26 Mar
2000
26 Mar
'00
9:16 a.m.
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. The Coercivity of the media will
effect signal level in both reads and writes, but won't make up for 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, 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.
Under worst-case conditions the signal is written to the inner tracks by the
96tpi drive, which attenuates write current on the inner tracks, and it is
read by the 48TPI drive. The diskette was a bulk erased diskette and was
formatted in the 96 tpi drive. The region of the diskette that we're using
was written by the 48tpi drive and subsequently the new data was written in
the 96TPI drive.
Clearly, there is a risk that the 48tpi drive won't be able to distinguish
the narrow channel of cleanly written data from the 96tpi drive, among the
straddling field of residual signal at nominally the same frequency but out
of phase with the sync field, address mark, data, and CRC written by the
96tpi drive. Consequently there's a real risk that there will be a data CRC
error if not an address mark misread.
Remember, in order to get the drive to perform properly at 96TPI, the head
gaps had to be made narrow enough to avoid interference between adjacent
tracks. This was accomplished by a combination of changes in head
design/manufacture, write current levels, and read amplifier gain. I submit
that it's the heads that make the difference that we normal mortals perceive
as "not working" when the 360K drive can't read the 96tpi's writing.
The
signal to noise ratio is MUCH to low.
Dick
----- Original Message -----
From: Fred Cisin (XenoSoft) <cisin(a)xenosoft.com>
To: <classiccmp(a)classiccmp.org>
Sent: Sunday, March 26, 2000 1:16 PM
Subject: 360K in a 1.2M drive (was: Parallel port hard drives?
> It doesn't matter whether the diskette is a
1.2 MB one or a 360K type,
but
Give us a break! YOU know better!
YES, IT WILL MATTER!
And it is NOT necessary for any idiot to say, "But I use the wrong type of
diskettes all the time."
360K is 300 Oerstedt. 1.2M is 600 Oerstedt.
My personal experience is that using a 600 Eorstedt diskette in a 300
Oerstedt application results in a significantly reduced lifetime for the
data. Sometimes minutes!
> the drive which wrote it make a great deal of difference. 1.2 MB drives
can
> read 360k diskettes just fine, but you can't
expect a drive designed to
> operate on half the track width (twice the density) and even more bit
> density, to erase the writing of a 48 TPI 360K drive properly. It works
OK
> the first time, and less well the second ... and
so on until it fails
> because the residue left behind by the 48tpi drive generates too much
noise
for the 96 tpi
drive to discriminate between noise and signal.
If you HAVE to use a 1.2M drive to write 360K diskettes, it is best to
start with a virgin disk. And ALWAYS bulk erase it (preferably with a
better bulk eraser than the one on your refrigerator door). THEN format
it as 360K ("/4" in DOS). If the diskette is EVER written to by a 360K
drive (even "ERASE"ing a file), then bulk erase and FORMAT in the 1.2M
drive again.
You have to take these steps if you want reliable writes to 360K in a 1.2M drive, even if you might sometimes get away with not
doing it right.
--
Grumpy Ol' Fred cisin(a)xenosoft.com