HP 110 "garbage" on screen

[ThomasF]

(05-11-2023 12:40 PM)Jeff_Birt Wrote:  If you were to type 'a' ($61) and bit 6 stayed low you would get $21 which is '!'. This does not look exactly like what is showing up on your LCD but perhaps something similar.

As already mentioned before, this seems to be exactly what happens.

All numbers and ":;<=>?" (column 3) shows up correctly (since bit 6 is low (0x30-0x3F), actually all characters from column 0-3 shows up correctly, but all characters in column 4-7 (ie 0x40-0x7F) is shown as from column 0-3 (ie. 0x00-0x3F) instead.
So "qwerty" in the attached picture is shown as "17%249", and line 2 should be read as "Line: 4 Column: 21".

But the interesting part is that this only applies on every other character.
If we have a 16-bit bus, then 1 stuck bit could show up like this, but not with an 8-bit bus.

If RAM is collected from two different chips or dies (ie even bytes from one and odd from another), then we might also explain it, but it only seems to apply to the display (not RAM in general).

Anyone has schematics for the HP110? Service manual? It might give a clue ...

Just my 2 cents ...

Edit: Answering my own question ...

I found schematics on this site (where else ... ?): HP110 documents

Page 14 (of Tony Duells schematics document) shows the LCD-driver, and there is actually a 16-bit to 8-bit latch here.
The latches (82C82 - U11-12, U23-24) latches 16-bit data down to 8-bits to the LCD.
So if I had the device in my hands, I would check the pins for bit 6 or bit 14 on these, and if you are lucky you will find a bad solder or something that shorts any of these pins.

Good luck bit-hunting!

Cheers,
Thomas