Date   : Tue, 08 Sep 2009 21:19:30 +0100
From   : robert@... (Rob)
Subject: Acorn World '09: Final Layout & Wiring Plan

On 08/09/2009, Michael Firth <mfirth@...> wrote:
> The 74x139 option sounds emanently do-able, I think I might have a go at
> that.
>
> Presumably if I shuffle where the difference IC76 signals are used I can
> re-arrange the RAM and ROM banks?
>
> So something like:
>
> 15 RAM a
> 14 RAM b
> 13 RAM c
> 12 RAM d
> 11 ROM 1a
> 10 ROM 1b
> 9 ROM 1c
> 8 ROM 1d
> 7 RAM e
> 6 RAM f
> 5 RAM g
> 4 RAM h
> 3 ROM 2a
> 2 ROM 2b
> 1 ROM 2c
> 0 ROM 2d
>
> Could be arranged, with a bit of thought, mapping 2 x 27512 ROMs and 1 x big
> RAM into all the ROM spaces?

Yep. IC76 pins 12 and 11 are bits 2 and 3 of ROMSEL, so feed those
into A & B on a '139 and the four outputs select which block-of-four
banks you are interested in.  feed the one you want back into S21-SW
to enable the on-board sockets.

If you're wanting to use a 1Mbit RAM and a 512Kbit ROM together, plus
the on-board, then just use both halves of the '139, using one to
select banks of four, and the other to select banks-of-eight as my
original post.

make sur you understand what this is doing and don't take my word for,
e.g., pin numbers, as this is all off the top of my head with nothing
more than a uqick squint at the circuit diagram!



>
> Thanks for the suggestion - need to get myself a 74LS139 and start playing
> :-)
>
> Regards
>
> Michael
>
>
>