Re: Overlay Revisions

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <41qf3g$gjb@opal.southwind.net>,
Randy Shackelford <shack@onyx.southwind.net> wrote:
> 
> My just-purchased ROM 3 box is sitting here with the lid off, and it
> has the - A mega II. It appears to be a newer ROM 3 than the one I
> got three years ago, which has sockets too short for one of the ROM
> chips so four more pin things were added at the end. This ROM 3 has
> sockets longer than the chips so four pins on each socket are empty.
> Wonder what that's all about. 

My ROM 3 has just the right number of pins for its ROMs (28 pin chips
in 28 pin sockets).

The motherboard does have a position for a larger socket in at least
one of the positions (not in front of the machine so I can't check
right now).

Any fiddling around with larger sockets and/or chips will have been
during development stages, when EPROMs were being used for the
firmware before the PROM or masked ROM versions were burned.  A 128KB
EPROM is a 32-pin chip, while a 128KB ROM can just fit into 28 pins
(no write enable or programming voltage inputs needed).

Incidentally, what is the part number written on the ROMs in each
machine?  I'm still trying to track down a copy of one of the rare ROM
3 versions so that I can analyse what changed.  I don't have the
numbers handy right now.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand