Lisa "twiggy" specs (Was: selling a blank floppy disk

From: Don Maslin <donm_at_cts.com>
Date: Sat Jan 2 17:22:04 1999

On 2 Jan 1999, Eric Smith wrote:

> "Fred Cisin (XenoSoft)" <cisin_at_xenosoft.com> wrote:
> > Did they have the same number of sectors on each track ("constant
> > rotational speed"), or did they have different numbers of sectors on
> > different tracks ("constant linear velocity")?
>
> The Twiggy disk uses a zoned-CAV system like modern hard drives, except
> that it changes the spindle motor speed and keeps the data rate constant.
> Thus the number of sectors per track varies.
>
> There are 46 tracks for data (numbered 0 to 45), and track -1 which is
> used for speed calibration.
>
> I'm assuming that track 0, the outermost track for the lower head, has
> a radius of 2.25 inches, the same as a standard 5.25 inch drive. The
> Twiggy uses a track pitch of 62.5 TPI.
>
> The zone information is given in this table, which is compiled from
> the Lisa Hardware Manual; I've added the peak flux density which I
> computed in an Excel spreadsheet.
>
>
> 512-byte peak
> sectors rotation flux
> per rate density
> track track (rpm) (fcpi)
> ----- ------- --------- ---------
> 0-3 22 218.3 9721-9933
> 4-10 21 228.7 9550-9989
> 11-16 20 240.1 9588-9973
> 17-22 19 252.7 9552-9955
> 23-28 18 266.8 9509-9931
> 29-34 17 282.5 9463-9907
> 35-41 16 300.1 9414-9981
> 42-45 15 320.1 9453-9749
>
>
> Note that for Apple's (6/8) GCR format, equivalent to (0,2) RLL,
> the minimum flux density is 1/3 of the maximum flux density. For MFM, the
> minimum is 1/2 the maximum. This means that the media has to work for the
> range of 3138-9989 FCPI.
>
> Standard 5.25 inch floppies spin at a constant rate, and transfer data
> at a constant rate, so they have a much lower flux density on the outer
> tracks.
>
> For 360K format, the maximum flux density ranges from 3537 FCPI on side 0
> track 0 to 5877 fcpi on side 1 track 39, for an overall range of
> 1768-5877 FCPI.

Does that imply that the flux density on side one differs from side zero
at the same cylinder? If so, why.
                                                 - don
 
> For 1.2M format, the maximum flux density ranges from 5895 FCPI on side 0
> track 0 to 9870 fcpi on side 1 track 79, for an overall range of
> 2947-9870 FCPI.
>
> Obviously, then, my inital guess that high-density media is required
> would appear to be correct.
>
> > GCR would tend to provide a flux transition density of about 1.5 times that
> > of FM, and slightly less than that of MFM.
>
> Apple's (6/8) GCR is exactly 3/4 as efficient as MFM.
>
> Eric
>
Received on Sat Jan 02 1999 - 17:22:04 GMT