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homebrew computer - where to start?

Discussion in 'Electronic Basics' started by [email protected], Sep 3, 2005.

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  1. Guest


    i've been a dedicated computer nerd for about twelve years now. i
    started with a cast-off tandy "laptop" that ran BASIC and have been
    hacking ever since, through 386/486/pentium/etc.

    now i find myself much more interested in computer systems that appear
    historically before my introduction to computers, and i'd really like
    to get to know these older computing methodologies more intimately.
    i'd like to try to build a computer from scratch - build my own
    processor, etc. - to gain a greater familiarity with the underlying

    so where do i start? poking around on the internet, all i can seem to
    find is vendors trying to sell "homebrew" computer parts which
    basically involves piecing together readymade components.

    i know that the definition of "computer" covers a pretty wide continuum
    right now, but what i'm interested in building is just the basic
    machine: an electronic device that runs programs, whether it has a
    display, printer, or just an array of LEDs as its output.

    maybe someone knows a book or something that covers this material.

    thanks for your time!

  2. jm

    jm Guest

    I doubt you're talking something so basic as the once offered (perhaps still
    is) Radio Shack computer for kids. It had a bunch of LEDs that flashed with
    logic signals of sorts. Even further advanced but on same level as LEDs
    flashing or whatever - was the SWTP computer. SWTP - South West Technical
    Products. A box about the size of a bread box or so - with LEDs on the front
    (or an LCD readout - can't quite recall which) and a bunch of switches on
    the outside. In a subsequent model I believe they offered a keyboard or pad.
    I'm now 48 and this was all when I was 16 or so, so memory is faint there.
    That "would" take you back to the very basics. From there came the Tandy
    computer which I believe you've had already, as well as the Vic and
    Commodore models and the TI-99 and then Apples. Someone with a better memory
    than mine can elaborate. The Vic and TI-99 were pretty basic if memory
    serves me correct. Heathkit/Zenith also offered such a computer set up as I
    believe you're looking for. Maybe you can find one on E-Bay. National Radio
    Institute (NRI) did a computer something like this for their course in
    computers. I seen one recently - unbuilt with books on E-Bay. Not sure what
    it sold for. Lafayette Radio offered a similar version of the Radio Shack

  3. What an easy life you've had! In my day, I had to build it up solder
    joint by solder joint. 256 bytes of static RAM was all I could afford
    on the board. Metal bat handle switches developed calluses on your
    fingers. ASCII was entered with 7 switches and a push button -- those
    keyboards were for the silver-spoon, fancy-pants rich boys. ;) Used
    an AM radio nearby to help debug the programs by listening to the
    Well, you are in great luck, to be honest. There are good books today
    as well as cheap FPGA boards and free software you can use to lay out
    your own cpu. You almost won't even need a soldering iron.
    Try "Bebop BYTES Back: An Unconventional Guide to Computers" from
    Doone Publications as a very good and very detailed introduction. For
    an introduction to VHDL and Verilog, try "HDL Chip Design" by Douglas
    Smith (who, last I heard, had moved from England to Alabama.) I'd
    recommend looking on the web for FPGA boards -- there is BurchEd and
    Xilinx and a variety of others making such boards. Some of them are
    in the $50 to $100 range and very, very good. Make sure you get
    software with them for writing VHDL or Verilog and where you can do
    floorplanning, later on (not now), and the like.

    I'd recommend starting out with a simple board that includes some
    switches and some 7-segment LED displays. Try writing some VHDL code
    to perform various simple functions, like binary addition. That will
    get you started on some of the basic features of an ALU (and you won't
    need to understand sequential logic in VHDL, yet.) Work on busing
    data around, addressing and registers, etc. You can use the internal
    RAM in the FPGA for your program and data memory.

    Just get doing it. It's not hard to do something good enough to get
    the basic ideas across and learn from. It is so much easier to do,

  4. Michael Gray

    Michael Gray Guest

    Build your own processor?
    Are you sure?
    How much time and money do you have?
  5. Rich Webb

    Rich Webb Guest

    How far back (and why?)? You could start with Babbage's Difference
    Engine, I suppose.

    And then there's the IMSAI, still alive over at
    I always thought they looked very "techy" but ended up getting a
    Digital Group Z-80 system instead. Blazing 4.5 MHz, 18 KB static RAM:
    What I'd really recommend is a quick course in Verilog or VHDL and an
    inexpensive development board like this (there are others out there, of

  6. Well, Jake,

    A starting point is a matter of choice. Some early calculating machines used
    cogwheels and functioned pure mechanicaly. The first electronic computers
    used electron tubes by the dozens and required more power then your mains
    connection can provide. The first computer I worked on was build with
    discrete transistors, ferrite cores and lots of wire all packed in five 19"
    rack enclosures higher then a mans length. The first one I build for myself
    has a Z80 processor on 4MHz, 2k of RAM and 2k of EPROM. The latter contained
    a monitor program derived from the NASCOM. I build, also from scratch, a
    separate I/O card for it containing a UART that communicated with a dumb
    terminal. The next step was an I/O card that could write to - and read from
    cassette tape. Still works when I hook up a PC running a terminal emulator.
    At about the same time you could buy Apples or one of its clones. You could
    buy an empty board and fill it with components. Some time later you could
    buy empty PC- and peripheral boards to do the same. AFAIK the last computer
    building that required soldering. These days you can assemble your own
    machine even without a screwdriver. IMHO you can't go back but to the first
    microprocessors like the 8085, Z80, 6800, 6502 and some others I don't know
    well. Some stuff, like the Z80, is still available. Don't know about the
    others. Nevertheless, I don't think this is the way to go. The old times
    will not come back you know. I advise to look around in the world of
    microcontrollers. They have processor, RAM, ROM and I/O in one package but
    fiddling with the bits, assembler programming and even soldering are still
    required. There is a wide range of them from six pins SOT-23 to forty and
    more pins DIP all with eight bits processors. The latter at least as
    powerfull as the old Z80 and its contemporaries. If you want more there are
    much more powerfull sixteen bits micros available as well. You'll find more
    info then you ever can read on the web but you still can do plenty of things
    others have not done before.

    petrus bitbyter
  7. Charles Jean

    Charles Jean Guest

    If you can stand starting at the microprocessor level, try getting
    your hands on "Build Your Own Z80 Computer" by Steve Ciarcia, BYTE
    Books, McGraw-Hill, circa 1981, ISBN 0-07-010962-1.

    Sounds like it might be just what your 'e looking for.

    "Sic hoc legere scis nimium eruditionis habes."
    (If you can read this, you're overeducated.)
  8. My first build-up was the Altair 8800 from a kit of parts. Years
    before the Z80 was available. I remember looking with envy at the
    8085 when it finally came out, because it had been so simplified from
    the complex clocking monster that the 8080 was. And that was before
    the Z80. The Z80 was so simple to design a board for. Nice!
    Well, I've some 65SC02's laying around. And I'm sure I could find an
    8080A and a clocking chip for it, if I looked. ;)
    The only problem I have with your answer to the OP is that the OP
    specifically asked this:

    It was the 'build my own processor' part of the above phrase that
    really sounded to me like asking about how to design an ALU with
    latched input paths and output, access to registers for latching, a
    memory address latch, tri-state buffers on a common internal bus, etc.
    I could have been wrong, though.

    But that is a great deal of fun to learn about and do -- especially in
    this day of cheap FPGA boards from a variety of sources in attractive
    variations on a theme. It's almost dead simple to get started now
    writing VHDL and getting a simple cpu up and going, entirely of your
    own making. You don't even have to worry much about routing and floor
    planning, as the tools will do a uniformly lousy job automatically but
    one that still gets something working for you. And with the huge
    resources available on these chips today, who cares if it all gets
    ruthlessly squandered by the planner while you are learning VHDL?

    It will be interesting to see if the OP is really more about learning
    the internal basics of how a cpu works inside or more about learning
    the larger picture of a cpu-memory-i/o system. There are so many
    choices now that can teach at almost any level of interest -- from
    internal cpu design all the way up to pasting down one-chip-wonders
    that include all the I/O, code space, data space, and cpu processing
    power and leave very little left to learn about.

  9. I've never looked closely at the book, but my impression was that it
    was to build a working system, circa whatever year it came out.

    No matter how fancy a system was back then, it's going to be nothing
    compared to what you can find in the garbage today. Thus I think
    there's little reason to build a computer to that extent.

    I think there's still plenty of reason to have something on the level
    of the KIM-1, ie a calculator style keyboard and readout, a good monitor
    that can single step etc, a cassette interface to save programs, and
    some sort of general purpose I/O so you can play with things. There
    were plenty of such single board computers back then, I use the KIM-1
    as an example because I had one as my first computer.

    But the single stepping meant you could run programs and see what
    happened at every step. You could even just put in a line of code,
    and check that out, really useful to getting a real feel for
    what the op-codes were supposed to do. Since there was so little
    in there, you didn't need to learn a whole lot of GUI stuff before
    you tried out your simple program to add some numbers. The monitor
    did have what you needed.

    The general I/O meant that you could play with real things, like
    hook some LEDs onto it and learn how to control them. Or have
    inputs from something, to control the program.

    These computers are of such a simple level that they are easy to build,
    and you will get ample use of it (as opposed to trying to build a full
    blown S-100 bus computer from 1976, which would have real limitations
    today if you wanted to run applications). In some ways, it's even easier,
    because whereas the KIM-1 needed 8 ICs to get 1K of memory, you can
    scrounge up a static RAM that fills the address space that will draw
    less current and require much less wiring. If you can live with
    hooking it up to a terminal (ie your home computer running a terminal
    emulator), then you can toss the readout and keyboard. Many of
    those old boards allowed for hooking up a terminal to an RS-232 port,
    though many of us didn't since the terminal cost more than the computer.
    Likewise, one could use the home computer to store the programs, which
    beats out a cassette interface in terms of speed and reliability.

    ONe great project from the era was in Byte, though I can't remember the
    year, or even a general idea. 1978 and 1980 somehow come to mind. Someone
    wrote an article about bootstrapping an 8085. Jam a NOP onto the data
    bus, so the processor advances the address bus while doing nothing. By
    single stepping, this means you can load the RAM without any bootstrap
    ROM. The input mechanism was a piece of circuit board that he'd carved
    up with a hacksaw to make pads, and a "stylus" to connect a wire to
    the needed pad. Some LEDs on the data bus. Not much else. Build
    up the simple hardware, he just used wire to do it if I recall, and
    you can start playing right away. None of that fussing with a bootstrap
    ROM, but something you can infinitely play with it. Get some coding in,
    and then add that monitor ROM. Or dig up the listing from an old single
    board computer, and use that.

    SOme years later, Byte put out a book about the 68000, and it had
    an article (which I think had never run in Byte because I never
    saw it elsewhere) using similar techniques to bootstrap the 68000.

  10. jm

    jm Guest

    Not "really" being wrapped up in computers back when the Z80, the Altair and
    so on came out, I had forgotten about the KIM and so on. My question is -
    trying to sort out my memory here - was the Altair 8800 sold be SWTP? OR a
    different monster altogether? I DO recall the Altair now that it has been
    mentioned - but may be getting it confused with South West Technical
    Products - products.

  11. Art

    Art Guest

    May even run across some links to the Altair 8080 systems that preceeded
    many "user Friendly" devices. These were once offeres in a kit of parts and
    fair instruction manual. Basic, if my memory serves me, was the op-sys du
    jour. Others may corect me if in error?? Item I had actually used Octal
    Nixie Tubes for Hex Dec output which we manually decoded.
  12. Tha Altair came first, being on the cover of Popular Electronics for
    January 1975.

    That's what set off the whole thing.

    Of course, there was the Mark-8, an 8008-based computer that was on
    the cover of Radio Electronics for August 1974, but while there was
    interest in that, it somehow did not get the excitement and the attention
    that the Altair did.

    Likewise, Scelbi had an 8008-based computer before the Altair, I can't
    remember exactly when, but at least for me, and likely others, it
    wasn't noticed till after the Altair hit. I know the Scelbi was advertised
    in the back of QST, but I only saw the ad when I looked for it after
    the Altair hit the market and the ad was mentioned.

    The SWTP 6800 came a bit later, I can't remember exactly when, but
    of course it had the advantage that it was terminal based. No fiddling
    with switches and strings of LEDs on the front panel, and surely their
    absence made it simpler to build.

  13. The Altair 8800 was 'advertised' on two issues of Popular Electronics
    circa the beginning of 1975. You can see it here:

    I bought mine in January 1975 and assembled it over the next month or
    so. It was sold my MITS, which was a company that used to make
    calculator kits, back in the days when calculators were expensive and
    selling kits actually made some money. They also had contracts with
    NASA, I think. But after the huge budget cuts of 1970, after we got
    to the moon, and the gasoline crisis, all hell broke loose and
    companies like MITS went into red ink and started looking for
    something else to do. Luckily, Intel had just had Intersil drop a
    contract (but largely paid for, already) for a cpu for a terminal, for
    which they were making the 8008, if I recall. Intel was rolling that
    into the 8080 to sell as a micro just about the time MITS was starving
    and looking for work. The Altair 8800 was the result.

    But I'm open to corrections on the above. That's just what my poor
    memory reminds me of.

    SWTP was, at least from my point of view, a later-on company. I
    hadn't heard of them until well after the Altair 8800 assembly I did.

  14. 8800.

  15. No, MITS had nothing to do with NASA.

    They were a small company that started by selling some rocketry related
    electronic kits, and used "MITS" to suggest MIT/Massachusets Institute
    of Technology.

    They went the usual route, selling kits and counting on articles in
    the hobby electronic magazines to run articles (which they wrote)
    as promotion.

    When calculators were feasible, they started making them as kits.
    The November 1971 issue of Popular Electronics has their calculator,
    I think it was their first, on the cover. They were a new thing
    at the time, and even though expensive they were still a good price
    if one needed such a thing, compared to the cost of what came before.

    But as things evolved, the big companies came into the market, just
    as with digital watches (I can't recall if MITS made those) where
    small companies were in early and then lost to big companies. MITS
    couldn't compete with TI and the other big companies in the calculator
    business after they came into the field, and then nearly went bankrupt
    as a result. Legend says the Altair was a last minute attempt to
    save the company.

    It wasn't Intersil, it was Busicom that wanted a calculator IC. Intel
    convinced them a general purpose CPU would be the best solution, and
    the result was the 4004. The 8008 was developed for a terminal
    manufacturer, but not Intersil, who of course manufactured
    seminconductors, mostly analog. By the time they came out with the 8080,
    they weren't designing it for anyone.

    And as I've pointed out, others had already come out with "home computers"
    before the Altair, and while we hobbyists likely hadn't hard of
    the 8080 before the Altair hit the cover of Popular Electronics,
    I think they already had a level of familiarity and usage in professional

  16. But one thing about the Altair is that it needed so much to get going.
    The front panel (which wasn't just the switches and the LEDs, but I
    gather rather extensive TTL), and the CPU board had no memory so
    you needed an extra board for that, and of course then you needed
    the motherboard so you could connect the boards.

    That bootstrapping project leaves it relatively simple. Even something
    like the SWTP 6800 CPU board could be standalone I think,
    because it had memory however tiny and the monitor in ROM and a software

    One reason the Cosmac Elf was so popular, relative speaking, was that it
    didn't take many parts to wire up, and you could have it running as soon
    as it was wired. It was made for bootstrapping without ROM. The problem
    there is that 1802's were never that common back then, and scrounging one
    up will likely require much effort at this point.


  17. SWTP was already in the kit business before kit computers hit the
    market. They sold kits of parts for a lot of Popular Electronics
    articles from the late '60s, on.

  18. MITS was "Micro Instrumentation Telemetry Systems".

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  19. Okay. Then it was only the ordering of my own experience here and not
    fact. Thanks!

  20. My memory is definitely fading! Thanks! I seem to recall reading
    some 'history' on MITS circa the late 1970's that included some
    comment along those lines. But I appreciate the correction and I'll
    try and kill that 'recollection' of mine!
    Ah! Perhaps that was what my memory distorted over the years...
    I remember seeing some tiny partial-column ads from them and thinking
    about buying one. But it was too much for me at the time. I just
    kept using a slide rule.
    I've heard that, too.
    No, I think you are thinking earlier than I was thinking. Isn't
    Busicom a Japanese company that started the whole ball rolling with

    Anyway, I was thinking actually later than the late 1960's. And it
    was definitely something to replace a lot of discrete logic in the
    body of a glass terminal. Not a calculator.
    Fully agreed, here.
    Who was it, then? Do you know?
    Yes, that's what I have heard as well. Something along the lines of
    'productizing' what they had left off with on the terminal project.
    Since I was just 19 at the time, I wasn't in those circles.


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