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

----- Original Message ----- 
From: "Baltissen, GJPAA (Ruud)" <ruud.baltissen@apg.nl>
To: <cbm-hackers@musoftware.de>
Sent: Thursday, May 19, 2011 9:34 AM
Subject: RE: PET 2001 fix Part 3 - RAM/ROM board etc.

<snip>
>Two things go through my mind:
>- The 74100 latches the address to be read by the video circuit. What if
>the circuit stops the latching too early (but the rest goes on)? At the
>next line the circuit starts with the too early latched address. - What if
>the circuit starts one bit too late with clocking the 74107/D5 and 74177
>counters?

>My problem: I keep on encountering confusing things, like the combination
>at the top of the schematic of the vertical synv and the video on signal. I
>only can hope that the above ideas trigger someone elses mind to find a
>possible reason for the problem.
----------
I have the same problem; I didn't think it was the '177s, but my head starts 
to hurt when I try to work through the timing and decoding logic to find 
what it might be instead.

I can't even think of what to look for with a 'scope, but since Philip also 
has a working PET I've suggested that he compare signals between the two at 
various points in the video logic; maybe he'll find a difference that will 
give a clue.

An interesting puzzle for better minds than mine though; anyone else have an 
idea what fault could give the described symptoms:

If a line ends with ...ABCDEF, then the next line continues with DDEFGH... 
instead of GHIJ...

mike 


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