BBC Acorn - test-drb@ccmp.vtda.org

13 Dec 2002

On Dec 13, 21:11, Tony Duell wrote:

...
  This reminds me of something that suprises me. Namely
that the Model A
 missed out some fairly simple/cheap chips like the buffers for the 1MHz
 bus and/or Tube, the RS423 buffers (the rest of the serial I/O system was
 fitted on the A becuase it was needed for the cassette I/O), etc. I would
 have thought that the cost of fitting sockets and having 2 different
 boards in production would have exceeded the cost of these chips... 
The idea was to keep the cost as low as possible while making it simple to
upgrade.  At that time the sockets would have been much cheaper than the
chips.  I don't know how the decision was made, nor by whom (though Acorn
was a fairly small comapany at that time), and it may have involved the
BBC.

...
  > I doubt if it's that simple.  UK models
don't use mylar ribbons, they use a
...
  > single 17-way notched IDT cable with a 0.1"
pitch, and with a single-row
...

 Unless thr speech upgrade is fitted. 
True, in which case there's an additional 10-way 0.1" pitch IDT cable on
the left of the keyboard.

...
  Very early Beebs had linear PSUs with 3 7805
regulators in them. One fed
 each of the +5V outputs. You (Joe) do not have one of these, though. 
:-)  Horrible things.  Acorn had an exchange scheme, and they destroyed the
black ones they got back.  There were actually three black models, two matt
black, one gloss, IIRC.  One (I forget which) was unofficially known as the
"adaptor and exploder" becasue it had a habit of doing the latter.

...
  IIRC, the ground pins are linked on the mainboard, the
+5V pins are not,
 so you have to have all 3 sets connected for it to work. 
Later boards did have the +5V joined up, just not with very fat tracks in
places.  So on all but the earliest ones, it sometimes "mostly" works if
one set isn't connected (or is dirty).

-- 
Pete						Peter Turnbull
						Network Manager
						University of York