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

From: Philip Lord <>
Date: Sat, 14 May 2011 02:55:34 +0900
Message-id: <>
Thanks André,
I only have limited tools. Logic probe, eprom burner and multimeter.

I'll have to snag the keyboard out of my working pet to try the 'LOAD' test.
What other ways can I test if the CPU is working?


On May 14, 2011, at 2:39 AM, André Fachat wrote:

> Looking at the further discussion, you are concentrating on the video part, but is the CPU actually running ok? I didn't see that, may be I overlooked it. But you mentioned burning new ROMs as the original ones were dead.
> If the cpu is not running ok, this could explain the random characters on one side and also the "snow", as some random code would not wait for the vertical retrace when writing to the screen memory.
> Did you try checking for example with a tape and typing LOAD and pressing the Play button that the tape starts turning?
> I'd concentrate finding why the CPU isn't working and looking at the video later.
> André
> -------- Original-Nachricht --------
>> Datum: Fri, 13 May 2011 19:27:34 +0200
>> Von: "André Fachat" <>
>> An:
>> Betreff: Re: PET 2001 fix Part 3 - RAM/ROM board etc.
>> -------- 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". 
>> André
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Received on 2011-05-13 18:00:27

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