11 Jun
2014
11 Jun
'14
12:44 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Rick Bensene
Sent: 11 June 2014 17:15
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Amazing Score
The week before last, my wife and I took a trip to the Missoula, Montana
area
to do some dog-related activities (my wife competes
with our German
Shepherds in Canine Agility and Sheep Herding). Along with that, I had
recently re-connected with my old High School computer science teacher,
whom I hadn't seen since 1978 (I graduated in '77), who lives in the
Missoula
vicinity.
When we arrived at my teacher's home, it was like we picked up right where
we left off, even though many years had passed. It was so wonderful to
see
him again.
After we'd settled in and visited for a while, he took me down to his
basement,
and there sitting on a table were two IMSAI 8080
computers!
Back in my junior year of high school, I pestered my teacher to purchase a
computer kit to be built by the advanced computer class as an educational
project. I researched the available 8080-based computer kits (Altair and
IMSAI)
and was convinced that the IMSAI was a better choice
due to its more
robust
design. My teacher didn't have the money in his
budget to buy the kit, so
instead, he bought it with his own money - a clear indication of his
commitment
to education. He was an amazing teacher, and I credit
him for stirring
up, then
cultivating my interest in computers.
We received the kit late in the school year, and got started on building
it, with
the cardinal rule being that anything that was done
had to be
explained and understood by our teacher. We got through building the
power supply, chassis, and front panel, but didn't complete the CPU and
memory board (ROM/1K RAM) during my junior year. The machine was
finished up early in my senior year, and with a few minor issues (LED
installed
backwards, a couple of resistors that were swapped,
and a few bad solder
joints) it worked. It was a great learning experience for the team of
five kids,
including myself, that built it.
The school year after I graduated, a second kit was purchased, and the
same
process followed to build it, as well as a some
additional boards (memory,
video) purchased for the original machine to augment it.
The two machines sitting on the table were those two machines from the
school. My teacher had rescued them and took them home when he left the
school district, , and kept them all these years. The machines were
obsoleted
in short order when the school district purchased a
bunch of Apple II's
for a
more standardized computer curriculum across the
district.
When he showed the IMSAIs to me, he said "They are yours, take them home
with you". I was stunned, but also extremely happy and thankful.
The machines were a bit hodge-podged. One was stuffed with boards, and
the
other was minimalist.
The first machine, the one that I was involved with building, had a the
original
CPU, ROM/RAM, Front Panel, and 3P+S I/O board, along
with a Processor
Technology (PT) Video Board, PT CUTS cassette interface, a 32K DRAM board,
and two 16K RAM boards. The second machine had only the Front Panel,
original CPU board, and a 64K DRAM board.
The first machine was in nice shape, but three front panel toggles were
missing. The second machine had some scuffs on the front panel, but all
of the switch toggles were there and in good shape. Along with the
computers, there was a big stack of original documentation for them.
When I got home with these treasures, I focused on getting the machine
that
I'd
help build running. I first went through the power
supply, and found that
even
after all of these years, the big caps were still
good, and it was
delivering clean
unregulated voltages within specifications.
Then the front panel, CPU, and the ROM/RAM board were installed, and it
was
slowly powered up with a variac. It came up properly,
and after a RESET,
it was
clear that there were some issues. The front panel
was "twitchy",
operating
erratically. Setting an address in the front panel
switches and operating
EXAMINE would result in some bits being on where
the switches were off. The problems were fixed by reseating all of
the ICs on the front panel in their sockets...they were all secure, but
the
sockets
were not the best quality, and there was probably a
little oxidation here
and
there that caused some problems. A couple of the
switches were also a
little
flakey, which were traced to "less than
optimal" solder joints that were
easily
cleaned up.
After a little tinkering, I could reliably deposit data into the 1K
memory, and
examine it back accurately. A little memory test
program verified that
the
memory was working well, and that the CPU was working
as it should. I then added more memory, first installing the 32K DRAM
board and verifying it worked, then adding another 16K board and making
sure
that it worked. I then put the PT video board in, and
could poke data
into the
display memory from the front panel, and it too
worked
great. Given that three switch toggles were missing, I tried putting
the front panel from the other machine in, but it was quite messed up, so
I just
stole three of the toggles from that front panel to
make the original
front panel
complete.
I have yet to get the CUTS board installed and checked out, as well as
getting
the 3P+S board reconfigured to use RS-232, as it was
configured to use
current
loop for an ASR-33 teletype that was used on the
system originally. Then,
I am
going to dig into the second system and see if I can
get it running, and
find
some kind of serial I/O board.
Needless to say, I'm really happy to have these great old machines. It is
really
special to me that I'm reunited with the system
that I instigated and
helped to
build so many years ago.
Rick Bensene
The Old Calculator Museum (along with some old computers, too)
http://oldcalculatormuseum.com
What a great story!
Maybe I should visit *my* old teacher, perhaps he's got the DECSYSTEM20 I
used to use at the local polytechnic in his garage :-)
Regards
Rob