Re: How to play an old floppy disk.
- From: Franc Zabkar <fzabkar@xxxxxxxxxxxxxxxxx>
- Date: Tue, 10 Jun 2008 19:31:56 +1000
On 10 Jun 2008 07:21:35 GMT, Arno Wagner <me@xxxxxxxxxxx> put finger
to keyboard and composed:
Previously Franc Zabkar <fzabkar@xxxxxxxxxxxxxxxxx> wrote:
On Sat, 7 Jun 2008 03:48:44 -0700 (PDT), "mscotgrove@xxxxxxx"
<mscotgrove@xxxxxxx> put finger to keyboard and composed:
HD 5.25" drives were dual speed as the 1.2MB disks span at 360 rpm,
and not 300 rpm.
If you mean that HD drives would spin at 360RPM for 1.2MB diskettes
and at 300RPM for 360KB diskettes, then I don't believe this is true
for all HD drives, if any. My copy of IBM's original technical
reference for the PC AT states that the High Capacity Diskette Drive
had a rotational speed of 360RPM. There is a circuit diagram of the
main FDD PCB, but not for the motor control board. Nevertheless there
appears to be only one signal going to the motor PCB and that is Motor
ON.
This is not a multi-speed motor. 360rpm for everything,
as you state. Since 300rpm is the floppy standard, this
requires a special floppy controller that can deal with
two different data rates.
AFAICS it just requires a single PLL that can lock to a wider range of
frequencies. I haven't yet seen a controller that couldn't handle a
720KB 3.5" drive. Its data rate is 2700 sectors per minute. A 360KB DD
5.25" drive has the same number of sectors/track (9), and spins at the
same speed (300RPM). Therefore it has the same data rate and should be
electrically indistinguishable from a 720KB 3.5" drive (apart from a
lesser number of tracks). A 360KB diskette in a faster drive should
pose no problem since its data rate would fall in the middle of the
PLL's range. Of course I'm assuming that the controller doesn't have
two different PLLs, one for each drive type.
They are also 96tpi rather than 48tpi.
Not the early ones. They were definitely 48TPI.
See http://en.wikipedia.org/wiki/Floppy_disk
"Finally in 1983 DOS 2.0 supported 9 sectors per track rather than 8,
providing 180 KB on a (formatted) single-sided disk and 360 KB on a
double-sided."
Do the maths and you'll find that equates to 40 tracks per side.
According to IBM's original technical reference for the PC AT, HD
5.25" drives were 96TPI. Confusingly, the manual also lists *exactly*
the same specs for DD drives as for HD drives (eg 1.2MB formatted
capacity, 600-650 Oersted media coercivity), even though it states
that the DD drive is limited to 320/360KB capacity. <shrug> I suspect
this is an error.
DD is actually 640/720kB when done dual-sided and it is 96TPI.
See above. BTW I do have Nashua DSDD and DSQD 96TPI diskettes.
The HD 5.25" disk emulates the DD 8" disk.
A 360KB formatted capacity is achieved with 2 sides, 40 tracks per
side, 9 sectors per track. There are 512 data bytes per sector.
Alternatively it is done wit one side and 80 tracks. This
requires a DD disk.
A 1.2MB formatted capacity is achieved with 2 sides, 80 tracks per
side, 15 sectors per track.
A 720KB formatted capacity (3.5" drive) is achieved with 2 sides, 80
tracks per side, 9 sectors per track.
A double-sided DD disk.
A 1.44MB formatted capacity is achieved with 2 sides, 80 tracks per
side, 18 sectors per track.
And HD 3.5" disk, that spins at 300rpm. (Don't think there were single sided
HD disks.)
When I last 'played' with floppy disks some years ago I do remember
that some modern disk controllers did not handle all old densities.
It is possible you have an issue with the controller rather than the
physical drive.
With the correct controller, all HD drives will read DD disks
Michael
A 720KB 3.5" diskette rotating at 300RPM would produce a data rate of
2700 sectors per minute. A 360KB diskette rotating at 360RPM would
produce a data rate of 3240 sectors per minute. A 1.44MB diskette
rotating at 300RPM would produce a data rate of 5400 sectors per
minute. Therefore it seems to me that a controller that can read a
720KB 3.5" diskette should easily handle a 360KB 5.25" diskette, if
the issue is the range of the PLL/VCO in the clock/data separator.
WIth a 5.25" DD drive, no problem at all. With a 5.25" HD
drive, the 20% faster speed may just be within the range
it can still syncronize on.
That would only be a problem if the controller used two different
PLLs. Otherwise it should be easier to sync with the faster drive. See
above.
You cannot write though.
AFAICS, the same PLL is used for reading and writing. If you can do
one, then you can do the other.
However
a PC floppy controller can deal with 360rpm by increasing
the speed of everything. You just need BIOS support and need
to configure your drive as 5.25" HD.
Arno
If you could spin your 3.5" drive at 360RPM, then your BIOS or OS
wouldn't know the difference.
In fact I just tried configuring my 1.44MB drive as a 1.2MB FD, and my
1.2MB as a 1.44MB FD, and was still able to read 1.44MB and 1.2MB
diskettes in their respective drives. However, I wasn't able to read a
5.25" 360KB diskette when the 5.25" HD drive was configured as a
1.44MB FD.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
.
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