The Acorn 6502 Microcomputer Kit An Interview with the Designer

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The Acorn Microcomputer was designed by Sophie Wilson, then an undergraduate at the University of Cambridge, England. Here, she answers some questions about those early days. See also the timeline and people overview.

  • How did the Acorn Microcomputer get started?

    I had designed something similar for myself, and was in the process of helping Hermann [Hauser] with his ideas for an ‘electronic pocket book’ (what we might nowadays call a PDA). In the course of showing that my designs for it would work, I showed him my schematics for my own machine and was challenged to build it. So I did… With my own white ceramic 6502, too. That was “just” the equivalent of the CPU board of the System 1 with LEDs and keyboard (all on the same bit of Veroboard) – the cassette interface was added later. I think Hermann was overly impressed when it worked first time!

  • No bugs in the monitor program?

    No bugs in the first, smaller, version of the software (256 bytes of code blown into PROM by Nick Toop’s PROM blower). There were a couple of problems with the cassette interface software because I got the order of bits the wrong way round (from the CUTS¹ standard). But hey, it was only 512 bytes: you can’t make any mistakes in that even when you write it by hand, hand assemble it and hand enter it into the PROM blower! Besides, it could debug itself to a fair extent (given that it basically worked).

  • Was the design based on/derived from an earlier machine? Kim-1, Apple I, etc.?

    Not exactly based on anything. Most of its heritage was from an automated cow feeder that I’d designed for a Harrogate company the previous summer (1977). Quite an advanced thing, really – it had a (waterproof) number pad, big 7 segment LEDs, OS in non-volatile EEPROM, and the trademark 6502. Both were from my own designs for something for myself, and they came from the aether.

    The most hair-raising thing was the cow-feeder’s programme. I didn’t own a PROM blower, so I had to write the whole thing by hand and send it off to a company who hand entered it into a machine and sent me back the PROM. That worked first time, too. Mind you, it was even smaller, being a boot loader that allowed the cow-feeder’s EEPROM to be initialised.

  • You were an undergraduate student at Cambridge then. What were you studying?

    Maths, followed by Computer Science.

  • Why did you choose to design the computer around the MOS Technologies 6502 microprocessor?

    Because it was there. Because it was new. Because a few of the other members of the Cambridge University Processor Group [CUPG] were going on about it being easier to interface to circuits. I guess I don’t really know precisely why I chose the 6502 – maybe we just had an affinity!

    It cost quite a lot back in 1977/78 when I bought mine – which was a wonderful white ceramic part with gold (coloured?) legs and lid.

  • Why did you use the RAM I/O chip, instead of a UART or something similar, when the machine had separate RAM?

    Because Hermann had them around: Science of Cambridge used 8154s on its MK14 kit (National SC/MP based) and so they were available when we needed something for the 6502. They were fairly cheap and the extra RAM was a bonus, even though it meant converting from 6502 clock/write to the read strobe/write strobe that they used.

  • Hermann Hauser (from Kings College, Cambridge University) had recently founded Acorn Computers Limited in Cambridge, with Chris Curry, correct?

    Actually, that came later. The initial work was done for Hermann’s own company “Cambridge Processor Unit” (that’s an Austrian’s idea of a joke). Hermann went for the System One and came up somehow with the Acorn name, then Clive and Chris had an argument and Chris left Science of Cambridge and joined Hermann at Acorn: the first thing that we worked on with Chris was the Atom.

    At the start, CPU had consultancy contracts for fruit machines. Initially these had been SC/MP based, but they got moved to 6502s. I was first approached by Hermann at a CUPG meeting – he wanted someone who knew about low power technology, since he had this idea for a “electronic notebook”. I designed an anti-theft device for the fruit machines (piezo lighters [were] being used to knock out electronic devices, so I put in a wideband radio receiver to stop the fruit machine paying out mistakenly: later on the acceptance test for the machine involved it being plugged into the same power line as an arc welder and sparks being struck – it passed!). After that Hermann wanted to see my designs that might work for the electronic notebook and asked “will it work?” “Of course” “so build it”.

  • When did you build the prototype?

    Summer holidays, 1978. Then I went home and drew circuit boards on the dining room table (and floor!) and wrote the manual. All by hand, of course.

    Christmas 1978 I must have written System BASIC.

  • When did you first show it to Hermann Hauser?

    During the time it was built! Hermann was very interested in it. It certainly worked before I went back to Yorkshire before the start of the term.

  • Who designed the hardware?

    Me for the bottom board. Me, Stephen Furber and maybe Kim Spence-Jones for the top board (cassette interface). (Hmmm – maybe KSJ was a little later – he certainly did some of the work on the analogue bits of the BBC machine cassette interface.)

  • Who drew the Schematics? [initials: CBT]

    Christopher Brian Turner aka Chris Turner: became Chief Engineer of Acorn.

  • Who laid out the circuit boards?

    Me for the bottom board. External company laid out the top board.

  • What else do you remember about the Acorn System 1?

    Packing them in boxes (upstairs at 4a Market Hill): the whole company would stand around tables (a production square) and put in the right components (me, Hermann, Hermann’s then fiancée, Stephen, Chris). We all did pretty much anything: I ended up as Hermann’s secretary before we could afford one!

    There used to be problems with answering the phone: one chap would ring up and say “I have got an Acorn, it does not work” often enough for it to become a legend. We got very tired of kits – the highlight being a guy who assembled his Atom with glue because he knew that heat (solder) would damage them – so that coloured the BBC machine a lot.

  • (The Acorn Microcomputer kit was based on the Eurocard printed circuit board format, which meant that it was readily extended. Acorn went on to produce a series of machines, the System 2 through the System 5, based on the format. Acorn then used elements of the design for the Acorn Atom in March 1980, one of the first home machines that came with a QWERTY keyboard and a case.)

    When was the last Acorn System x shipped?

    We designed the BBC machine using System 3s (I did a lot of character design work with a prototype System 80 column video card) and still had System 4/5 stuff going on in 1982 – perhaps then.

¹ CUTS: Computer Users Tape Standard.

[Interview edited from e-mail conversations between Mike Cowlishaw and Sophie Wilson, in January and February 2002. Thanks, Sophie.]