strange HP-97A

[BobVA]

It appears to be mostly tapped into connector XA2P1, in particular:

Code:


1  KBA
2  KBC
3  KBD
4  KBE
5  STR
6  RCD (update - courtesty of Katie's post noted below)  connected to the via above pin 4
9  SG
10 SH
13 SF
14 SE
15 SA

The KB lines are the four that signal which of the four keyboard "rows" have been pressed.

The SA-SF lines are a subset of the LED segment lines. The ones present are the decimal point and the segments that would form an "F" if they were all illuminated. (SD, SB, SC are missing).

The red and black wires are probably power, and I'm not sure of the others.

The STR line advances the digit selection on the display and, I think, also serves as the column strobe for the keyboard, so by watching it and the KB lines I think you can tell what key has been pressed, but I'm not sure.

So, I think that connector could read the keyboard. (I don't know if those connections could are sufficient to simulate a key press.) As to why it would want to sample a subset of the display segments, I don't know. Perhaps whatever the designer wanted go know could be inferred from just those lines?

Interesting, though! Thanks for posting this!

Bob

Update - from an excellent article by Katie located here (that I should have googled first! :-) )

Quote: Inspection of the Keyboard PCA Schematic Diagram in the HP-97 service manual shows that a 1-of-14 counter/decoder chip (labeled as the cathode driver since it servers that purpose too) is used to scan the rows of the keyboard matrix.
These 14 rows times the 4 columns (KBA, KBC, KBD, KBE) account for all 64 keys on the HP-97. The 14 outputs from the cathode driver chip are normally high and only go low when the row is being scanned. If a key on that row is pressed it will complete the circuit to one of the 4 column lines.
So, pulling a column line low at the moment when the needed row is selected is equivalent to pressing the corresponding key.
In this way, keyboard input can be simulated by an external circuit. The cathode driver chip is clocked by the rising edge of STR (strobe?) line and reset by the falling edge of RCD (reset cathode driver?) line. Knowing this it is straightforward to program a microcontroller chip to simulate key presses using these 6 control lines.

So it appears to be able to write (and presumably read?) keystrokes. The subset of LED segments selected is still puzzling, though.