Date   : Sat, 16 Mar 1993 08:02:57 WET-2
From   : "Walter Meyer" <WMEYER@...>
Subject: Re: Minimum configuration?

> Has anyone any idea what the ABSOLUTE minimum chip set needed to get a
> running system?

> Obviosly I need a 6502, and I think I need a 6845 to do the memory refresh
> (they left the RAM in, as its all soldered).

Yes.  Hopefully the ram works - if not, either the machine will not
start, or you will see al kinds of interesting lines running
vertically accross your screen

> I can obviously live without the model B stuff, but do I need all of the
> model A chipset? Can I live without a Video ULA (If I don't want video -
> though I probably do), a system 6522?(I don't want sound, and probably don't
> want a keyboard - I'll install a serial ULA, and use my current machine as a
> keyboard).

You'll need the video ULA (as it is used to divide the 16MHz clock
from the crystal for the rest of the system).

Regarding the system 6522: I once built a "bbc-like" computer, with
all the necessary hardware, except the ULA and the system 6522, with
the hope that the OS would run (and that I could program it in
basic).  However, it seems that the OS does not run if it can't see
the system 6522.  (If I remember correctly the ULA is mostly read-
only, so most probably it will not be noticed.)


> I do remember that they don't power up without a keyboard, but figured that
> that was simply because the break key was on there (or some other simple
> fix).

I don't think it is that easy - see above.

> The problem really has wto parts to it, as I;m sure that most of the
hardware
> I'll be dependant on will only be required if I run the MOS - how much more
> culd I get away with if I ditched that too (or patched it?).

I did that with the above card - disassembled the whole thing, and
started implementing all the calls used by basic (I wrote a routine
that prints out an error message every time an unimplemented
OSByte/OsWord call is made).  I think I still have the assembler
listing if you want it.

> I've a few ideas for things I could do with it - A second display
would be  > nice. Failing that I could use it for a Teletext server.
>
> It's pretty unfortunate that both the Video and Serial ULA's are missing as
> these will be hardest to get hold of. Anyone like to comment on my chances
> of getting this thing going? (Anyone got a pile of spare parts they're
> willing to part with?).

The video ULA seems to be one of the most regularly replaced chip in
BBC micro's.  (I would guess about 20% of all faults?)  I personally
have five micro's that need new ULA's.  I have ordered some from
Watford Electronics (who advertise having them) in December, but I
havn't received any reply / goods yet.

Comparing prices in S.A., it seems that the ULA would be by far the
most expensive chip.  The 6522's and the 6502 is relatively
inexpensive.

> Ian Stephenson

Good luck

Walter



Walter Meyer
Department of Physics
University of Pretoria
South Africa

voice  (+27) (12) 420 3508
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