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The BBC Micro

Acorn BBC Microcomputer model B Issue 4

Start-up screen

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My current Beeb is pretty much identical to the first computer I ever owned, back in 1991 or so. The primary differences are that it has a classic and more versatile 8271 floppy controller installed (we fitted a later 1770 to my first computer), it has horrible internal expansion, and the video processor chip isn’t fried. While the 8271 floppy controller is more primitive, it opens you up to a wider choice of disc filing system software, in my case Watford Electronics’ version 1.42 Disc Filing System.

Solidisk Sideways expansion kit

This particular computer has some truly horrible internal expansion that was bad even for the early 80s. It turns my otherwise professional-looking computer into a DIY kit! The case modding and overclocking fashion these days has nothing on this commercial memory upgrade from Solidisk Technology which, unlike my awesome floppy drive mod, was a serious upgrade. This is not AfroTech even if it looks like it. Fortunately, Watford Electronics came out with a much nicer solution that didn’t involve the horrors below.

The fun started when I discovered that my computer would no longer start up. The first step was to take the computer apart and have a look inside to see what had gone wrong this time:

The lid of the BBC Micro’s case BBC Micro running with the lid off BBC Micro running with keyboard detached from the case

Dirty connections with the Solidisk expansion kit had been the cause of boot problems in the past, but this time it was a flaky keyboard connection that was starting the system from stopping. While I had the case off, I thought I would take Meagan some photos of the Solidisk expansion since she was curious about it, and they’re worth making public. Note that the camera is lying about the computer having a banana-shaped keyboard.

The expansion kit comes in several parts. The most obvious one is a granddaughterboard the length of the case:

The granddaughter ROM/RAM board Close-up of the granddaughter board

The granddaughterboard is the board that carries the extra memory and provides the extra software chip sockets. The grey wire on this board is supposed to be soldered to its parent board but the previous cleaning effort to get the computer to start resulted in its detachment. This board is now dead.

This board slots into an edge connector on a daughterboard that itself sits in one of the computer’s four Sideways ROM sockets. Sideways ROMs contain application and utility software, file systems and drivers that together occupy a 16 kB paged slot of the 6502’s address space above the RAM area. This software is run straight from ROM for extremely fast loading, permanent background access no matter what cassette or disc is inserted, and to offer considerable memory savings. The computer can address up to 16, 16 kilobyte Sideways pages, and each ROM is allocated one page. The motherboard, however, only has space for four sockets, and the small Solidisk board provides one extra.

Both expansion boards Close-up of the daughterboard The daughterboard disconnected Underside of the daughterboard The Sideways ROM sockets

If you patch up the motherboard, you can also write-enable any Sideways bank, and this expansion kit provides, one assumes, a 16 kilobyte Sideways RAM bank. Clearly the best use of this is rampant software piracy, as the computer came with discs upon discs of ROM images. Not "romz" as in Amiga disc images, but actual ROM images. Just load an image into the Sideways bank, reset the operating system and it’s available for use. A serious use of this memory is software testing, as it saves blowing an EPROM for each test run.

Fitting an extra board or card to a computer is all you have to do these days to perform an upgrade. Memory upgrades on the BBC Microcomputer also involve patching up the motherboard. In the case of this expansion kit, the daughterboard is wired up to all sorts of places:

Eleven flying leads soldered to various components Close-up of the soldered-on flying leads

There are eleven flying leads:

The upgrade also involved fitting a switch to the rear of the case, wired to the motherboard somewhere with no obvious relation to the rest of the system. This is normally a write-protect switch for the Sideways RAM to stop it from being accidentally erased:

Sideways RAM write-protect switch wired to the motherboard Sideways RAM write-protect switch seen from the rear

The expansion system leaves me puzzled. If you remove the granddaughterboard from its slot, the computer functions normally. If you remove the daughterboard from its socket, the computer will boot but fail to find the BASIC ROM and the system will halt with “Language?” What’s more disturbing is the following disconnected flying lead, which surely would be causing problems?

A disconnected flying lead

A computer should not run with an obviously disconnected wire. And yes, that chip is indeed the processor.