Date   : Sat, 17 Oct 2009 09:28:24 +0100
From   : bbc@... (Dave Curran)
Subject: ROM upgrades on the cheap

2009/10/16 Rick Murray <rick@...>:
> Mark Haysman wrote:
>
>> You just connect pin 27 of IC101 to pin 27 of IC100 instead of the R/W
>> signal. You still need to make the cut to isolate it of course.
>
> So, in essence, the EPROM sockets would be wired up in *EXACTLY* the
> same way!? Ooookay, I'm not sure how the memory decode works to tell one
> from the other. I think it's time I print out Jason Tribbeck's redrawn
> schematic and stop looking at the scanned one...
>
OK, this is from memory, but the sideways rom decode is based on 4
bits that are latched in ic76, giving a possibility of 16 addresses.
The lower 2 are used in the standard beeb to control the rom select
lines on each of the 4 rom sockets. Because it ignores duplicate roms,
those appear as c-f, even though they are actually selected as 0-3,
4-7 and 8-b as well.

The two spare latched address lines on ic76 are wired up to the two
highest bits on the eproms to perform the bank switching, so the
rightmost rom is chip selected for 3, 7, b and f as before, but now a
different image is presented because of the addressing within the rom
image.

To make the sideways ram, it is either a choice of having 2 banks and
using a 32K sram with one of the address lines and the write latch, or
4 banks with a 64K sram (or larger as I couldn't seem to find any)
with 2 address lines and the write latch.

I'm not sure why they didn't do that on the original beeb. No extra
components are required, only a bit of extra tracking, it maintains
compatibility with 2764 and  27128 roms, but gives considerable scope
for expansion. I was certainly pleased to be able to remove the
watford 12 roms board which had been in there previously, it now looks
a lot neater and the power consumption is down as well.

> Anyway, cool, that's simple. Even *I* can get that right! :-)
>

I saw Mark's page, and it seemed like it should be a simple but
powerful upgrade that I thought was within the capabilities of my
dodgy soldering, so I had a go. I don't know if the instructions were
wrong or more likely, I copied them wrong, but I have the two address
lines reversed, so the images within the eprom need to be in the order
of 1, 3, 2, 4. This was a bit of a faff when building up the images,
but makes no difference in operation.

> Have you thought about writing up a HOWTO? I am looking at
> http://www.pdcurran.co.uk/BBC/DSCF2800.JPG too.

Ah, you found my hidden pictures, curse the inquisitive viewer! I
uploaded a few more when I put the originals up, in case there was any
follow up. I suspect the two horizontal blue wires should cross over
before they get to IC76, then the addressing would be in the correct
order. I might get around to writing this up, but it is on a very long
list of things to do.

> Yes, I love those extra pins floating there! When the chip is in place
and hides the glue, it's quite surreal.

I did try to cut down an IC socket to just the top four pins, and was
hoping to install it to make a complete (looking) 32 pin socket, but
it didn't quite fit and there were height issues. The plan was then to
use the stuck on pins to hold an ic socket and then put the chip in
there, but it actually fitted better direct and removed any chance of
getting in the way of the keyboard.

Thanks,

Dave