Re: Why can't Gs 800k drives read 720k IBM disks?

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <797758948.AA00397@tlp.apana.org.au>,
Peterm <peterm@tlp.apana.org.au> wrote:
> * On 18-03-95, David Empson was waffling to All about:
>
>  DE> MFM disks do not use variable speed rotation.  This is the
>  DE> main reason they only hold 720k.

I should also add that they have the same number of sectors on all
tracks (whereas the Apple 400k and 800k disks have 8 sectors on the
innermost tracks and 12 on the outermost).

> This doesn't explain why cards like CTI's FDC-10 for the Apple II can
> produce 800K ProDOS MFM 3.5" diskettes using standard IBM-style drives.

It probably varies the bit width, which simulates the speed variation.

> I believe that the real reasons the IBM-style drives were 720K
> were: 
> a) IBM's inherent conservatism. 720K was easily achieved and worked well.

... and was MUCH cheaper than making a variable speed drive.


-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand