HP 1820-0107 pinout

I have finally found some documentation about HP 1820-0107 custom IC from 1970's. It is (hidden) in the HP 3720A manual.

HP 1820-0107 as documented in HP 3720A manual.

That document describes it as "Interface buffer gate" and draws the chip pinout as follows:
Well, I've never seen a NOT gate with two inputs. I confirm Vin is a Vcc pin (+5V in my case). In a custom chip Vout could make sense, but why so close to the Vin pin?!

In the next page of the same document I see this:

which makes more sense considering two Voltage pins and the NOT symbol. Further down the manual, the schematic diagram the corresponding ICs (A6MC21, 25, 31, 33, 35) are drawn as interface inverting gates:

Alright, now this makes sense! It explains the presence of resistors on data lines in various places of the HP 9403A. But, most important:

1. the 1820-0107 is not a flip-flop of any sort (easier to reverse engineer the circuit)
2. the input pin requires pulling down to ground (which explains why I haven't found yet a common +V line on connectors towards the external world)

 Back to the PCBs now!
 

More work on the HP 9403A

Lately I spent few hours trying to understand the logic behind the HP 9403A device. Trying to reverse engineer its logic without a schematic diagram or documentation on all IC's (I'm missing what a 1820-0107 does) has caused me mixed feelings about its future: I could just keep the Nixies, or continue to look for a way to talk to the device.

I managed to understand how the display board(s) work, at least up to 80%, and to light a different number on a chosen digit. I also reverse engineered the connections from the front panel and found out that buttons have backlights of different colors, which makes it kinda cool.

Looking at the input interface board (see picture below) I confirmed that the HP 9403A is a passive device, like a modern keyboard or display: it is totally controlled remotely (from an unknown and equally undocumented device).

HP 9403A input board.

So, for now the big thing stays in one piece. I am going to trace connections between boards and then to the external world. Then I will try to send control signals.

Time will tell. Oh, by the way, I keep assuming that the device works as designed!

HP 9403A first look inside

So .... time to look inside the HP 9403A device! I didn't do it in Friedrichshafen last June because the screwdriver I had was not the right fit and I was afraid to destroy the head. With the right tool it turned out that screws were secured with a dab of glue, then off they came easily. Once removed the thick aluminum top lid I was enchanted by the shining boards, gold plated ICs, perfectly grouped and routed wires as if it was build yesterday, while the thing is about 45 years old and has certainly passed few owners.

The absence of dust inside is easily explained: there are no holes, so no way out for the air and no way in for contaminants.

There is a lot of room inside, a power supply, an optional PSU, no microprocessor whatsoever at first glance. Board are mounted with servicing in mind, or mostly quick replacing. 90% of integrated circuits are branded HP and carry their specific mysterious 1820-xxxx part number. Unfortunately Nixies are hidden behind a part of the chassis and I cannot see their model. Nevermind. Since everything looks clean I decide to power it up.

As a precaution I screwed back the lid in place and brought the 9403A on the balcony so that exploding stuff would stay inside and any smoke would not pollute home. Well, I was not surprised that nothing bad happened and Nixies lit up:

1-2-3 ... 13 tubes! Neat. They have gas. Further experiments will tell if they have cathode poisoning or broken digits.

Any interaction with the keypad or front panel keys do not lead to any change on the displayed value. Given that there is no intelligent part inside I was expecting it too. Next step is accessing the Nixies to see their model.

Seven clocks and counting

As of December 2018 I have built seven clocks:

1. Binixie with two Nixies B5092
2. Adapter chinese LED clock kit to 4x IV-6
3. HP LED on my bedside
4. Repurposed Olivetti Logos 262PD desk calculator
5. Repurposed dual-needle meter
6. Self-designed 4x ZM1332/NL5870 Nixie
7. Single IN-14 clock with AC timebase

Four of them have gotten a proper case, while two are a repurposed commercial device that already had a box.

I would like to repurpose an old 3.5 digit multimeter: will it be #8?!