My four-yorkshireprogrammers contribution

Gregory Ewing greg.ewing at canterbury.ac.nz
Thu Mar 4 11:01:06 CET 2010


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



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