29 Mar
2000
29 Mar
'00
2:12 a.m.
----- Original Message -----
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Monday, March 27, 2000 12:10 PM
Subject: Re: 360K in a 1.2M drive (was: Parallel port hard drives?
which
single
> 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 > 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
> > 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.
"ordinary"
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
> 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