[vcf-midatlantic] Homebrew Computer Systems from Bob the donor

Jeffrey Brace jeffrey at vcfed.org
Thu Jun 24 01:01:22 UTC 2021


>From "Bob the donor":

"{FirstComputerAssembled.jpg} is what it looked like in working order, not
just the box of parts I sent with bernie.  The rocker switches were for
entering the boot-loader for the cassette interface, which used a 565 PLL
to convert the FSK audio to bits for the UART.  IIRC, it used the same
tones as were used on the ham bands for TeleType chats (because I also did
that).

{FirstComputerCircuitBoardsTop.jpg}  This was my first computer because I
was introduced to the MOS Tech 6502 in 1975 and thot it was Really Neat.
That IC cost me $25 back then; it's the 40 pin white ceramic item on the
'middle' board.  The half board, far left, is the cassette interface.  To
its right is the memory board (note the 2102s, partially installed at this
point).  There was only 2 KB of RAM, both because of the price and also my
rather small programs.  Various 'glue bugs' are below the RAM and on the
next-right board with the processor.  Below the uP are parallel interface
ICs and the UART.

  The rightmost board is the video generator, which used a
character-generator IC (the largest bug).  It required another 2 KB of
2102s, used in two banks that alternated access, because they were so
slow.  IIRC, the display was 32 lines of 64 characters.  I later removed
this part, added a UART and powersupply so it worked as a standalone video
display.  That was in the box of parts, and should still produce
serial-to-text.

{FirstComputerCircuitBoardWiring.jpg}  Is pretty self-explanatory.  A mouse
would be right at home!  I added a few more wires every evening after it
was too dark to continue finishing the house construction.

Entering the boot-loader every time quickly became a bother so I soon added
some NiCads and a small secondary powersupply to keep the main 2KB alive.

It was quite a project for how little I did with it; a word processor that
could output thru a Model 19 TTY, and also display ham RTTY when I didnt
want the noise of the 19.  It turned out that I was far more interested in
building than in coding, which is why this computer didnt last and you got
only a box of some parts of it.

The computer in the metal case is an Apple 2 clone board.  As you've
discovered, I had to 'bend' the board to make it fit the neat enclosure.
The price of Apple (or any) 5.25" drives was high, which is why I 'went
hardware' again and installed an 8-Track drive and circuitry to make use of
it.

A larger pulley on the motor provided a faster tape speed (probably around
8 or 9 ips, but I dont remember exactly), dual UARTS read/write to the
stereo head (writing square waves at this speed required something like 45
volts IIRC).  This required special code, which was in the battery-backed
CMOS RAMs that replaced the original Apple ROMs.  I copied the useful parts
of those ROMS into the RAM so the computer still behaved like an A-2, but
my code and the 8-track 'imaged' the RAM and system state, then restore it
all on demand.  No doubt the battery is long dead from decades in storage,
so it's all junk now.

As bernie and I were loading it into his car, I noticed the back-panel
connector for a paper-tape drive.  I no longer have that or I would have
included it.

So far, I have not found any code listings or diagrams, but will forward
anything that turns up."

Here are the pictures folder for anyone interested with the new pictures
added:
https://drive.google.com/drive/folders/1DhjdsPnDN9s5vkCTalxUilHZGvctiVtt?usp=sharing

=========================================
Jeff Brace
Vice President & Board Member
Vintage Computer Festival East Show-runner
Vintage Computer Federation is a 501c3 charity
http://www.vcfed.org/
jeffrey at vcfed.org


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