Re: ROM 00 to ROM 01 upgrade

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

Steve Q <sqnospam@superbio.notrash> wrote:

> I've got a ROM 00 "Woz Limited Edition" boat anchor that I picked up cheaply
> some time ago.  I got on the horn with AllTech a few days ago and asked
> about the ROM 01 upgrade that they've been known to carry, and I was lucky
> enough to hear "We have one left."  OK, it's on its way to me, and at least
> I'll be able to boot GS/OS on this thing.
> 
> My question is this:
> 
> I'm not much of an electronics person, but a good friend -is.-  Let's say
> that I'd like to copy these ROM 01 upgrade chips, so that if I'm ever in
> this predicament again, I can simply roll my own (and for personal use...I'm
> certainly not looking to pee on anybody's lawyers or anything like that).
> Is this doable?

It is difficult, because the IIgs uses a type of mask programmed ROM
which holds 128 KB in a 28 pin chip.  Standard 128 KB EPROMs are 32 pin
chips, so they don't fit on the IIgs motherboard unless you build a
socket adaptor.

The ROM 3 motherboard has provision for installing 32 pin EPROMs, and
this was used in prototypes and some early production machines.

There may be a rare type of 28 pin 128 KB EPROM, but I haven't heard of
any such chip.  The problem is that the 28 pins are completely occupied
by the address bus, data bus, power, ground and a single chip select
signal.  An EPROM also needs a programming voltage input, so a 28 pin
128 KB EPROM would need to supply the programming voltage through the
Vcc input (normally +5V).  Most modern EPROMs use +6V or +6.25V on this
pin during programming, and need +12V or thereabouts on a separate Vpp
pin.

> PS:  Is there any way to identify a ROM 00 versus a ROM 01 versus a ROM 03
> simply by looking at the case (assuming motherboards haven't been
> transplanted)?

The only visible external difference would be the serial number, and I
don't know what ranges apply to each machine (there may be some
overlap).

The cases are identical, apart from the "Woz limited edition" which is
printed on early machines (all of which were originally ROM 00, but
might have been upgraded to ROM 1).  The motherboards are
interchangeable.

The best solution is to pop the lid and look at the motherboard.  The
ROM 3 has several obvious differences from ROM 00/01, the main one being
that it has two ROM chips instead of one.


One other point mentioned by other people in this thread is the video
chip which was typically upgraded as part of the ROM 00 to ROM 01.  This
is the VGC (Video Graphics Controller), which is a socketed PLCC (square
pack), so assuming you can get the replacement, you will need a PLCC
extractor tool to replace it.

Early versions of this chip had an error which caused them to display
flickering pixels (usually pink) when displaying monochrome double
hi-res graphics mode, and some also displayed them in 80-column text
mode.  The replacement VGC fixes the problem.

This problem is purely cosmetic - if you can't get an upgraded, the
machine will work fine, but will look strange when displaying the
affected video modes.

If I remember right, the VGC version affected by this fault has a part
number which ends in either -A or -1.  Later revisions (-2, -3, -B and
-C) are OK.

-- 
David Empson
dempson@actrix.gen.nz