IBM 9331-011 8" External Floppy Drive - eBay 183038271095

Chuck Guzis cclist at
Fri Feb 2 00:17:48 CST 2018

On 02/01/2018 09:39 PM, Fred Cisin via cctalk wrote:

> When I first got a Micropolis drive, I found "100tpi" hard to believe.
> But, interchange attempts showed that by the inner tracks, it was not
> 96tpi, and nor a multiple of 48tpi.
> My homemade developer, and my patience to keep trying weren't good
> enough to be able to successfully do other than take their word for how
> many.
> (Seeing the difference between 96tpi and 100tpi should be easier than
> telling the difference between Leica thread (39mm x 26 Whitworth threads
> per inch) V the early Russian Fed (39mm x 1.0m DIN thread)
> some of the earliest Canon imitations (39mm x 24tpi thread))

Other than a brief encounter with an SA400 (single-sided 48 tpi. 35
cylinders), the Micropolis 100 tpi drives were my first real run in with
5.25" drives.

Great drives, heavy, built like a tank--and expensive.  I've got a
late-model 96 tpi Micropolis that illustrates their NIH mindset--the PCB
 pivots as you close the door latch.  Still used the precision-ground
leadscrew positioner.  Buffered seek; something like 4 steps per
cylinder, if you can believe it.

ISTR that if you stick a formatted 100 tpi ddisk in a 96 tpi drive and
do a read ID, the first cylinder reads as 6.  That is, the 100 tpi disk
is offset a bit more toward the outside of the disk.  You can read a few
cylinders after that, before the misregistration causes things to get weird.


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