Re: IIGS ROM chip questions

[David Wilson <david@uow.edu.au>]

Wayne Stewart <waynes@telus.dotnet> writes:
>Dan Foster wrote:
>> What's the speed? 120ns? 150ns? I know the DRAM was 120ns but less sure
>> about the ROM.
>> 
>> Would a 27C010 do? (Assuming one had a 32-to-28-pin socket adapter,
>> especially if on a ROM 01 system.)

>150ns is fast enough, the part number on one of the EPROMs in a ROM 3
>is M5M27C100K-15. Actually 150ns if fast enough for DRAM too. Quite a
>few of the earlier IIgs memory cards from Apple had 150ns RAM soldered
>on.

The 2nd ROM on the ROM3 has its A16 address line latched by a 74LS74
clocked by Phi2. By my calculations, a 150ns part is just fast enough
(it would work with the typical 74LS74 delays but not the maximum ones).
I am assuming that the ROMSEL lines are at least this fast.
Of course this is not the only time Apple have used marginal timings on
ROMs (the 450ns 2513 chargen in the Apple II does not get 450ns).
-- 
David Wilson  School of IT & CS, Uni of Wollongong, Australia