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!