BBC Acorn

On Dec 12, 7:40, Joe wrote: 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! 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). 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.

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. 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.

When you power up a working Beeb, it should make a double "beep" and then display something like BBC Microcomputer 32K Acorn DFS BASIC

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 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).

-- Pete Peter Turnbull Network Manager University of York