Date   : Mon, 07 Feb 2005 23:26:07 GMT
From   : "J.G.Harston" <jgh@...>
Subject: Re: BBC SWRAM module

I've had a closer look@... on Michael's site.

The ROM between the MOS and BASIC ROMs is clearly VIEW 2.1

The line from the keyboard to the mystery components clearly goes to an
/input/ of a 74LS02. I conclude that the cassette motor relay is used
to write protect and write enable the RAM. Also, there are two lines
coming from the ROMSEL cluster, not the one I thought I had seen. That
suggests to me that two extra address lines are being taken from ROMSEL.
That would suggest more than 32K RAM - 64K RAM?

There appears to be a link from S37 on the PCB. That is the CS line
for ROM socket 14. It looks like the mystery bundle 'claims' socket
14 as well as the socket 15 that it is plugged into.

This is what I can work out from the picture so far:

ROMSELy------????

ROMSELx------+-----\
             | NAND +---
R/nW---+-----+-----/
       |                       ROM14---+-----\
       +-----+-----\   +---+-----\     | NAND +---????
             | NOR  +--+   | NOR  +----+-----/
Cas----------+-----/   +---+-----/

Mike, can you post more details on what pin is connected to what?

-- 
JGH - mdfs.net