The BBC Micro
Acorn BBC Microcomputer model B Issue 4
- MOS Technology 6502A 2 MHz CPU
- Microvitec Cub 14″ monitor
- Acorn MOS 1.20 (16 kB ROM)
- Level 2 BASIC (16 kB ROM)
- 32 kB of RAM
- VLSI VC 2069/201647 video processor in conjuction with a Hitachi 6845 CRT controller (global effective resolution 640×512, maximum native resolution 640×256)
- Mismatched, joined pair of 40/80 switchable external 5.25″ floppy disc drives, powered by the computer
- Texas Instruments SN76489 four-channel audio generator
- Solidisk Sideways expansion kit with Sideways RAM fitted
- Nifty unsprung joystick that stays where you put it
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:
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 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.
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:
There are eleven flying leads:
- Two soldered to the processor (black, brown) except one has come off
- Two apparently soldered to the floppy disc interface (yellow, orange)
- Three soldered to one of the two 6522 Versatile Interface Adapters (red, brown, black)
- Four soldered to the Sideways configuration jumpers (red, orange, yellow, green)
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:
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 computer should not run with an obviously disconnected wire. And yes, that chip is indeed the processor.