Date   : Tue, 08 Sep 2009 14:23:15 +0100
From   : philb@... (Phil Blundell)
Subject: Acorn World '09: Final Layout & Wiring Plan

On Tue, 2009-09-08 at 13:33 +0100, Rob wrote:
> On 08/09/2009, Phil Blundell <philb@...> wrote:
> >
> > 512Kbit SRAMs seem to be somewhat hard to buy, so for 64K you'd need to
> > use either a pair of stacked 62256s or half of a 1Mbit device.  I guess
> > the former is probably easier.
> 
> You'd have to invert at least one address line and gate it with the
> selects, to use stacked chips, surely?

Right, but that just requires a single 74HCT139 or some such.  Connect
the original nCS to the decoder's B input, and the extra ROM select bit
to the decoder's A input, then you can use the Y0 and Y1 outputs as the
chip selects for the two SRAMs.

> I'm sure it's much easier just to stick a single 62256 in each of two
> sockets, both modded as shown above..

Heh.  Yeah, if you have two spare sockets then that's probably true
enough.

p.