Date   : Tue, 08 Sep 2009 17:43:39 +0100
From   : philb@... (Phil Blundell)
Subject: Acorn World '09: Final Layout & Wiring Plan

On Tue, 2009-09-08 at 14:14 +0100, michael.firth@... wrote:
> Half of a 1Mbit device is probably a lot easier than trying to combine
> two 62256s, as for the latter you have to muck around with logic gates
> to get the chip selects working properly, rather than just a little
> pin re-shuffling.
> 
> To use half of a 1Mbit device you should just have to tie the spare
> address line either high or low, and I think move the power pin up, as
> devices bigger than 512kbit will be 32 pin, rather than 28 pin.

Unfortunately quite a lot of 1Mbit devices have a completely different
pinout to the Beeb sockets.  Compatible ones certainly do exist but
they're getting hard to buy (at least in DIP packages), whereas 62256s
still seem to be fairly common.  But yeah, if you have a suitable 1Mbit
part on hand then it would be easy enough to wire up.

> I've got some 8Mbit RAMs doing service as 64Kbyte SRAM in one of my
> machines (and a friends) - we had a huge tube of them at work, so
> using a couple was easy, if very wasteful on capacity.
> 
> If anyone can come up with a simple circuit that will let it appear as
> 8 banks rather than 4, I'd be interested to see it...

If you can sacrifice a second ROM slot then it should be as easy as just
using an AND gate to combine the two chip selects, then feeding the low
order bit of ROMSEL to one of your SRAM address bits.  That way you'd
have RAM in, say, banks 0, 1, 4, 5, 8, 9, 12, 13 and ROMs in slots 14,
15.

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.

p.