'06
11:08 a.m.
A freind of mine had a nasty problem with a cheap DMM.
The range switch
wenter intermittant (without him realising it), and it claimed the input
voltage was 0, when in fact it was full mains (he was working on some
mains wiring, and wanted to check it was dead. Yes, I know the _correct_
procedure is to check with it live, check again when you've isolated it,
then check the meter on a seperate live source to guard against this, but
how many people do that?). After being thrown across the room by the
shock, he went out and bought a Fluke. They're cheaper than funerals :-).
He could only afford the cheapest model, which didn't even have current
ranges, but as he said, current is the least-often measured quantity.
Voltage and resistance the ranges I (and most people I know) use all the
time.
I would never trust a single reading from a single instrument to tell me it's
safe to get "hands on" with mains - If you happened to pop the meter fuse
last time you were using it and knocked off before you realized it (I've done
this) it gives you a nice zero reading - I want to at least see it reading ON,
then disappear when I switch it off, and even then I remain cautious.
NEVER trust a zero reading until you have seen a "normal" reading in the
same session.
Always had a healthy respect for mains - even the 120v variety we get
over here - I seem to be somewhat lower "impedance" than most - in my
younger days I measured less then 1k from fingers of one hand to the
fingers of the other hand - guys used to laugh at me in the EE lab testing
leds on a 12v supply - Grab one rail in one hand, led in the other and touch
the free leg to the other rail - I could feel it too! - In retrospect probably not
one of the smartest techniques I ever came up with. Once I passed about
30 I started increasing in resistance though... Still wouldn't want to grab a
handful of mains however...
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.
I've never found this to be a real problem. If you're seriously worried
about the voltages, then use the 'scope. The logic probe is a useful
instrument to look for sillies, like a data pin shorted to ground or
something. It'll also tell you if a memory chip is ever being enabled, if
the clocks are running, and so on
It;'s not common for a chip to fail in a way that it gives illegal output
voltagess, although I have had 74Hxxx parts do this. Most logic analysers have one threshold. Anything over
it is a '1',
anything under it is a '0'. The LpgicDart is good in that it has 0, 1,
illegal states for the inputs, you set 2 thresholds (it has pre-set ones
for TTL, 5V CMOS, 3.3V CMOS, ECL).
One of the main reasons I like the scope - you always get to see what you
are actually measuring. There's been plenty of times I've immediately said
"that doesn't look right" ... when a probe would have simply shown a pulse.
I think it's what you're used to using. I
prefer the analyser, since I
can look at many signals (an entire microcode address bus, for example)
at once. The triggering on aq good analyser is more versatile too. But if
you prefer the 'scope, use it..
This is probably a better argument for either case than any - at least "begin"
with what you are most used to. If I were debugging a microcoded CPU
implementation and the scope failed to reveal obvious problems (missing
selects, bad signals etc.) the LA wouldn't be far down my list of things to
do next.
Absolutely. Read the manual (old HP and Tek came with
very fine manuals
that are well worth reading), play with the instrument. Use it on stuff
where you know what you should be seeing. Misuse it too. I don't mean
applying the mains to the input of the logic analyser. I mean things like
on a DSO, applying a signal of a higher frequency than it claims to
handle. What does it do. If it aliases it down in to the passband,
beware. It'll do this sometime 'for real' and you will get confused (yes,
I have seen a DSO that does this sort of nasty, no I wouldn't want to use
it!).
An a similar theme (and I'm surprised how many "engineers" I have had to
have this discussion with) - It doesn't make a lot of sense to be doing these
kinds of measurements if you don't know what to expect. Know what is
"reasonable" for the measurements you are taking. To tie this in with your
point above - you need to know that the signal you are measuring will be
within the range of your tools before you can trust the readings - and you
should know what to expect to a LOT narrower range then "DC to 200Mhz".
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.parse.com/~ddunfield/museum/index.html