On Mon, 13 Sep 1999, Tony wrote:
> I picked up an HP3421 (I think that's the number -- the data
> logger/multimeter thingy) with HPIL and HPIB on it for a couple of pounds
> at a factory sale aa few years back. Nobody else knew what it was. I
> didn't really know what it was, but it said HP, and looked interesting
> :-)... Still not figured out how to make it do something useful, but it
> returns a sane ID when probed on the HPIL so I guess it's doing something.
That was a great find, especially at that price. I have a couple
of these with the optional HPIB interface.
The most useful card for these beasts is the 10 channel
multiplexer/actuator assembly (HP44462A), usually configured with the first
two channels as actuators . The multiplexer works like this:
when the relay on a given channel is closed, it connects its corresponding
LO and HI lines to the LO/HI lines in the main chassis (which, by the way,
are the same ones available in the front panel). Relays (channels)
configured as actuators connect nothing to the instrument's LO and HI lines;
they just close a switch for external use. Warning: because of this
design, it is possible to tie two channels and the main LO/HI
terminals together by closing two relays at once. The software allows
this (it can be useful in some situations) but it is possible to
inadvertently connect two high-current circuits together, possibly creating
a short. It is also important to remember that when a relay closes,
the corresponding measured voltage is available at the front panel terminals
of the instrument. I like to connect my HP3468A there, since the 3421A
has no display.
The 3421 is really simple to use. Since it has no front panel, it is
always in "remote" mode, and it doesn't respond to the hpil commands
"remote", "local", or "local lockout". It has a set of high level
commands and a set of lower level (or "advanced") commands.
The high level commands allow pretty simple operation, and are mnemonic;
here are some to get you started (note: <argument> is mandatory and
[argument] is optional):
DCV [channel list] Take a sequence of voltage measurements on the
optionally specified channels (up to 30)
ACV [channel list]
TWO [channel list] Two-wire ohm measurements
FWo [channel list] Four-wire ohms; needs special connections and two
multiplexer assemblies.
TEM [channel list] Take a temperature measurement ("T" thermocouple
assumed in the corresponding channel). Each
multiplexer assembly has its own cold junction
compensation.
FRQ [channel list] Frequency reading
CLS <channel> Close an actuator relay or multiplexer channel
(protected; first opens any other closed multiplexer
channels)
OPN <channel> Open " " ...
UC <channel> Unconditionally (no protection) close a channel
When the optional channel list is not specified, the commands
DCV,ACV,TEM,TWO and FWO leave the current state of the relays
undisturbed, so readings are taken from the last channel that was
closed. When the channel list is specified, the last channel that
was closed remains closed. At startup, no channels are closed,
so if no channel list is provided, readings are taken from
whatever is connected to the front panel terminals.
The 3421A can operate for about 8 hours on its internal
sealed lead-acid battery, a Panasonic LCR-306P, 6V, 3.2AH . Unfortunately,
these are no longer produced. I put smaller, 2.4AH batteries in mine,
similar to those used in the HP9114A/B drives. These you can get from
Digikey.
One thing _not_ to do with an HP3421: if you remove a
multiplexer assembly and turn the thing on, it will notice
that it is missing, and the next time you plug it in, it
will label the assembly as "uncalibrated", requiring you to complete
the calibration procedure (all calibration is done in software).
I have not had a lot of time to play with my 3421's recently, but
back in January I built a subwoofer with a three-chamber acoustic
labyrinth. I needed an automated way of evaluating the frequency
response of the thing, since tuning the relative volumes of the
chambers is an iterative process. I hooked a WaveteK 111 voltage
controlled function generator, the voltage control provided by
a DAC08 driven by an HP82165 HPIL-GPIO converter, plus a good mike,
an HP3421A, my trusty HP71B and an HP82164 HPIL-RS232 converter.
So, to take a response reading at a given frequency, first the HP71B
would take 10 frequency readings from the 3421A (connected to the
Wavetek 111), average them, and adjust the voltage control using a
Regula-Falsi algorithm (just a couple iterations required since
the Wavetek 111 is pretty linear) until the desired test frequency
was being output by the function generator. Of course, the subwoofer
was fed by an amplifier connected to the funtion generator.
When the right frequency was being output, amplitude readings were
taken from the mike, and after repeating this for several
frequencies, the response data was downloaded via RS232 to my pc
for further analysis in matlab. After quite a few iterations, I
was able to get the response to where I wanted it. Could not have
done it without the automated rig.
Carlos.
Received on Mon Sep 13 1999 - 12:34:48 BST