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

From: Marko Mäkelä <>
Date: Thu, 12 May 2011 11:41:05 +0300
Message-ID: <20110512084105.GB3328@x60s>
On Thu, May 12, 2011 at 05:15:52PM +0900, Philip Lord wrote:
>Ethan recently sent back my two boards, one fully fixed and working, 
>and one partial fix with lines running vertically down the screen, 
>probably caused by a stuck bit somewhere.

If I remember correctly, on the PET the high-order bit of the screen 
memory byte tells the video circuitry to invert the character. Most 
other Commodore models would have two character sets in the font ROM, 
one normal and another inverted.

Because you seem to have both inverted and non-inverted characters on 
the screen, the stuck bit should not be D7 on the main data bus. The 
vertical stripe at the very right would suggest that if there is a stuck 
bit, it could be D0 on the character generator ROM chip that is stuck at 
5 volts. Can you confirm or deny this?

This is just guessing without looking at the schematics. As far as I 
understand, the PET should always display the contents of the character 
generator ROM. You should not be seeing the screen RAM contents on the 
screen directly; the screen RAM just selects which characters to 

If the character generator ROM is OK (can you test it on the working 
PET?), then you should be able to tell the screen memory contents by 
looking at the displayed characters.  A quick look at your picture shows 
these four character patterns on the screen:

00000001 (each line like this, 8 lines)
00000011 (each line like this, 8 lines)
11111100 (each line like this, 8 lines)
11111110 (each line like this, 8 lines)

These are valid PETSCII characters. I am too lazy to look up the codes.

Best regards,


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Received on 2011-05-12 09:00:16

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