27 Mar
2000
27 Mar
'00
10:30 a.m.
On Mar 27, 19:57, Tony Duell wrote:
There is some wonderful confusion in this thread
:-)....
Yes :-)
The first is the standard density 48tpi drive. This is
what PC owners
would call a '360K drive'.
[...]
The second is the standard density 96tpi drive.
[...]
The third is the high density drive, known to PC users
as the 1.2Mbyte
drive.
[...]
From this, it seems to me that a 1.2Mbyte drive will
_not_ reliably
read/write a 360K format on HD disks. It will use the wrong (too low)
write current. HD disks should be used for 1.2Mbyte formats only.
Of course when a 1.2Mbyte drive is set to use 'standard' disks, it in
fact behaves like the second type above -- a 98tpi (80 cylinder) drive
and not a 360K (40 cylinder) one.
Tony has put this very well. It's exactly what I was getting at.
Furthermore, the media used in 96 tpi standard density disks is, in my
experience, the same as that used in 48 tpi standard density disks. It
even says so in my CDC drive manuals. However, as we all (ought to) know,
the media for HD disks is very different.
According to all the data I can find (which is not
much), the centre
lines
of the 48tpi tracks and the centre lines of alternate
96tpi tracks
coincide. This means (amongst other things) that you can use the same
alignment disk for both types of drive. It also means that a 96tpi drive
can reliably read a disk formatted and written on a 48tpi drive (only).
When the 96 tpi drive's head is on a suitable track it is reading along
the middle of the track. It's going to read good data. That's why there's
no problem reading disks that have been formatted/written in 360K drives
using 1.2Mbyte drives.
Now lets consider going the other way.
Tony and I are in very close agreement here as well. The only point I'd
add, is this:
Disks which have been written in 48 tpi drives, and not subsequently been
bulk erased, may be read in a 96 tpi drive; and if then overwritten by a 96
tpi drive, they will still be readable perfectly reliably by the 96 tpi
drive, but often not by the 48 tpi drive. Just as Tony (and others) have
said (and as I've been telling people for nearly 20 years). However, there
is a good reason that the process is sometimes observed to work to some
extent, and that has to do with the way the tracks are written/erased.
When the track is written, data is only written into the centre portion
(about 50% of the nominal track width). Either side of that, the erase
coils in the head write a narrow guard band, which occupies most of the
remaining track width. It may happen that the guard bands written by a 96
tpi drive are wide enough to erase a large proportion of the signal written
by a 48 tpi drive, and a 48 tpi drive might just manage to separate the new
signal from the relatively low level of the residual old signal.
I'm *not* advocating this as an excuse to write 40-track (or 35-track)
disks in an 80-track drive without prior bulk erasure. But it explains why
some people have found it works for them (even before manufacturers
streamlined the range of heads they made), though probably only with
limited permutations of drives, and limited (at best) reliability.
I'm well aware of the problems in switching between 48 tpi and 96 tpi. In
the early '80s, I made and sold a little addon for micros that might need
to read 40-track disks in 80-track drives. Because I was well aware of the
problem, I arranged it so that it could not only switch on double-step for
one drive, but could separately switch one of the drive selects between the
normal connector and an extra one. The device was normally used on BBC
Micros with twin 80-track floppies, and the extra connector was to connect
a third, genuine 48 tpi, drive.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York