22 Jun
2012
22 Jun
'12
12:46 a.m.
David Riley wrote on Thu, 21 Jun 2012 15:52:27 -0400
On Jun 21, 2012, at 3:20 PM, Eric Smith wrote:
Here is a picture of the motherboard:
http://www.merlintec.com/download/mac_unitron_motherboard.jpg
The video had died and I guessed that it was the CRT so I swapped it
with the one on a Mac Plus. They weren't 100% mechanically compatible,
but fortunately the Unitron chassis had been designed for the original
CRT and had an adaptor to use the slightly different tube (the phosphor
is a little different too). That didn't do anything, so I swapped the
analog boards too and then it worked.
Anyway, I can't quite read what is on the PALs but it seems they are
Jecel Assumpcao Jr. wrote:
I know I would, mostly from a historical perspective. JEDEC fuse
maps are pretty easy to turn back into equations. I did a clean room implementation of the PALs in
1987 that was better
(27% performance increase over the original) but don't have a copy of
the sources (I do have access to a machine with the actual PALs with
my design, however).
I'd be interested in trying them if you can read them and post the JEDEC
files as some point. I suspect probably a few other people would be
interested as well. 
from MMI and "Made in Singapore". This makes
it more likely that the
Unitron would have burned the security fuses so the chips
can't be read
in a PAL programmer. On the machine I worked with them had Lattice GALs
instead and while those can also be protected they were less likely to
do so.
Of course, the only way to know is to try. I don't have a PAL programmer
handy and wouldn't like to open up the machine again without a good
reason, but it would certainly be worth doing it if I could fix the
original analog board and restore the original components to the
machine. So this isn't something I will do right now, but it might
happen by the end of this year.
I did find the very first draft of my design (it had been forgotten in a
drawer for over a decade, which is why I didn't turn it over to Unitron
like I did all other material. The big change was that Apple's design
uses 4 clock cycles for each memory access and mine uses 2. This ended
up making all the state machines smaller and more uniform.
Ah, someone gathered a bit of this information on this page:
-- Jecel