Just found this: In the PET only seven bit of the data read from memory are
used for character ROM addressing. The 8th bit is instead directly fed to an
inverter, inverting the pixel stream if set - thus inverting the
character. This
means it has no special drawing characters like the C64 and others.

So it is definitively a 2114 problem, I know how to find the defective chip
by calculating the position of the defective characters on the display.

2009/5/8 Anti Florian <anti77@gmail.com>

> Thanks.
>
> Actually i found out that these garbage columns are on invert (a white
> block apears). By typing some characters will display the characters inside
> them on invert. I assume then that the data store on the RAM are correct (at
> least the first 7bits from PETSCII), so it may not be a 2114 problem
> afterall. The only invert line is inside the CRTC. I have no clue right now
> how inverse characters are generated on the PET, but i think it's safe to
> assume, that an address line shorts with the INVERT line from the CRTC,
> hopefully not inside the IC... It may be that the inverse logic is stored in
> the remaining 48 bytes (4 rams 1kx4=2048bytes, display of 80x25 =2000
> characters)?
>
> 2009/5/8 <ruud.baltissen@apg.nl>
>
> Hallo Florian,
>>
>>
>> > The one with an output pin always on high, right?
>>
>> No. If it was a data line issue, the whole screen would be affected.
>> Just a column means an address line is affected. in this case I would
>> say A4. And only the upper half of the screen, that means the error it
>> is restricted to one pair of 2114's ie. it isn't one of the
>> buffers/drivers connected to the 2114's.
>> Although it may look like garbage, it should be possible to find out
>> whether bit 0..3 or 4..7 are affected.
>>
>>
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