Date   : Tue, 08 Sep 2009 14:14:52 +0100
From   : michael.firth@... (michael.firth@...)
Subject: Acorn World '09: Final Layout & Wiring Plan

> -----Original Message-----
> From: 
> bbc-micro-bounces+michael.firth=bt.com@... 
> [mailto:bbc-micro-bounces+michael.firth=bt.com@...
> .uk] On Behalf Of Phil Blundell
> Sent: 08 September 2009 13:18
> To: bbc-micro@...
> Subject: Re: [BBC-Micro] Acorn World '09: Final Layout & Wiring Plan
> 
> On Mon, 2009-09-07 at 23:43 +0100, Jonathan Graham Harston wrote:
> > > Phil Blundell wrote:
> > > > FWIW, if you have at least one free sideways slot then 
> it's fairly
> > > > straightforward to fit 32K of SRAM (and probably 
> feasible, though
> > > > somewhat more fiddly, to fit 64K).  Of course, if you 
> have enough
> >  
> > http://mdfs.net/Info/Comp/BBC/SROMs/32K
> 
> Right, exactly like that.  
> 
> 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.
> 
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.

It should be a very similar process to the "using 1Mbit EPROMs in a BBC Master"
advice, as is available at:

http://www.adsb.co.uk/bbc/adaptor.html
and
http://www.mdfs.net/Info/Comp/BBC/SROMs/MegaROM.htm

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...

Regards

Michael