The drives used in the original IMSAI disk system were
Calcomp 140's (this
was with the 2-board IFM/FIB disk controller set). In later
versions that
supported double density (with the later DIO/PDS 2-board
disk controller
set), these were changed to identical looking (but
electrically different)
Calcomp 142's. I have the original Calcomp OEM and service
manuals here
.... and I have them both in hard copy and as scanned PDF
files (someone
really lucked out .....).
[The drives used by DEC were made for them by Calcomp and
had the same
faceplate, but I am not sure if they were the same drives or
not ... I don't
think that they were, but I'm not sure of that.]
In the early chassis, Imsai used a Calcomp "PLO"
phase locked loop data
separator board for each drive (e.g. two of them ... it was
a PC board about
4" x 6"). In later chassis, they figured out how
to use one single PLO
board for both drives. In still later implementations, the
PLO was
eliminated entirely and data separation was done in the disk
controller back
in the computer (this did not happen until the DIO/PDS
controller replaced
the IFM/FIB controller).
Also, there was a "short" chassis (early) and a
"long" chassis (late).
Originally, the chassis had terrible cooling, and a cooling
mod kit was
developed that consisted of some die-cut cardboard vents and
baffles as well
as an added fan. The short chassis was too short to house
the fan, so when
the cooling mod was retrofitted, the fan went on the OUTSIDE
back of the
case. The later "long" chassis was 2" deeper
and had the fan on the inside.
In addition to the original Calcomp manuals, I also have an
original IMSAI
manual on the disk SYSTEM (the original IFM/FIB controller
version, not the
later DIO/PDS controller version). Unfortunately, I have
never scanned that
to PDF (it's a bound manual ... I'd have to destroy it).
Note that in the original 2-board disk controller, the IFM
board went
through a lot of revisions, and versions earlier than Rev. 6
were junk. The
IFM board is a complete 8080 compute system with it's own
8080 CPU, RAM
(2111's, I think) and ROM (two 2708's, I think). I have
source code for the
IFM firmware, I believe. This board did DMA into the
"main" 8080 system
memory. It was, for the time, a very elegant design.
Unfortunately, it was
slow, it REQUIRED the full original CP/M interleave of 6 to
work.
It was (and therefore is) possible to replace the IFM/FIB
controller with a
single density Tarbell controller (Western Digital 1771), I
did this in a
few systems. The result was a MUCH better disk system. In
this mod, the
PLO board(s) were removed entirely, raw unseparated clock
and data were fed
back to the [Tarbell single density] controller and the
system was 6x faster
(could work with no interleave at all), more reliable, lower
power and one
board instead of two. To do this you put a dip plug into
the socket on the
Calcomp drives where the PLO had plugged in, that was the
only change to the
disk box, which connected to the main IMSAI (or other
computer) with a
25-conductor flat ribbon cable (DB-25 at both ends). Then
you needed a
custom cable from a DB-25 socket on the back of the IMSAI to
the Tarbell
controller, and custom wiring of the Tarbell controller,
which was not much
more than a "disk controller prototype board" to
begin with. I MAY still
have my notes on the Tarbell jumpering for that
configuration. The may, in
fact, be in the PDF file I created of the Tarbell single
density manual that
was posted on Howard Harte's site.
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