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Years ago an acquaintance of mine had a digital voltmeter which plugged into one of the ports on the HP-41CX. He used it with some simple circuitry to monitor and record fermentation temperatures for his home-brewing hobby. And I recall seeing an ad--maybe in the EduCalc catalog--for an IL compatible A/D converter.

Anybody remember the manufacturers of these devices?

Are you truly referring to 3rd party accessories or are you
referring to the
HP3468A/B HP-IL multimeter
HP3421A HP-IL Data Acquisition Unit
which were both made by HP and (I believe) the only two
"Instruments" that were ever advertised in the same brochures as the HP-41's were (but the brochure indicated
that they were NOT sold by regular calculator distributors but instead by Test Equipment dealers).



I found two products in 1988 and 1989 EduCalc catalogs (my oldest ones) and nothing after. One is Interface Instruments ADC71A. The description says "It's a 16 channel input, 8 channel output, 4-1/2 digit, analog-to-digital interface that connects to any HP interface loop." Four ranges: 40, 160, 640 mV, 2.5 V. One optically isolated 8-bit digital output port. Thermocouple reference junction built-in. List $750, price $649.95. The other is made by HP, "Stock #IL-800 [#210 I/O Interface] List $590 ..... $544.95" Features listed are 32 digital output lines, two TTL 8-bit input ports, an 8-bit DIP switch that can be read via HPIL, "Analog input section has 8-bit resolution or 8 channels" (?), 7 timers, 7 counters (all 16 bit), "NOTE: this is an inexpensive, unpackaged PC Board for professional use."

Christoph Klug describes a number of commerical HP41-compatible measuring instruments in his book 'HP41 Input/Output Board'.
Here's a brief list of them : HP3468 Digital Multimeter (HPIL interfaced); HP3421 Data Logger (Also with HPIL Interface); CMT-200 Data logger (Corvalis Micor Technology, this unit has 1 8 bit input port, one 8 bit output port, 2 handshake lines, plugs directly into the HP41);
CMT-300 Digital Multimeter (also Corvalis Micro Technology, AC/DV volts, AC/DC milliamps, Ohms, again plugs directly into the HP41); Interloop I/O board (32 digital output lines, 16 digital input lines, DIP switch, 8* 8bit analogue input channels, counters, HPIL interface); Interloop Stepper Motor Driver (Drives 4 stepper motors, HPIL interface); Roland Walcher Elektronik Mikro-Logger W41 (13 bit ADC, 8 analogue inputs, 12 bit DAC, 8 bit input, 4 bit output + 4 relays. Also takes option cards for special applications, HPIL interface);
Roland Walcher Elektronik Logger W51B (Similar to the W41, from what I can see, HPIL Interface); and a couple of very specialised units that are not going to be of general interest.
My guess is that the unit you are thinking of is the CMT-300.

Thank you all. The CMT-300 sounds like the item.