MRAB wrote:
Mk14 from Science of Cambridge, a kit with hex keypad and 7-segment display, which I had to solder together, and also make my own power supply. I had the extra RAM and the I/O chip, so that's 256B (including the memory used by the monitor) + 256B additional RAM + 128B more in the I/O chip.
Luxury! Mine was a Miniscamp, based on a design published in Electronics Australia in the 70s. 256 bytes RAM, 8 switches for input, 8 LEDs for output. No ROM -- program had to be toggled in each time. Looked something like this: http://oldcomputermuseum.com/mini_scamp.html except that mine wasn't built from a kit and didn't look quite as professional as that one. It got expanded in various ways, of course ("hacked" would be a more accurate word). Memory expanded to 1.5KB, hex keyboard and display (built into an old calculator case), cassette tape interface based on a circuit scrounged from another magazine article (never quite got it to work properly, wouldn't go at more than about 4 bytes/sec, probably because I used resistors and capacitors salvaged from old TV sets). Still no ROM, though. Had to toggle in a bootstrap to load the keyboard/display monitor (256 bytes) from tape. Somewhere along the way I replaced the CPU with a 6800 - much nicer instruction set! (Note for newtimers -- that's *two* zeroes, not three.) During that period, my holy grail was alphanumeric I/O. I was envious of people who wrote articles about hooking surplus teleprinters, paper tape equipment and other such cool hardware to their homebrew micros -- sadly, no such thing was available in NZ. Then one day a breakthrough came -- a relative who worked in the telephone business (government-owned in NZ at the time) managed to get me an old teleprinter. It was Baudot, not ASCII, which meant uppercase only, not much punctuation, and an annoyingly stateful protocol involving letters/figures shift characters, but it was heaps better than nothing. A bit of hackery, of both hardware and software varieties, and I got it working. It was as noisy as hell, but I could input and output ACTUAL LETTERS! It was AMAZING! As a proof of concept, I wrote an extremely small BASIC interpreter that used one-character keywords. The amount of room left over for a program was even smaller, making it completely useless. But it worked, and I had fun writing it. One thing I never really got a grip on with that computer was a decent means of program storage. Towards the end of it's life, I was experimenting with trying to turn an old 8-track cartridge player into a random access block storage device, using a tape loop. I actually got it to work, more or less, and wrote a small "TOS" (Tape Operating System) for it that could store and retrieve files. But it was never reliable enough to be practical. By that stage, umpteen layers of hackery using extremely dubious construction techniques had turned the machine into something of a Frankenstein monster. Calling it a bird's nest would have been an insult to most birds. I wish I'd taken some photos, they would have been good for scaring potential future grandchildren. My next computer was a Dick Smith Super 80 (*not* System 80, which would have been a much better machine), Z80-based, built from a kit. I had a lot of fun hacking around with that, too... but that's another story! -- Greg -- http://mail.python.org/mailman/listinfo/python-list
