BBC Acorn

From: <(>
Date: Thu Dec 12 13:37:00 2002

On Dec 12, 7:40, Joe wrote:

> It does appear to be a B. It has two rows of (mumble)16 memory chips
so it has 32k of RAM.

It probably is a B if that's the case (and the part number certainly is a
"B" part number) but just having the RAM doesn't make it so. The chips are
4116s, BTW. As Tony said, there are lots of differences between an A and a
B, but all the upgrades are/were available and there is no difference
between an upgraded A and a B (except perhaps the date codes on some chips,
how many are socketed, and maybe a little flux residue on the board).

I thought I had a document listing the exact differences between UK and US
versions, but I can't find it :-( However, the differences are small:
different MOS (mainly video timings to be programmed into the CRTC),
different colour crystal, different BASIC ROM (American spellings added,
version number changed from II to III). IIRC the US version also had
Econet and speech synthesis fitted (otions on the UK version). It didn't
sell very well, and eventually a lot were re-imported to the UK becasue
there was a boom in demand around 1986-87, which rather surprised Acornm,
as they'd had two successor models since the original Model B in 1981!

> What is "DFS OS"?

Something is wriong if it says "DFS OS", I think. See below.

DFS = Disc Filing System ("disc" has a "c" in English :-)). A
single-density dis[ck] filing system, with a fairly simple flat directory
structure. It can handle two 5.25" floppies, which may be any mixture of
double- or single-sided, 40- or 80-track, so long as they have a standard
SA400-type interface.

Capacities are 200K for 40-track discs, 400K for 80-track. The two sides
are treated as separate disks (so Drive 0 and Drive 2 are opposite sides of
the same physical unit, and 1 and for the other unit).

> I was surprised to find that it uses an intel 8271 for floppy drive

It's based on an earlier Acorn design. It's possible to replace the 8271
and some parts with others, plus a daughterboard carrying a 1770. You need
the "1770 DFS" to drive it, still single density (in fact, compatible in
almost every way) but you can then also add the ADFS (Advanced Disc Filing
System) ROM to get double-density and a fully hierarchical directory
structure. Capacities range from 160K (40-track, SS) to 640K (DS
80-track), called "S" (small), "M" (medium), "L" (large).

As you may have guessed by now, if you didn't already know, many parts of
the MOS are modular, and it's possible to add many other ROMs with
languages, service utilities, and filing systems. FS's I can think of off
the top of my head include TAPE (cassette filing system, built-in), RFS
(ROM Filing System, also built in), DFS (you've met), ADFS (see above), TFS
(Telesoftware, with a teletext adaptor), NFS (Econet networking), DNFS
(combined DFS, NFS, and 2nd Processor handler), VFS (Video Filing System,
of Domesday fame), "Advanced" versions of some of these, and various
third-party equivalents.

> It powered up up but isn't recognizing any keys except the BREAK key.
 I ran it for over an hour then one of the rectangular tantalum filter caps
in the PSU blew. Fixing the PSU shouldn't be a problem. Does anyone know
what they use for a keyboard interface (if it even uses one)? or if there's
any schematics posted anywhere for it?

You want to look at a couple of sites in particular, starting with The BBC
Lives! at and The BBC Documentation Project
at There you'll find many things, including the
Service Manuals (look under "Essentials"), which include the schematics.
 Also 8BS Software at , where you'll find a nicely
annotated set of pictures of the BBC B motherboard
( Chris Richardson at 8BS lives not far from me,
and he's scanned a lot of my manuals and contributed them to The BBC
Documentation Project.

When you power up a working Beeb, it should make a double "beep" and then
display something like

BBC Microcomputer 32K

Acorn DFS



If you get nothing, check power. If you get a continuous tone and not much
more, or just get a flashing cursor at top left, check the keyboard cable,
IC3, and other keyboard-related stuff.

The way the keyboard works is quite neat. Like almost everything on a
Beeb, it's interrupt-driven (unlike Apples and PETs, which are much slower
-- even allowing for clock speed differences -- becasue they use polling
loops). I wrote the following to explain it on a BBC mailing list
recently, when someone had a faulty one. This particular fault was
heat-related, hence the notes at the end; note that the most common fault
is a broken or mis-fitted 17-way ribbon cable between keyboard and
motherboard or a cracked track near a key:

A 74LS161 (or 74163, they're interchangable for this purpose) 4-bit
pre-loadable counter is clocked at 1MHz, and its outputs drive a 7445 (or
74145, again they're interchangable for this purpose) one-of-ten decoder.
 For each state "0" to "9" generated by the counter, one of the
open-collector outputs of the 7445 is pulled low, and the corresponding
keyboard column goes low. If a key in that column is pressed at the time,
one of the row inputs of a 74LS30 is also pulled low, which signals to the
system VIA on its CA2 handshake pin, and generates an interrupt.

When the MOS services the interrupt, it uses PA0-PA3 to drive the 74LS161
like a latch, and tests each column in turn. For each column, it also
selects each row in turn. Each row is connected to one input of a 72LS251
multiplexer, and by using PA4-PA6 to address the MUX, the MUX output, which
is conected to PA7, can be sampled. When the MUX output is low, you've
found the row and column of the key that was pressed to generate the

The most likely fault is a cracked track near the counter or MUX; I've seen
similar situations where a crack right beside an IC becomes heat sensitive
(when it warms up, the metal expands and -- counterintuitively -- widens
the crack. Second most likely is the chip itself.

If all the keys in one column are faulty, suspect the decoder (7445). If
all the keys in several columns are bad, suspect the counter (74LS161). If
some keys in a column are good but some are bad, but all the keys in a row
are bad, suspect the MUX (74LS251).

If it's just a mixture, suspect the VIA (which is usually socketed, so easy
to swap with the user VIA).

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Thu Dec 12 2002 - 13:37:00 GMT

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