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

Sorry, emails crossed and you answered my question before I asked it ;-)

A crude way would be to measure the voltage on the various 6502 pins, 
especially the Reset, Sync and Ready pins.

There's a thread on VCF right now that might be relevant.

m

----- Original Message ----- 
From: "Philip Lord" <random6000@mac.com>
To: <cbm-hackers@musoftware.de>
Sent: Friday, May 13, 2011 1:55 PM
Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc.


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?

Phil


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" <afachat@gmx.de>
>> An: cbm-hackers@musoftware.de
>> 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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