What BBC Microcomputer features were requested specifically by BBC engineers?

Question

When requirements for the BBC Microcomputer were developed as part of the BBC Computer Literacy Project, an outline specification was created by an independent consultant, the late John Coll. This specification was then fleshed out by the BBC and given to manufacturers who were interested in tendering for the project. Acorn Computers subsequently won the contract with their innovative design, and the rest, as they say, is history.

Now, whilst many of the machine's features were specified either by Acorn (for their own needs) or by the BBC (as required for the literacy project), some features were specified by BBC engineers entirely for their own requirements. One infamous (and well-documented) example of this was the requirement for linear power supplies to prevent (supposed) interference with broadcast equipment when used in television studios. Since the linear PSUs ran very hot, this requirement was reluctantly dropped by the BBC, and switched-mode PSUs were subsequently used.

I remember reading years ago that other requirements were specified by BBC engineers, but I cannot remember where I read it or what they were. One fairly obvious facility would have been the possibility of providing an external Genlock signal (via an add-in board), so that the video output could be synced with the studio equipment, and such boards were manufactured by various companies. However, I would love to know if any other requirements were specified by BBC engineers for their own purposes.

Answer 1

Linear power supply was not a BBC engineering requirement by the time I joined Designs Dept in late 1981. Perhaps it was dropped well before actual production; I don't know if that was pushback from Acorn or experience with a prototype.

But by late 1981, shipping some with linear PSUs was an unhappy compromise caused by : (a) demand much higher than expected and (b) designing the SMPS and getting it into production would be a couple of months late (and miss the pre-Christmas launch altogether).

So, to keep the overheating down a bit, the first production was Model A, with half the memory, and slightly less power consumption.

I never saw a BBC Micro with 8" floppies though they were ubiquitous in the BBC at the time, as well as standard for the "Richard Russell Board" (his own home computer design, Z80 based, which grew up to a 64k CP/M system. About 100 of these were made, mostly by BBC engineers. I built one, instead of getting a BBC Micro).

Also the "disk connector shall be a 34-way type" in the hardware spec (linked in the other answer) strongly hints that 5.25 inch floppies were intended; 8" drives generally used a 50 way connector.

One aspect which (if I remember correctly) was a BBC requirement - probably Richard's own - was that BBC Basic had to have a useful block structured syntax, as far as could be achieved while still being BASIC. The aim was, for an educational machine, to teach best practice at the time. You could program in a Pascal-like style that was a considerable advance on other BASIC dialects around then, and it's probably why BBC Basic still has its fans today. This was probably the most important and far reaching BBC requirement in its impact on users, who mostly did at least some programming - and perhaps on the UK as a whole.

(Source : Richard Russell was my project leader at the time).

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I've been asked to expand on the Richard Russell Board.

It was probably unknown outside the BBC, and slightly pre-dated the BBC Micro, it was fairly new when I joined in 1981.

Sadly, I left mine in an attic about 15 years ago during a house move. Mine actually turned out to be a bootleg - apparently Richard loaned the PCB masters to Research Dept and they made some in house, one was left over. It wasn't actually used at the BBC per se, it was more of a homer "home office" in BBC jargon) project, but it inspired the ZEUS modular system which ran a lot of the BBC transmitters - and the broadcast clock and other graphics at the time. ZEUS modules configured for S/W development were known as ZELDA - Z80 Loader/Developer/Assembler - I think that (1981)was before Zelda was a game character.

The BBC had a fairly enlightened attitude to "home office" projects, as long as they were kept out of hours and didn't drain expensive components. They gained a lot out of the process, like the ZEUS (Z80 Universal System) following the RTR Board.

It was a SBC with 32K DRAM (4116, the original 3 rail type). These used +12V for the storage, +5V for the I/O and logic, and -5V at a ridiculously low current to bias the substrate. It was important that the -5V supply came up before the +12V, or all 16384 transistor turned on at once, with fatal consequences. I fitted a relay powered by -5V to connect the 12V supply.

Later, a second board had 32K more, either for monochrome high res bitmapped graphics, or 64K CP/M. But the main board used Z80, I think 8251 SIO, 8255 parallel I/O, and the ubiquitous 6845 for video. Two 64-way connectors along one side gave you everything. I may still have the schematic somewhere but can't find it just now.

Most had 8" floppy drives (dual was luxury!) and it booted to BBCDOS in under a second. BBCDOS was pretty simple, but robust. It used 6.1 filenames (6 character name, 1 character extension) and a robust chained sector file system. The last two bytes of a 128-byte sector were a pointer to the next sector. No interleaving, so a 2.5 MHz Z80 could read 6 tracks/second while the original IBM PC was throttled to about 1 track/second.

And yes, he wrote his own BBC Basic interpreter for it. It wasn't 100% compatible - naturally, the built-in assembler understood Z80 instructions rather than 6502. That went on to sell as a CP/M version, the ... wait for it ... Sinclair (!) Z88 portable, Tatung Einstein, and eventually ported to x86 for MS/DOS (still available).

Mine spent most of its time running CP/M, where I had the FTL Modula/2 compiler.

Answer 2

Seemingly predating that StarDot thread by a few years, I found this list of documents that includes "HardwareSpecification.txt", "BASICOutline.txt" and a few others that are "(c) Copyright BBC 1981" rather than copyright Acorn, or dated 1982 when the BBC Micro actually appeared. There are various details not from the StarDot thread, including:

• unused gate inputs will not be left open-circuit;
• with the sole exception of the Econet interface, integrated circuit sockets will be fitted in all unequipped positions;
• dimensions: 400mm wide by 300 mm deep by 60mm high approx;
• the case will be of two-part construction, the upper shell supporting the keyboard PCB and the lower shell the main PCB and power supply. Electrical connection between the two parts will be by flexible ribbon cable;
• [it is] essential that all points which could be at mains potential when power is applied be inaccessible to the "standard finger";
• it must be possible to generate all ASCII codes (0/0 to 7/15) by using the SHIFT and CTRL keys in conjunction with the other keys;
• a RESET (or BREAK) key will not be provided on the main keyboard;
• both pound sign and number symbol (hash) must be included ... the RETURN key will be a different colour from the rest;
• the power supply will withstand an overload indefinitely and will protect itself from damage through overheating, even at 260v mains. The power supply will NOT be a switched-mode type;
• a 1v pk-pk (75 ohms) composite video (PAL coded) output will be provided on a 75 ohm BNC socket accessible at the rear of the machine. The socket will not be fitted in the basic machine;
• a cassette modem will be incorporated ... the demodulator will be insensitive to input level variations of up to +6dB or -12dB and must recover the UAR/T clock from the tape in order to track short and long-term speed variations, It must cater for an instantaneous speed error of at least 10% WITHOUT relying on the inherent insensitivity to speed of asynchronous data, i.e. bit-centre sampling must be maintained;
• a parallel printer output to Centronics specifications will be provided. The 6522 I/O device will be fitted in the basic machine but the buffer components and connector need not be fitted as standard although provision will be made for these on the main PCB;
• provision will be made on the main PCB to fit a floppy disk controller plus data separator and buffer devices to allow interfacing to one or two mini-floppy or 8" floppy drives ... both hardware and software must be capable of supporting 8" disks to the IBM 3740 specification, although this may necessitate fitting an additional ROM which is not present in the basic machine. There is no requirement for double-density operation;
• an elapsed-time clock will be included in the basic machine, having a resolution of 10ms. The clock can be set and tested under software control;
• there will be 8 selectable display formats as follows ... (followed by exactly the BBC Micro's modes, including the requirement that 'text' modes be bitmaps);
• in modes 0 to 6 the "colours" are selectable from a palette of 16 effects being black, red, green, yellow, blue, magenta, cyan, white and the same eight colours automatically flashing;
• the CPU is a 6502A running at a 2MHz clock rate except when accessing some input-output devices, when the effective clock frequency is reduced to 1 MHz; and
• the level of R.F. radiation from the machine is to be minimised. The exact maximum level is to be agreed by the BBC.

It sounds to me like that document might be after Acorn has become the de facto supplier, given the specificity of the processor, the display modes, the implementation of econet, etc, but I'm pretty sure that even the Model A came with a composite BNC and that 8" disks were never supported, and certain that the return key ended up being the same colour as all the other keys and that break is not only present but shift+break is the intended way of launching almost all disk-based software. Various other specifications I haven't quoted also weren't eventually met. There's also still quite a lot like that final item that is reserved for later specification by or agreement with the BBC.

So this is a statement of specifications intended to be met, not a description of the machine as completed.