S100 Z-80
I then built an S-100 system, with commercial
backplane, memory and an 8080 CPU, quickly moving
up to a Z-80. First, I just had a borrowed terminal and
a paper tape reader, then I built my own serial
glass TTY by copying a commercial CRT terminal
logic in wire-wrap. I think it was a copy of
a Beehive model. I then got a Dura typewriter that
had a parallel electrical interface as well as
paper tape reader and punch. It was an IBM
Selectric made for use with computers and word
processors. Slow and noisy, but it could
print out a program listing. I got a Persci
dual floppy drive with a voice coil head positioner.
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Circa 1978, from my apartment in Audubon Park
(Brentwood, MO).
At the upper right is the paper tape reader.
Below that is the power supply for the Persci
dual floppy disk drive, which is to the left
of the P.S. An HP power supply
below that, then the black rectangle is the
Z-80 console LED display. Below that is the
S-100 bus backplane and cards.
Below the Persci floppy drive is a shortwave
listening receiver and an RTTY converter.
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10 MB Winchester hard drive
From an ad in EDN or similar magazine, I was able
to buy a Winchester disk with SASI controller from
Memorex on an introductory offer. I think it cost
$1600, which was a lot of money, but it was a great unit.
I wrote a software driver and simple parallel interface
to emulate the SASI bus, the Z-80 had instructions
that could read/write a sequence of bytes from an I/O
register to/from memory in one instruction, thes was
great for this purpose. I discovered you had to
read only 256 bytes at a time and then break out of
the transfer and check the ready status, as some
of the 1K byte CP/M blocks could straddle tracks
on the drive and it wouldn't be ready. It performed
SO much more reliably than floppies, as well as being
faster and capable of holding 10 MB! WOW, that was
a lot of disk for a home computer in 1981 or so!
I had a major accident while still setting it up, the
spindle drive transistors were on the disk drive, and
had 24 V on them. These touched the SASI controller
board with sparks and smoke. I started debugging with
the finger method. When your finger got burned,
replace that IC. I then had to use a scope to look for
bad TTL levels, and hoped the microcode ROMs didn't
get fried. They didn't, and after a few more iterations
it fired up again. I quickly made sure proper standoffs
were keeping the boards from ever touching again.
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This is my home computer room, perhaps about 1983?
I have labeled a few things. At the very left is a
shortwave receiver (on its side at that moment) sitting
on top of a short equipment rack, and a navy RTTY
converter below it. On the table, at left there is a
color graphics terminal (really a piece of junk, I
was lucky to get some cash for that boat anchor).
In the middle is an IBM 3101 CRT terminal.
To the right edge,below the big tape drive is the S-100
system. I never got that Bucode tape drive connected
to a computer, although I got the capstan, vacuum columns
and reel servos working.
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X-Y CRT display
I also got a Sanders 14" X-Y display that was used in
an old hospital info system. It was basically a computer
screen CRT with slow phosphor, but used X and Y
linear amps to drive the beam around, and a simple
character generator. I hooked up some 12-bit DACs
and a photomultiplier tube with a fiber optic
pigtail to use as a light pen. I had a program that
displayed 10 numbers on the screen. When you pointed
the light pen at a number, it would turn into a tracking
cross and you could drag the number around the screen.
When you arranged all the numbers as you wanted, you
hit a key and it calculated 8 Bezier splines through the
control points. I also had a program that could
create the points on some common mathematical curves,
I think one was the Lemniscate of Pascal. It took about
a minute to compute the list of points, and then drew
them at maybe a 1 Hz update rate.
Printers
At my home computer, I got a Centronics 101
dot matrix printer (it was one of the first,
with an all-steel cabinet and clutches that
drove the print head back and forth).
I quickly traded it with a guy that had a HUGE
Honeywell drum printer with attached tape drive,
used as an off-line printer at State Farm Insurance.
I got it running and made some test tapes at work
to make it print something. While fooling around
with it, I did get it to dump the last page
printed in its previous life, which was a dunning
message to some poor schnuck who hadn't paid his
bill. The printer had a core memory as the line
buffer, and the last contents were still in it.
Wish I could find that page, but I haven't seen it
in a long time.
After getting some idea of the signals between the
tape drive and the printer, and having all the docs
too, I managed to get it working on the CP/M
system. It was so huge it had to stay in the basement,
so I had to run down, turn it on, run upstairs,
print out whetever files I needed to, and then
run down to get the printout. But, it was incredibly
cool to have a 300 line/minute printer on a home
computer!
9-track tape drive on my home system
I also got a Pertec key to tape system from
surplus. It had a stock 7" Pertec 800 BPI
tape drive and a controller with a keyboard and
a field of lights for every upper-case ASCII
character. So, you could read a tape and
step through the contents a character at a time,
and enter data into memory and then write a record
to tape. You can see it in the picture above,
to the left of the lower reel of the red tape drive.
Above it is the original rack of cards that came
with it. I think I pulled about the first 4
cards and used the remaining ones.
I found a convenient place to cut into the system
and built an interface to read and write data
to the drive. The Z-80 couldn't handle read after
write, so it backed up and reread every block
after writing it. I would write out a tape
every night and take it into work to check the
quality of the tape format. After a few days, it
was basically producing perfect ANSI-D tapes
compatible with the VAX. I used it mostly for
backups. So, I had a 300 LPM line printer,
10 MB Winchester disk and 9-track mag tape
on a CP/M system!