Date   : Tue, 11 Feb 2014 20:24:44 +0000 (WET)
From   : bbcmicro@... (Peter Coghlan)
Subject: Paged ROM board

My original Model B has developed a fault in the ROMSEL latch, IC76 (74LS163).
It can't find any paged ROMs and reports "Language?" every time at startup.

On looking it up, I found that a 74LS163 is really a loadable counter but Acorn
suppressed the counting part of it, using it as just a 4 bit latch.  Mine has
decided that it now wants to live out it's designers original intention as a
counter and is going to count, no matter what Acorn intended :-(

(I hope I didn't cause this by loading down one of it's unused outputs by
connecting it to A14 on three 32K EPROMS to give me six ROM images without
requiring any extra hardware.)

Rather than experience the pain of sourcing a replacement 74LS163, I went to
have a look at what was in the box of Beeb related spares and found this:

http://www.beyondthepale.ie/board.jpg

I can't recall where I got it, probably in a box of miscellaneous stuff in
an electronics shop.

The only identification on the board is "SIR COMPUTERS Ltd 1983" except for the
giveaway legend "BBC ROM" beside the two pin header where a flying lead is
plugged in and a tiny etched "rom 2" at the bottom left.  Given that there are
also twelve 28 pin IC sockets labelled IC0 to IC11, it appears this is a
sideways ROM board for the Beeb.  When I google this, I get is lots of matches
for Sir Clive Sinclair :-( and a few references to a company in Cardiff making
Acorn related equipment.

SKT1 and IC23 appear to be wired in parallel and match the pinout of a 6502.
The sticker placed over the IC23 pads suggest that they are not intended to
be used (unless maybe the shop put it there) and a short ribbon cable adaptor
of some kind must be used to connect SKT1, the 6502 and the 6502 socket on the
Beeb mainboard.  The three fixing holes don't seem to line up with anything in
a model B so it would appear the board can only be stuck inside the lid.

On the plus side, it has a 74LS373 latch so it would appear to be able to
work without a functioning ROMSEL in the Beeb.  It seems that the flying lead
might plug into S21sw to enable the four ROM sockets in the Beeb (which would
then be usable if there was a functioning ROMSEL to select them).

Does anyone know anything about this board?  In particular, can anyone save
me having to figure out what the jumpers labelled A to L are for?

Regards,
Peter Coghlan.