Re: Do it yourself upgrade: IIgs ROM 01?

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

DeeJ Price <deej@gte.net> wrote:

> >Even if you could burn a 128KB EPROM, it won't physically fit in the
> >motherboard (you would need an adaptor plug to get the signals into the
> >right locations).  128KB EPROMs are 32 pins, while the IIgs ROMs are 28
> >pins.
> 
> I am confused by this. Admittedly I dont know alot about EPROMS and chips
> and things, however I was planning on attempting this myself. So I got my
> hands on an EPROM programmer which will do 128K EPROM. Now I am looking at a
> ROM chip on an old adaptec card I had laying arround, its part # is 27c128
> and it has a window in the top(I am assuming this means I can erase it, I
> got an eraser too). Anyway the point to this story is that this chip has 28
> pins and would fit perfectly in the GS rom socket.

Well, yes; but note how EPROMs are numbered: the 'xxx' in '27Cxxx'
refers to the capacity in kiloBITS, not kiloBYTES.  A 27C128 is a 16
kilobyte EPROM.  Just a tad too small for this application.  :-)

The IIgs needs a 128 KByte ROM, but has a 28-pin socket (and there are
no 28-pin 128 KByte EPROMs, only 32-pin ones).

128 KByte EPROMs generally have a part number of 27C010, 27C1000 or
27C1001 ("1000 kilobits", roughly), depending on the manufacturer.

27C1000 and 27C1001 are part numbers used by SGS-Thomson.  We use the
27C1001 at work, and it follows the JEDEC standard pinout for EPROMs.
We also use the Atmel 27C010, which has an identical pinout.

The 27C1000 has a nonstandard pinout that is intended to be used when
prototyping for a 28-pin masked ROM.  It is still a 32-pin chip, but has
some signals rearranged to match the masked ROM.  To use this, the
motherboard must have a 32-pin socket during prototyping.  The
production motherboard only uses a 28-pin socket (mounted at the lower
end of the 32-pin position) into which the masked ROM is inserted.

This is how the IIgs motherboard is implemented, at least with ROM 3 (I
don't have my ROM 1 handy, so I can't check it now).  If you look at the
top of the ROM sockets (towards the front of the computer), you will
observe an additional four pins on the motherboard, and the white legend
indicating the ROM extends to cover these pins.  [I don't recall seeing
this in the ROM 1.]


There is also a "27C1024" EPROM available from SGS-Thomson.  This has
the same number of bit cells (1024 x 1024 = 1048576), but is 16 bits
wide (thus 64Kx16) and is a 40-pin chip.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand