Re: ROMs for IIgs

[david@uow.edu.au (David Wilson)]

Wayne Stewart <waynes@intergate.bc.ca> writes:
>Andrew Trotman wrote:
>> I've searched the web and it appears that no-one sells ROM 03 for the IIgs
>> and it isn't possible to burn it into an EPROM either as 28pin EPROMs don't
>> exist
>>
>> It this right?
>>
>> Andrew.

The reason no one sells the ROM03 is that it only works in the 1MB IIgs
motherboard (it is two chips) and they all came with one installed. The 256KB
IIgs motherboard came with either a single ROM00 or ROM01 chip and can not
be upgraded. The only ROM03 chips not in motherboards would be spare parts
from Apple but we may have passed the period Apple keeps spares for discontinued
IIgs computers.

>If you look at the front of those 28 pin sockets you'll see that the
>motherboard was
>designed to take either 28 or 32 pin sockets. The 32 was in case Apple decided
>to
>use EPROMS , which they did sometimes. While changing the socket'd be a pain,
>adding a couple of pins to the end wouldn't be difficult.  The number on the
>EPROMs
>I have are M5M27C100K-15. I don't have the means to get individual ROM images
>but I can send you a collective ROM image of the two if you can work with
>that. If not
>I can likely scare you up a set of ROMs but it'd have to wait till I next dig
>into my
>storage which may be a while.

>    Wayne

The only problem with using 32 pin EPROMs in a //gs, even if it has 32pin
sockets is that the pinouts of the ROMs are not quite the same as those of
EPROMs.

1Mb/128KB  EPROM	1Mb/128KB ROM
----------------	-------------
1	Vpp			--
2	A16			--
3	A15		1	A15
4	A12		2	A12
5	A7		3	A7
6	A6		4	A6
7	A5		5	A5
8	A4		6	A4
9	A3		7	A3
10	A2		8	A2
11	A1		9	A1
12	A0		10	A0
13	D0		11	D0
14	D1		12	D1
15	D2		13	D2
16	GND		14	GND

17	D3		15	D3
18	D4		16	D4
19	D5		17	D5
20	D6		18	D6
21	D7		19	D7
22	/CE		20	/CE
23	A10		21	A10
24	/OE		22	A16
25	A11		23	A11
26	A9		24	A9
27	A8		25	A8
28	A13		26	A13
29	A14		27	A14
30	---		28	VCC
31	/PGM			--
32	VCC			--

As you can see, all the pins match up EXCEPT pin 24/22. On the Apple ROM
it is A16 while on the EPROM it is /OE and A16 has moved to pin 2.
If you look at the circuit diagrams in the back of the IIgs hardware
reference manual (1st edition) the signal is even called ROMOE/A16 or something
similar. Does the ROM03 have a jumper to relocate the A16 signal?

There may be a 28 pin 128KB EPROM (the same problem occured at the transition
from 24 pin 2732 (4KB) to 28 pin 2764 (8KB) - Motorola had a 24 pin 68764 (8KB)
which managed to make the /CE pin do double duty as an enable and as the VPP
pin when programming) but I do not know its part number or manufacturer.

An adapter can be constructed out of a 28 pin plug and a 32 pin socket:

plug	socket
1..21	3..23
22	2	A16
23..27	25..29
28	32	VCC
	22+24	common /OE with /CE
	1+31+32	common VPP and /PGM with VCC (I think - check this with
		EPROM data sheet (271001).
	30	leave open or connect to VCC 
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia