RE: PET 50Hz editor ROMS

On Thu, 7 May 2015, Martin Hoffmann-Vetter wrote:

>>> Yes, it think too, this 970150-07 must be near to 901474-03 with
>>> changed crtc registers. But it's possible, they changed anything
>>> else, too.
>>
>> I think it more likely a 901447-29 with crtc initialization added.
>
> Yes, you are right. The 901474-03 is a 80 column rom. On a 9" screen a 40
> column is better. So the 970150-07 must be a 40 columns version, too.
>
>>>> This could also be used with an adapter to drive external
>>>> monitors. The 50Hz PAL is probably for an external monitor,
>>>> otherwise there is no reason to make it PAL compatible. In which
>>>> case, monitor size doesn't matter.
>>>
>>> Isn't it? This 901474-04_ can be for an european system board
>>> replacement with 50 Hz. This must be working with a 9" crt. The
>>> changes about 970150-07 and 901774-04_ must be the 50 / 60 Hz
>>> difference. So 901774-04_ looks to 970150-07 as 901774-04 to
>>> 901774-03!
>>
>> Was there a different system board for the European thin 40 or was
>> there just a different video board?
>
> On the normal thin 40 the video board is on the system board. I think, the
> old boards without crtc runs in europe with 60 Hz, too. So no different roms
> exists for basic 1.

Every PET I have seen had a separate video board mounted at the bottom of 
the CRT enclosure. It was essentially a monitor video output circuit using 
ramp generators instead of oscillator circuits. It took the digital 
output from the system board (data, H. sync, V. sync) and used it to 
generate the image on the screen.

>> The system board would not need to be different
>
> Yes, i think, too.
>
>> and the power transformer could be wired to be either
>> 220 or 110 V.
>
> That's an other question. Many first PET have an additional transformer for
> the 220 to 110 V transfoming!

That may be.  I looked at my oldest PET and I cannot see an extra wire on 
the transformer.  I also do not have a scematic for the power supply of 
the oldest PET.

>> The ramp generator for the horzontal sync would have to
>> be different.
>
> Why? The crt is the same and the system board to drive the crt are also the
> same. For minimal interferences only the vertical sync is defined to the
> frequency of the main power.

I looked at the schematics for the vertical drive circuit.  It is driven 
entirely from 12 V DC and the sync signal from the system board. So, it 
would not be affected by different line frequency.



wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org

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