HP-10 disassembly/reassembly

[teenix]

(02-14-2020 06:33 PM)[kby] Wrote:  Tony,

Your response implied that they connect such that connection "n" connects to the CPU on the pin labelled "Kn." But it also appears there are Kn connections with overlapping values of n to the display. Is that correct, and, if so, why are the display keyboard and CPU connected on the same pins of the CPU?

Thanks again.

I have changed the HP-10 pin numbering and uploaded the circuit files zip.

Yes that it correct. The same multiplexing method is used in the HP-29C for example.

Each time a cathode driver pin is LO, the LEDs in the corresponding display will get updated from the anode driver. At the same time the A B or C lines are tested. It is possible now that either or all of these lines are LO depending on what keys are pressed.

For example, if K11 is LO, then the LEDs in the 11th display are being updated from the anode driver. If A B C = LO HI HI, then '6' is pressed, if = HI LO HI then '3' is pressed, or if HI HI LO then '.' is pressed.

It is possible that A B C = LO LO LO. In this case the microcode might decide to take the first A B or C that it tests to be the active key, or it might produce a dodgy keycode and the software ignores it.

The DSP, ALL and PRT switches are tested in the same way from the 'E' line by using K6 or K10.

This type of multiplexing can present a problem with the circuit when multiple keys are pressed which is why diode isolation is included on the A B C and E lines from the keyboard.

cheers

Tony