Clearly there's a lot of "folklore" muddying the water here. You're right,
this is somewhat off topic for the original parallel-port hard drive
question. While I was sleeping, someone drove the thread in the direction
of solving rather than circumventing the original problem, and that was
related to a problem engendered by writing 48TPI media for use on a
Commodore machine, or some such, on a 1.2MB 5.25" floppy drive ... I THINK
...
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.
Please see embedded comments below.
Dick
----- Original Message -----
From: Pete Turnbull <pete_at_dunnington.u-net.com>
To: <classiccmp_at_classiccmp.org>
Sent: Sunday, March 26, 2000 6:10 PM
Subject: Re: 360K in a 1.2M drive (was: Parallel port hard drives?
> On Mar 26, 16:16, Richard Erlacher wrote:
> > I'm not sure I am the one who should wake up here. I would repeat "It
> > doesn't matter whether the diskette is a 1.2 MB one or a 360K type" but
> it
> > does depend what sort of drive it is.
>
> No, Fred is right. There are plenty of drives designed to use single
> density or double density disks at 96 tpi, and disks cerified for such
use,
Those are different drives and have the same head gap size problems as the
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
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
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
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.
> and they work perfectly reliably for 80 tracks, SD or DD. Historically,
> such drives predate 1.2M (HD) drives. Provided you use SD/DD media in a
> drive using the correct write current, and providing there isn't any other
> leftover rubbish for the wider heads in a 48 tpi drive, any reasonably
> well-aligned 48 tpi will handle those disks just fine. However, no 48 tpi
> drive I have ever heard of is designed to use HD 1.2M media, and if you
try
> it, it will give trouble sooner or later. All the evidence indicates that
> the major factor is the magnetic coercivity of the media.
>
> > [...] the fact that the heads designed for 48tpi will write a
> > significantly wider swath on
> > the diskette than the 96tpi drive can erase. Consequently, and I'll bet
> > you've had this experience, you can format a bulk erased diskette to
> 48tpi
> > with a 96 TPI drive and have it work for a while.
> >
> > The more the diskette is written, by the 48tpi drive, the harder it
> becomes
> > for the 96tpi drive to erase its writing,
>
> Again, no. Providing you're only reading back the part overwritten by the
> 96 tpi drive, this is not true. It only matters if you both write with 48
> tpi, overwrite with 96 tpi, and then try to read back with a 48 tpi drive
> -- which is not what was being discussed. And it doesn't make any
> difference whether the 48 tpi drive writes once or one thousand times, the
> magnetic field strength is the same!
>
> > and, of course, the 48TPI drive
> > will have the most trouble because the signal not erased in the 96tpi
> drive
> > is perceived by the 48tpi drive as noise, while the signal written by
the
> > 96tpi is written at a lower level to begin with, since the drive relies
> on
> > the higher coercivity of the medium to generate a larger signal
amplitude
> if
> > that factor is to come into play at all.
>
> 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
them to prove that. .. cancelled checks, too ... The fact that so many
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
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.
The higher coercivity media were required because the signal sensed from the
heads is a function of the head gap area versus the media area and the
density of the magnetic field within that area, given that the velocity of
the media at the head is a constant. Speeding up the heads increases the
signal amplitude at the amplifier input. Now, if there is signal written at
the same track width as the maximum the head allows, the signal will be as
clean as it can be and maximal. If the signal track is WIDER than the head
gap, it is still maximal and still clean, as any crossover effects will be
synchronized with the signal, since the head is surrounded, ideally, on both
sides, by signal similar to what it is supposed to read.
When the head is wider than the signal track, the unerased signal from the
wider head which previously wrote the track is still present at the margins
of the track as seen by the narrower head gap. However, the head gap is
fully loaded with signal bearing media surface and "sees" only what it is
supposed to see. The wider head, however, when it "sees" the signal from
the narrow head gap also sees the signal from a wide head gap that differs
from the desired signal. This is NOISE (unwanted signal) and therefore
interferes with the operation of the read/write system.
> --
>
> Pete Peter Turnbull
> Dept. of Computer Science
> University of York
Received on Sun Mar 26 2000 - 19:52:38 BST