Looking at conversion of HP 2622D (250 system console) to 2621A

HP 1000 SYSTEM RESTORATION

Converting HP 2622D terminal to 2621A for use as third terminal

I bought a terminal on eBay that is a console terminal for later HP minicomputers, but is very close to the 2621A terminal I already own. It is the same ET shape (named for its resemblance to the hero of the Spielberg film) but appears to use a different character encoding.

I hoped that I could convert my 2622D to a 2621A if I could copy the ROMs from my working 2621A and place them in the other unit. I began studying the schematics and manuals that I could locate to see if the two models are close enough that this might work.

Alas, the changes are too extensive. I suspect that if I can find the differences between the keyboards for the 2622A and the special operator console version for the 2622D, I might be able to trick the unit into working sufficiently to use as a normal user terminal.

HP 2262A keyboard (one that I own)

HP 2262D keyboard (one my terminal expects)

The keyboards are not identical and I suspect that the keycaps are wired to respond to different select codes on each. That means if I push the key R on my 2262A keyboard, that is detected by a row and column activation which on the 2262D terminal might be a left arrow, or unassigned character.

In theory, HP has wired certain row/column combinations for keyboard identification - one type of keyboard might have a diode producing a steady 1 bit at a location while the others produce 0 always. A different row/column location might be a steady 1 to signify an alternate keyboard type.

The firmware in the terminal should recognize the keyboard type from these encoded bits and translate properly, but that does NOT happen with my 2262D terminal. Whether I hook up the extended 2262A (and 2261A) keyboard or the regular 2261A/2262A keyboard, it insists on interpreting keystrokes as coming from the 2262D keyboard.

Since I don't have copies of the firmware or a listing, it seems unreasonably hard to reverse engineer the terminal ROM to make it work properly. If I had a schematic of the 2262D terminal showing its keycap to row/column assignments, I could introduce a translator box between the terminal and keyboard. However, I have been unable to find the schematic anywhere.