Date   : Tue, 08 Sep 2009 18:56:39 +0100
From   : robert@... (Rob)
Subject: Acorn World '09: Final Layout & Wiring Plan

On 08/09/2009, Phil Blundell <philb@...> wrote:
> If you want to do something cleverer, say having your eight banks of RAM
> appear as 0-7 with the three ROMs in 13, 14, 15, that'd require a bit of
> surgery to the motherboard.  I think it should still be relatively
> straightforward though.
>
Shouldn't need much surgery if you can find a more or less pin
compatible device.  Off hte top of my head, just plug it into IC52
(rom 12) apart from the following pins:

CS (pin20)
RAM A14 take to IC76 pin 14
RAM A15 to IC76 pin 13
RAM A16 to IC76 pin 12

disconnect link from S21 SE-SW

Find another 74*139
Connect +5v and gnd
A(pin14) to IC76 pin 11
B(pin13) to +5v
2(pin 10) to RAM CS
3(pin 9) to S21-SW
G(pin15) to S21-SE

You could probably do this with a few NORs actually.  The 139 however
would let you use three 512Mbit chips (4 banks each) for 12 RAM banks,
if you had any of those, by using B to IC76 pin 12, and outputs 0 1
and 2 to the three CS lines.