RE: PET 50Hz editor ROMS

From: Martin Hoffmann-Vetter <>
Date: Tue, 5 May 2015 21:24:03 +0200
Message-ID: <007301d08769$0cfa7200$>
Hallo William,

> I checked my files and I don't have 970150,

That's nad news. Any one else?

> but at some time I coppied the register values.

Okay, now we need the other 2030 Bytes ... ;-)

> This was for a 9" PET which had the system board
> replaced.

So the old system board without CRTC was replaced with a board with CRTC.

> The ROM was an EPROM, obviously with the CRTC programming
> patched.

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.

> 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!

My first idea was too, the 901774-04_ was used to drive an external pal
montior. And yes, i remember we upgraded a thin 40 system with a 80 columns
video card and replace the 9" crt with a 12" zenith monitor. So this monitor
need pal sync. So we need an patched editor rom with pal frequency, too. But
i don't remember anything about the source of this rom.

> The internal monitors use ramp generators for the horizontal sync. As
> long as the supplied sync rate is close, the monitor locks on. The
> registers on the CRTC are only one byte in length, limiting the
> accuracy of the resultant sync rate. The values chosen by the
> programmers provided the desired appearance on the screen, knowing
> they could be a little off and still have it work.

Yes, that's true. So the accurated value isn't needed. But the high voltage
is defined over the horizontal frequency.


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Received on 2015-05-05 20:00:06

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