11 Oct
2008
11 Oct
'08
9:23 p.m.
On 11 Oct 2008 at 20:47, Mr Ian Primus wrote:
At one time, diskette "punchers" were a popular item at computer swap
meets. The price differential between HD and DD media at one point
was substantial. I think I paid over $50 for my first box of 10 Fuji
DSHD 3.5" diskettes. I found it easier just to stack up a bunch of
DD diskettes on my drill press and drill a hole through the entire
stack.
IBM PS/2s did
it optically.
Hehe. Or not at all... I remember using one PS/2 that just assumed
everything was HD unless told otherwise (or the disk was already
readable). Didn't even need to drill holes in the disks to 'upgrade'
them. Of course, these days, I find myself taping over the holes in
disks, since DD media is hard to find. I've made high density 5 1/4" drives into 80
track double density
(720K) drives by cutting the trace on the board that leads to pin 2,
and tying it to ground. This forces the drive into DD recording.
You're leaving something out here. While that forces the drive into
DD recording mode, the spindle speed doesn't change unless the drive
is also jumpered for "dual-speed" 300/360 mode. So there can be a
real difference between a "fixed" drive and a genuine 720K DSQD
drive.
Now, what I wonder, is how does the _controller_ know
what kind of
disk is inserted in the drive? According to the documentation I have,
the density select line is defined as a unidirectional signal TO the
drive, not from it. Having not worked as much with 3 1/2" drives, I
don't know, but I would assume that the drive senses the disk's hole,
and operates in that data rate and density recording, and the computer
simply picks up on the data rate and works accordingly. Or, perhaps,
in 3 1/2" drives, pin 2 is bidirectional, allowing the drive to tell
the computer the type of disk inserted. I don't know.
Grab some documentation on early 1.44MB drives. You could often
configure them for "auto sense" (the way most recent drives are by
default) or "remote sense" in which the status of the media sensor is
fed back to the host and the host selects the density via pin 2. Or
do it the way the PS/2 did and ignore the media type completely.
Some drives are smarter than others. Most 3.5" USB drives will
automatically sense 1.23MB media and adjust the spindle speek
accordingly (they do not, however allow formatting of 1.23MB media).
I've got a "legacy" interface drive from Toshiba that will also do
the same. Appropriately configured 3.5" Teac GF and some HF drives
can use a signal on pin 4 to change spindle speeds.
There's more variation among HD 3.5" drive signals that most people
realize, particularly in Japanese variants, where conventions used in
the NEC 9801 held sway. A bog-standard 1.44MB PC drive will often
refuse to work in that architecture. The 9801 used a very sane
approach to diskettes--the format of an 8" diskette is exactly the
same as the 5.25" and 3.5" varieties--77 cylinders, 2 sides, 8
sectors per side, 1024 bytes per sector and 360 RPM.
Cheers,
Chuck