----- Original Message -----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, March 27, 2000 12:10 PM
Subject: Re: 360K in a 1.2M drive (was: Parallel port hard drives?
> > I'm a firm believer in transporting data on portable hard drives rather
than
> > floppies, since they're MUCH less error prone and MUCH faster and
handier.
>
> Well, provided the system and the OS in use support such parallel port
> drives, they can be a good idea. But a heck of a lot more machines
> support floppy disks...
>
That's quite true, Tony, but for many files, it takes a heck of a lot more
time and floppies than portable/parallel port drives. I have mentioned
that my parallel port interface is a parallel<=>SCSI adapter, haven't I.
Those are a mite more useful, methinks, than the parallel port-only drives,
but that's just my opinion. I've seen some pretty well-done interfaces for
parallel port drives, though. They're more practical for PC's for which
drivers are readily avaliable than for, say, a PDP-5, for which the vendor
probably doesn't provide a driver.
>
> [...]
>
> > Those are different drives and have the same head gap size problems as
the
> ^^^^^^^^
> Strictly 'head width'. The head gap determines the bit density along the
> track that can be reliably read.
>
> > 1.2MB variety. The diskettes for those (720K 5-1/4" floppies) were also
a
> > class apart from the run of the mill. Nevertheless, the difference that
was
>
> '720K 5.25" disks', meaning 96tpi (80 cylinder) standard double density
> disks can be reliably used as 360K disks (either in real 360K drives or
> with the conditions that I specified in my last message). The reverse is
> not always true -- it appears that some '360K' disks generate too much
> noise to be reliably read by the narrower head in a 96tpi drive (I have
> seen this far too often).
>
> > essential was the drive, read/write hardware. This included both the
1.2MB
> > drives and the 720K drives in 5.25" size. The media had to be different
to
> > support the higher flux reversal density, and the heads had to be more
>
> The '720K' disk is the same coercivity as the '360K disk'.
>
> > sensitive to flux changes so that they could be driven at levels that
> > wouldn't engender too much crosstalk. Unfortunately, the heads with
which
> > 48TPI drives were normally equipped were not capable of this
sensitivity,
> > given the original task for which they were designed.
> >
> > Later in the evolution of the technology, media were alleged to be more
or
> > less the same, and drives eventually became the same, in the spirit of
> > economy, and one wasn't told whether the drives he was buying were
capable
>
> No reputable manufacturer of floppy disks ever used the same media for
> normal ('DD') and 'HD' disks. Period. It plain wouldn't have worked.
>
> Reputable floppy drive manufactuers, like Teac (but there are many more)
> put wider heads into 48tpi drives right up to the end. Some lesser
> manufacters (I don't have any names, but I _know_ this occured) put the
> narrow heads into everything. This caused all sorts of problems.
>
>
> > of the higher flux-reversal or track density or not. Likewise, one
wasn't
> > guaranteed that the media weren't the high-coercivity type, since it
didn't
> > hurt the older style of drives to use it. All this has been stirred
into
> > the mix of confusion. I would caution against drawing any conclusions
from
> > evidence gathered from 48TPI drives after 1.2MB drives became available.
> > The manufacturers were more interested in reducing their diversity than
in
> > making drives you couldn't use as "the other" sort.
>
> See above. And look at the service manuals for some floppy drives. You'll
> find out which parts _are_ specified as different between 48 tpi nd 96
> tpi versions. Of course if you use no-name floppy drives then you
> probably can't get the service manuals, but then you're asking for
> trouble by doing this anyway...
>
> > > No, it doesn't rely on the higher coercivity, not when it's writing
single
> > > or double density. It only does that for High Density.
> > >
> > First of all, it's NEVER writing or reading single density. Secondly,
the
> > "official" certified media for 96tpi use were always claimed to be of
> > greater coercivity than the ones intended for 48tpi use. I've got boxes
of
>
> Can you quote a specification which confirms that, because I sure can't
> find one. The '720K' disks (aka Quad Density, 80 track 5.25", etc) worked
> very reliably as 40 track (360K, etc) disks in every machine I tried them
> in. Going back to some quite early 5.25" drives.
>
> > them to prove that. .. cancelled checks, too ... The fact that so many
>
> I am not disputing they're different -- trying to use 48tpi disks in 96
> tpi drives (even at standard densities -- your '720K') is not reliable in
> my experience. The reverse has never given me problems. Unlike using HD
> disks for anything other than HD formats in HD drives, which never works
> properly.
>
> > people have "gotten away" (including me, by the way) with writing
"ordinary"
> > media at the higher track density only serves to confuse the casual
> > observer.
> >
> > As manufacturers found that they could build all drives with the same
heads
> > and electronics at lower cost than building two or three different
drives
>
> Reputable manufactuers, who sold service manuals for their drives, did
> not do this.
>
> > for a given applicaition, they started using the same heads in as large
a
> > portion of their drives as they could. It was just good business. The
> > media makers began capitalizing on these same economic decisions, and
> > started filling their orders with "better" emulsions, and selling the
excess
> > into their lower-grade market.
>
> Again, brand name 360K and 1,2M disks have different coercivities. This
> applies to ever box of such disks that I've ever bought (including ones
> I bought a year or so ago). No-name disks may well get this wrong, but my
> data is too valuable to wast time with those...
>
> -tony
>
>
Received on Wed Mar 29 2000 - 08:12:36 BST