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

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.