ard at p850ug1.demon.co.uk
Thu Mar 8 17:09:20 CST 2007
> On 8 Mar 2007 at 0:50, Tony Duell wrote:
> > Well, I've said many times the most important piece of test gear is not a
> > 'scope or multimeter, it's that thing between your ears. Your call :-)
> That reminds me--has anyone thought to clone the HP Logic Dart?
I once saw it listed in a catalague as made by Fluke, but that might well
have been a misprint.
> There have been numerous times when I've thought one would really be
I bought one when it was current, and I love it. For those who've never
used one, it's a handheld instrument whcih combines a continuity/diode
tester, a 3-channel 100MHx logic analuyser and something called
'investigate mode'. The last is the most useful of the lot. You have one
probe and a ground lead. Touch the probe on a point in the circuit, the
instrument displays the average voltage at that point (good for checking
PSUs), the frequency (counting theshold crossings per second, I guess),
for checking the master clock, and has a couple of LEDs that act as a
normal logic probe. Hit a button and it records the logic-level waveform
(high/low/undefined) at that point.
It's that last part that makes it much more useful than a normal logic
probe. For example, I had a 9820 calculator on the bench, this is a
bit-serial design. Running a logic probe along the outputs of the M
(memory address) register, which is a 16 bit shift register made up of 4
off 4 bit chips showed that the top 4 bits were changing, all the rest
were stuck low (it's loaded from the MSB end). OK, obvious, change the
snecond-from-top chip... Alas the fault was still there. The LogicDart
showed me that the timing of the outputs of the top chip was crazy with
pulse widths equal to half the clock frequency. Therefore the
second-from-top chip was working correctly, its input was always low on
at the active clock edge. Changed the top chip. the machine sprang to life.
What I'll do when the LogicDart fails I don't know. There's nothing
available now that comes close to its functionality and convenience.
There never was a sercvice manual available for it, and most of the logic
is inside 2 ASICs anyway (therre's also an 80188 CPU, EPROM, RAM, display
driver and a lot oa anloge stuff round the front end and PSU). Oh well...
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