Chuck Guzis cclist at
Wed Aug 25 18:23:39 CDT 2021

On 8/25/21 3:31 PM, Fred Cisin via cctalk wrote:

>> Later drives included both 100 TPI and 96 TPI (e.g. 1115-5 and -6).
>> For a 5.25" drive, a beast of one.  No wonder they quit making floppy
>> drives.

The 1115/16 series was Micropolis' last gasp at floppy drives, I think.
 Not the steel plate construction of the earlier drives, but cast
(zamac, I think) both body and diskette carrier.   The worm-drive
positioner pivots with the diskette carrier, just like the originals.  I
suspect that this contributes much to the accuracy of the track positioning.
The 1115 uses a MOS MCU (6502-based perhaps?)to implement buffered
seeking, which ameliorates the slower positioning times and allows for
some measure of "ballistic seek".

We had a problem finding suitable alignment disks for 100 tpi, so a
couple of the engineers got a 3/4" aluminum plate and an external
stepper coupled through a 100:1 right-angle precision drive coupled to
the floppy drive.  It took about 3 minutes to step from one end of the
disk to the other, but the setup worked.

Early on, with the 100 tpi units (about 1976-77, we had a problem with
the disk clamping.  Everyone was new to the 5.25" drive with its DC
motor, so naturally, one turned on the motor only when needed (much like
the later IBM PCs).  What happened when the spindle was stopped was that
the clamping was off-center frequently.  The hub area of the floppy
would get wrinkled and then it became a craps shoot to get the mangled
floppy to read correctly.

Our primary vendor was Dysan for floppies.   Initially, they introduced
a kit that enabled one to attach an adhesive-backed reinforcing ring.
It seemed to work, and Dysan began supplying floppies with the ring
installed.  Eventually, Micropolis determined that adding a microswitch
to the drive that detected when the drive door was being closed, turning
on the motor for a couple of seconds allowed for accurate clamping and
centering of the diskette on the clamping "cone".

By the time the 1.2M 5.25" drives came out, everyone was doing this, so
rings on high-density floppies were unnecessary.  Although there are
some exceptions (obviously), a good rule of thumb is if the ring is
present, the disk is DD; if absent, HD.


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