5 Apr
2010
5 Apr
'10
11:20 a.m.
On 4/5/10, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Does the machine work?
Yes, video works except for the CHRGEN ROM. If it worked in its
"normal" space, I'd just type chars to validate the Katakana mapping
to PETSCII.
If so, the ROM must be good (if the character
genereator was all 0's you'd get either totally black or totally white
charactoers (substitue whatever colour the CRT phosphor is for 'white',
of course).
That's exactly it... all chars that are in "normal video" are blank,
and all chars in inverse video are white (exactly as expected since in
PETs unlike C-64s, normal/reverse video is handled in hardware, not
with a 4K CHRGEN ROM).
One of the mask options with mask programmed ROMs
(apart from the ROM
contents of course) was the polarity (active high .vs. active low) of the
enable signals. It's quite possible this ROM has at least one active-high
enable, whch means it will not read out in most programmers. Depending on
the programmer you migth be ableo t make up a little adapter to tie the
appropriate pin high or low and then read it out.
I am not reading this in a programmer - the footprint from the adapter
is for the 6540, not JEDEC, and I have no programmer that knows what a
6540 is (the ROM on the adapter is a 2316, but it's soldered in place
and I was trying not to remove it to minimize the risk to the ROM or
the adapter). I stuffed the adapter into the $C000 socket and was
reading the contents with a Fluke 9010A in the CPU socket. BASIC and
KERNEL and real CHRGEN ROMs reads fine with this technique; I even
have Fluke signatures to verify ROM contents (and have identified two
bad ROMs from my parts bin). The ROM/adapter board in the $C000
socket reads all zeros, but that's consistent with its behavior in its
"normal" CHRGEN ROM socket, so I'm unsurprised I can't read it.
This board came to me with the CHRGEN ROM already bad. It has never
been known to work while in the owners nor my possession. I was
hoping to get a known valid ROM image so I could burn a replacement
2716 or 2732 and use either the original adapter or one that Jim Brain
has recently built (which has already been tested to work with real
JEDEC-footprint CHRGEN ROMs).
I have a photo of a Japanese keyboard map. I think I can use this to
cobble up a faux 901447-12 or near equivalent, but it sure would be
nice to get the data from a real ROM rather than guess.
-ethan