5 Feb
2007
5 Feb
'07
2:47 p.m.
I've got a CBM 8032 that powers up "fine", but the screen is filled
with "!" on
every other character position. When some letters are repeated, they
can change,
i.e. "nnnn" becomes "onon" which seems like (without checking the
ASCII charts)
that the "n" and "!" are ORed to become the "o". Anyone
have any ideas
what the
problem might be? Most of the schematics I have are buried someplace, so I
thought I would start here.
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.
And you guessed it,. Those RAMs are 2114s, a chip well-known around here
for failing in oadd ways.
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.
-tony