Date   : Tue, 08 Sep 2009 21:05:41 +0100
From   : mfirth@... (Michael Firth)
Subject: Acorn World '09: Final Layout & Wiring Plan

----- Original Message ----- 
From: "Rob" <robert@...>
To: "BBC micro mailing list" <bbc-micro@...>
Sent: Tuesday, September 08, 2009 6:56 PM
Subject: Re: [BBC-Micro] 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.
>
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?

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

Regards

Michael