"360K" - 300 Oersted. vs 1.2M 600 Oersted.
On Thu, 11 Jul 2019, Guy Dunphy wrote:
Can you believe that is the first time I have ever heard the actual figures for
the coatings?
I knew the 1.2M type had a higher coercivity, thus the incompatibility. Ditto with
3.5" 720K
vs 1.44M.
Then add to the useless information:
"720K" diskettes ("micro-diskettes") are 600 Oersted.
"1.4M" diskettes are 720 - 750 Oersted. Notice that it is not as much
difference between them as there is between the 5.25 variants. Therefore,
using a "720K" diskette for "1.4M" or vice-versa, while less reliable, is
something that you might "GET AWAY WITH". There were punches being
peddled for doing so! You could take a good quality "720K" diskette and
turn it into a mediocre/flaky "1.4M"
BTW, if you multiply out the space on it (2 sides * 80 tracks per side *
18 sectors per track * 512 bytes per sector), 1474560, the only way that
you can get 1.44 as the number of MBs is if you creatively redefine MB to
be 1,024,000 (1000K, 1000* 1024, 2^10 * 10^3)
A MebiByte is 1048576 or 2^20. A "1.4M" disk is 1.40625 MebiBytes.
BTW, ANOTHER incompatability between "360K" and "1.2M" was 48 tracks per
inch V 96 tracks per inch. ROUNDING THE NUMBERS (researching the correct
numbers,and redoing the calculations ACCURATELY is "left as an exercise
for the reader"),
a "360K" has tracks that are about 1/2mm spacing, that are about 1/3mm width.
96tpi is about 1/4mm spacing, with width about 1/6mm.
A 1.2M drive can read 360K by double-stepping. No problem.
If it also adjusts the write current, then it can format a virgin 360K
disk. The tracks will be sub-standard width, but will work. (1/6mm wide
tracks with 1/2mm spacing)
HOWEVER, if you take a track that has been written to on a 360K drive, the
1.2M drive, when it RE-writes that track will not erase the full width of
the old 360K track. You will have a 1/6mm track down the middle
of a 1/3mm track.
The result is a "360K" track that still reads just fine on
the 1.2M drive, but is not readable [reliably] on a 360K drive.
Visualize a car tiretrack that has been followed by a motorcycle. Two
motorcycles can leave a pair of tracks that resemble car tracks if they
are the only tracks. But, if there is already a car's tracks, the
motorcycles won't fully replace the car's tracks.
--
Grumpy Ol' Fred ci...@xenosoft.com
