5 Feb
2007
5 Feb
'07
4:03 p.m.
On 2/5/07, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Yep.
I've got a CBM 8032 that powers up "fine", but the screen is filled
with "!" on every other character position...
It's been a long time since I've worked on a PET, but some details have
stuck in my mind.
The first (obvious?) thing is that it appears that the LSB of some
locations in screen memory is stuck in the '1'' state. That would turn a
blnk into a '!' and a 'n' into an 'o' even in PETSCII Now, I seem to remember that on 80 column PETs, the 2K
video memory is
made up of 4 1K*4 chips and that alternate characters are stored in each
pair of chips (one pair of chips stores all the chracters in odd-numbered
locations on the screen the other pair the characters in even-numbered
locations). That would explain why you have problems with alternate
characters.
Hmm... I didn't know about the even-odd interleave, but that'd be easy
to check against the schematics.
And you guessed it,. Those RAMs are 2114s, a chip
well-known around here
for failing in oadd ways.
Exactly. In a PET, it's much more likely that a 2114 has failed than
a bus buffer or similar IC.
Without the scheamtics (and I have them _somewhere_,
they should be on
the web (does funet.fi still exist?)) I can;t be more specific. But I'd
gerab said schematics, find out which 2114 is at fault and change it.
funet.fi doesn't exist as such, but the archives do. They are now on
zimmers.net
http://www.zimmers.net/anonftp/pub/cbm/pet/
Enjoy,
-ethan