IMSAI Drive [Imsai original dual 8" disk drive system notes]

Barry Watzman Watzman at
Sat May 10 09:53:09 CDT 2008

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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