Re: assistance with disk ][ and modern 1.2M floppies

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

Kenneth Edward Smith <cg591@torfree.net> wrote:

> David Empson (dempson@actrix.gen.nz) wrote:
> : Obsbedia2 <obsbedia2@aol.com> wrote:
> 
> : > << You can't use 1.2MB (high density) 5.25" disks on an Apple II (short of
> : > getting a PC-compatible disk controller and high density 5.25" drive). >>
> : > 
> : > The Apple ][ compatible Franklin 2200 had internal 5.25" disk drives that
> : > had larger capacity because they wrote extra tracks (about 40 by my
> : > memory).  Would this also need Double Density disks, or could it use High
> : > Density disks?
> 
> : Still double density.  High density 5.25" disks have about four times
> : the density of double density disks: about twice as much data per track,
> : and twice as many tracks (80 instead of 40).
> 
> You're correct, as I've observed you appear to be consistently; but in
> this case incomplete.
> 
> Actually, the original A2 and compatible drives used single sided 
> single density diskettes; the double sided double density disks also work
> as you've noted but originally were far too expensive for common use by
> hobbyists...

I was under the impression that single density disks were not very
reliable (in Apple II drives) and double density was preferable.

This may have just been a quality control issue.

> it was rumoured at the time that SSSD disks were the result 
> of quality control procedures which, if a disk failed DSDD led to testing
> as SSDD, which if it failed led to testing as SSSD, which if it failed
> led to landfill...

I can easily believe this.

> SD disks were only guaranteed to hold a maximum of 40 tracks

But so are double density 5.25" disks, as far as I know.  On 5.25", SD
vs DD determines the maximum data density within each track, not the
maximum number of tracks.  (HD increases the tracks to 80.)

On a PC, I believe double density disks are required if you want to use
MFM (160K/180K for single sided, 320K/360K for double sided).  A
theoretical FM formatted 5.25" disk could use single density, and I
imagine it would hold half the data of an MFM disk (i.e. a maximum of
90K for a single sided disk).

(This seems a little dubious, since MFM only doubles the precision of
the bit placement, without packing the bits any closer together on
average.)

My memory is a bit hazy on 8 inch disks - I used them on a CP/M machine
for a while.  Assuming I remember the details correctly, a single
density 8 inch disk was 40 tracks, and a double density one was up to 80
tracks (our CP/M system used 77).

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