8" Floppies

From: Richard Erlacher <edick_at_idcomm.com>
Date: Fri Aug 31 22:28:44 2001

Well, what I apparently missed is that the intent was to use both sides of a
diskette in a single-sided drive, right? That is the only way that the
direction would have been reversed. It seems to me that DEC did rotate the data
in opposite directions in opposite sides on some models, didn't they? I've
never tried to use a floppy on a DEC machine, so I plead ignorance.

I'm not at all sure how any of this stuff would work out otherwise.

Incidentally, for 8" media at least, (since there's little confusion about
rotational rate and data rate) there's no reason why there has to be any
difference between SD and DD media, since the flux reversal density is the same
in FM as in MFM. The certification of the media for two-sided use, however, is
important, since it's conceivable that one side of a SS diskette could have a
contaminated emulsion, which would certainly be a defect disqualifiying it for
the two-sided use. VERY old drives needed nearly 250 ns of precompensation,
however, while newer ones needed between 125 and 180. I typically used a 6 MHz
oscillator to time the precomp on 8" diskettes, and that worked out remarkably
over a wide range of drives.

Back in the '70s diskettes often were priced at nearly $10 each, and we often
tried to save a buck or two by punching holes in the appropriate place in a SS
diskette's jacket in order to make it work as DS. This worked better than half
of the time. Punching the hole so the "other" side of a SS diskette could be
used SS produced disappointing results too. Neither approach was useable for
work paid for by others. I think that's because emulsions did get messed up
from time to time, and weren't applied to both sides at the same time. Maybe
that's not right, but there must have been some reason.

Dick

----- Original Message -----
From: "Don Maslin" <donm_at_cts.com>
To: <classiccmp_at_classiccmp.org>
Sent: Friday, August 31, 2001 3:32 PM
Subject: Re: 8" Floppies


>
>
> On Fri, 31 Aug 2001, Don Maslin wrote:
>
> >
> >
> > On Fri, 31 Aug 2001, Richard Erlacher wrote:
> >
> > > Well, if the written spec's can be believed, you can't SEE the difference
> > > between DD and SD media. That doesn't change the fact that they have
different
> > > spec's. Further, DS media are certified on BOTH sides, while the emulsion
on
> > > one side may be defective.
> > >
> > > It's true that DS media have their index hole located differently than SS
media.
> > > The index timing is different, according to some drive makers, albeit not
VERY
> > > different. At least one mfg spec's the DS drives with 160 microsecond +/-
50
> > > from the start of /Index to the data burst, while SS media are at 200.
I've
> > > never given much thought to this and how it affects things, but I am not
> > > comfortable with the notion of switching, mechanically, between the two
sensors.
> > >
> > > Dick
> >
> > Probably of little consequence, Dick, since his DSDD disks will not be
> > readable by anyone else unless the have performed the same modification
> > to a single-sided drive.
> > - don
> >
> Oops! Obviously, I meant and should have said "double-sided drive".
>
> - don
>
>
>
Received on Fri Aug 31 2001 - 22:28:44 BST