5 1/4 diskettes available

Fred Cisin cisin at xenosoft.com
Wed Jul 10 21:14:39 CDT 2019

>> They have hub rings, so they are probably 360K

On Thu, 11 Jul 2019, Guy Dunphy via cctalk wrote:
> And the index hole.

What about it?
There were some machines that didn't use the index (such as Apple and 
Commodore), but they didn't care if there was one.
There were some that used 10 or 16 index holes ("hard sectored").  In 
the unlikely event if these happen to be hard sectored, then others will 
gladly take them off your hands..

>  Double sided, 35 tracks/side,

That would be 48 tracks per inch.  Some early drives, such as the SA400 
(TRS80) and SA390 (Apple) were 35 track, but later drives extended that to 
40 tracks.  There were a few diskettes made with a shorter window, that 
could only manage 35, but otherwise 35 and 40 were the same.

> 16 sectors, 256 byte sectors, MFM, optical index hole sensor.
"Hard sectored" had a single optical index hole sensor, and 10 or 16 holes 
through the "cookie", with one hole through the "jacket"

On "soft sectored", there would be a "track" composed of multiple sectors, 
with headers, gaps, "sync fields", etc,  to set the start and end of each 
sector.  We can discuss the full structure of "IBM"/"WD" track structure 
if you want.

>  Total capacity: Listed as 270KB, but above gives 286,720 bytes. (280KB)
> So probably 360K disks would work?

Absolutely.  "360K" is the name given to double sided, 40 tracks, with 9 
sectors of 512 bytes per sector.  The choice of using 35 tracks, with 16 
sectors, and 256 bytes per sector does not squeeze as much onto the disk, 
but the disk is the same.

If that machine were to have been the most popular, then those diskettes 
would have been called "270K".

Single and double sided usually used the same cookie, so the difference 
was solely whether the manufacturer promised/guaranteed that BOTH sides 
were good.  Yes, there were rumors that the manufacturer would test all of 
the diskettes, adn sell the ones with 2 good sides as double sided, and on 
ones with a bad side would flip them over to get the good side into place 
to sell as single sided.  The reality is that diskettes were never 
expensive enough to be worth the labor of trying to salvage bad disks.

Except for some very early ones that had a short oval window, 35 track and 
40 track were the same.

In short, . . .
the disks that YOU need are called "360K".  (300 Oersted)

Do NOT use the 1.2M "high density" disks.  Those are 600 Oersted, and you 
need the "360K" 300 Oersted.  If you try to use the "high density" disks, 
they will either FAIL, or will seem to work, but be unreadable VERY soon.

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