Re: PET 2001 fix Part 3 - RAM/ROM board etc.

From: André Fachat <>
Date: Fri, 13 May 2011 19:27:34 +0200
Message-ID: <>
-------- Original-Nachricht --------

> So now I have a screen of random and slightly deformed characters (seems
> like the same or similar character layout on each start up though), with a
> pixel flicker through them. I've put pics and a short video up on the same
> page here:

Sorry to chime in so late only. The pixel flicker might be from the fact that the CPU accesses the video RAM. The oldest PETs had video RAM with max 1MHz operation only, so when the CPU accesses the video RAM the video display presented "snow". The CPU controlled access to the video RAM by sampling the vertical retrace on a VIA pin (VIA Port B ($e840, 59456), bit 5). This could relate to the fact that characters seem stable when the machine resets (switching off?)

Maybe the VIA has a problem here?

On a side note: the fact that switching this vertical retrace off resulted snow, but also faster screen output - no wait anymore - a certain POKE was found to make that VIA input pin an output - accidentally destroying some newer models, the "killer poke". 


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Received on 2011-05-13 18:00:13

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