HP-67 - 1612A03018 - Card Reader Advice Requested (was HP-67 - strange behavior)

[teenix]

(09-17-2019 07:29 PM)albertofenini Wrote:  Tony, when you say "the first ROM" do you mean the 1818-0268 so called ROM0
or the four little chips 1818-0231, 1818-0232, 1818-0550, 1818-0551 ?
Thanks for help, Alberto

Hi Alberto,

I'm not sure which ROM is which on the PCB, except for the larger ROM 0/display decoder.

Initially, the code executes through ROM 0, ROM 3, ROM 1 and ROM 0. Then it waits for a key press in ROM 0. This code must be functional or the [0.00] won't be displayed when you hold a key down on power on.

If you press a key, say [5], different code executes through ROM 0 then ROM 3 and back to ROM 0.

As the ROMs appear to be selecting properly, it may be bad data in a ROM address in these ROMs.

1 page in ROM has 256 words of ROM code.
ROM 0 has pages 0 - 3 ($0000 - $03FF)
ROM 1 has pages 4 - 7 ($0400 - $07FF)
ROM 2 has pages 8 - 11 ($0800 - $0BFF)
ROM 3 has pages 12 - 15 ($0C00 - $0FFF)
ROM 4 has pages 20 - 23 ($1400 - $17FF)

cheers

Tony