Prototype IBM DemiDiskette drive

[Eric Schlaepfer]

Yesterday I dug out my prototype IBM DemiDiskette drive and took some
photos: https://twitter.com/TubeTimeUS/status/1032066215647166464. It
caused a bit of interest on Twitter so I figured some of you here may also
like seeing it.

I don't really know a whole lot about it other than what my grandfather
told me (he worked on the team that developed it). Dates on the remaining
paperwork go from December 1979 through August 1980. It was supposed to be
a very low cost drive for the microcomputer market (target price IIRC was
<$100). Although it was originally developed by a team working at IBM
Austin, it was handed off to a different team apparently working out of
Rochester.

The disk capacity was not very large--I don't remember the exact number but
it was probably around 100K or less.

A few interesting observations:

* The stepper motor uses a spiral cam to convert rotation into linear
motion to drive the head.
* It is a single-sided drive.
* A microswitch senses the presence of the disk instead of an optical pair.
* There is no write protect notch or sensor.
* There is no index sensor.
* The spindle drive motor is a DC brushed motor with an encoder wheel for
speed control.
* Not shown in the pics, but the plastic "spot welds" holding the vinyl
jackets on the disks are intentionally widely spaced making the cookie
easier to remove for analysis.

It's not really something you get to see every day, that's for sure...

--Eric