At 07:39 PM 12/12/02 GMT, you wrote:
>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).
Nope, all the RAMS are soldered directly to the board and no solder flux. I'm sure it was built with 32K and that appears to agree with the US model number.
>
>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.
It's not running at the moment but when it was it said soemthing like:
"Acron version 2.1
DFS OS
BASIC OS"
There's also a EPROM that says "DFS OS" in it so I think it's supposed to say that.
>
>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
>controller.
>
>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.
I was just surprised to see the 8271. The only other systems that I remember seeing them used in are the Intel MDSs.
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 http://www.nvg.ntnu.no/bbc/ and The BBC Documentation Project
>at http://www.bbcdocs.com/ There you'll find many things, including the
>Service Manuals (look under "Essentials"), which include the schematics.
> Also 8BS Software at http://8bs.com , where you'll find a nicely
>annotated set of pictures of the BBC B motherboard
>(http://8bs.com/inbbc.htm). 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.
OK thanks for the pointers. I searched the net but found so many sites that I haven't had time to go throught them all.
>
>When you power up a working Beeb, it should make a double "beep" and then
>display something like
>
>BBC Microcomputer 32K
>
>Acorn DFS
>
>BASIC
Nope, that's not what I got. It didn't say anything about BBC or Microcomputer or the amount of memory and it definitely said "OS" twice. I did get the beep then the speaker had a slight buzzing in it. (Possible due the the failing capacitor in the PSU). I was in the process of checking the PSU outputs for noise when the cap blew. It was quite noticeable!
>
>>
>
>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.
Actually I didn't get a cursor. I wasn't sure if it was supposed to have one or not.
>
>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
>interrupt.
>
>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).
Thanks for the description. That's about what I expected. Sounds like I need to find a VIA. I need one for my spares anyway. There are two mylar(?) ribbon cables that connect the keyboard to the main circuit board. Is the wide one the column inputs and the narrow one the row outputs? (Keeping my fingers crossed that it's that simple!)
Joe
>
>--
>Pete Peter Turnbull
> Network Manager
> University of York
>
Received on Thu Dec 12 2002 - 18:44:45 GMT