Date : Thu, 10 Mar 2005 02:46:37 +0000
From : Mike Tomlinson <mike@...>
Subject: Re: Anderson Rom Board
In article <000d01c524d0$98b08570$d19bbd50@...>, Ken Graham
<ken@...> writes
> Seems the flying lead goes to link S19. As Bruce points out, not
> present on issue 7 boards.
> =A0
> I have left an image at www.stenswall.co.uk/images/anderson.jpg
Not a small board, is it? Some thoughts:
1. Although you say it looks new, the circuit design seems to be quite
an old one, i.e. I think it was designed early in the lifetime of the
Beeb. See below for reasons why.
2. The IC sockets used for the ROMs are of poor quality and very similar
to those used by Acorn on the early Issue 1, 2 and 3 machines (thus
suggesting the design of the board is older.) The board is otherwise
well made and well laid out. I can't see how the ribbon cable is
terminated to the board; is it soldered to the underside in some nasty
fashion? (A photo of the underside would be nice.)
3. EPROMs 0 to 3 have more jumper links than the others; the angle of
the photo obscures the jumper settings printed on the PCB, but I'd
hazard a guess that they are used to set ROM size (i.e. you can fit 2k,
4k, 8k, or 16k ROMs by changing jumpers.) EPROMs 5 to 15 have a "W"
link and another link selectable between "F" and "G" (see para 4).
4. The purpose of the flying wire can't be to pick up the R/-W line for
sideways RAM, as this is available from the 6502 which is unplugged and
moved to the ROM board. If it does connect to link S19 as you say
above, S19 is the output of the slow access control NAND gate (IC23,
74LS30) and is used to select ROM speed access at 1MHz instead of 2MHz
for older, slower EPROMs on early main boards. It's not needed for any
of the later 8k or 16k ROMs/EPROMs you'd fit to a BBC as these will work
at 2MHz, so link S19 is soldered for fast (2MHz) operation by default on
later (issue 4 and 7) main boards.
This reinforces my suspicion that the ROM board is quite an old design;
the vast majority of BBC ROM expansion boards didn't fit links to select
smaller ROM sizes or slower access speeds. The links for each ROM
socket are marked W, F, and G, with F linked for each. F for fast
access speed, G for slow? Not sure what W could be (link to enable
writing to allow static RAM chips to be fitted for sideways RAM?)
5. The two switches appear to be fitted to allow the ROMs in sockets 14
and 15 to be disabled, possibly to allow the machine to boot if sideways
RAM in those sockets becomes corrupted. Similarly, PCB on/off switch
positions are also present for ROMs 12 and 13, but the switches are not
fitted.
6. The board is fully buffered using a couple of 74LS244s and a 72LS245,
which is good design and nice to see. From tracing what I can see of
the PCB tracks, I think the eight TTL chips along the centre of the
board are related to the slow ROM access circuitry discussed in para 4.
7. Wonder why the socket for EPROM 4 is missing?
If you want to try it out to see if it works, I'd suggest the following.
Leave all the links in their default positions. Unplug IC1, the 6502,
from the BBC's main board (it's the middle of the three large vertically
aligned chips in the centre, between the 6845 CRTC controller and the
user 6522 VIA). Your ROM board already has a 6502 fitted, so you do not
need to transfer the 6502 to it. Plug the header cable into the vacated
6502 socket, ensuring you get pin 1 (the red stripe) aligned correctly.
Unplug the BASIC ROM (marked PB05) from the main board and transfer it
to the socket marked EPROM 15 on the ROM board. Make sure the switch
marked "EP 15 ON" on the ROM board is set to the ON position. Unplug
any other ROMs fitted to the machine, noting their locations, and set
them to one side. Do NOT unplug the OS ROM (probably marked PB04, the
leftmost one of the 5 in the bottom right hand corner of the main
board).
Leave the flying wire disconnected for now; I suspect it's only needed
if you make use of the slow ROM access feature.
Make sure the board is insulated (not touching anything it shouldn't be
- put a magazine, mouse mat or similar under it). Switch on the BBC and
see if it boots normally into BASIC. If yes, switch off and insert any
other ROMs removed from the main board into their corresponding
locations on the ROM board, switching on and doing a *HELP each time to
ensure they are accessible.
Add any other utility ROMs you wish into spare sockets on the ROM board,
work out how you are going to install the ROM board properly within the
BBC's case without it shorting anything out and so that you can close
the lid, and enjoy.
--
.sigmonster on vacation