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:
- the 1820-0107 is not a flip-flop of any sort (easier to reverse engineer the circuit)
- 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)
