3B1<->3B2

[Bill Mayhew]

Threre are many things that cause incompatibility between 360K 40
track drives and 1.2 megabyte 80 track drives.  Most notable is the
fact that the media in 1.2 meg drives must have a higher MOL (max-
imum output level) to compensate for the approximate 3 dB loss of
signal from the disk head (the tracks are half the width, hence
approximately half the amplitude is recovered in a read of an 80
track drive).  To accomplish this, 1.2 meg diskettes use a cobalt
doped oxide (to the eye, it looks black, rather than brown of 360K
disks).  This is akin to chrome cassette tapes.  Just for the
record, a 360K drive can not generate a field intense enough to
magnetize an HD type diskette.  The devices are thus incompatible
at the media level.

A 1.2 meg HD disk drive can, however, read a 360K diskette.  The
converese is not true; an HD drive can not write in low density
such that reliable reads are guaranteed on a 360K drive.  This
makes it possible to port from a 360K machine up to a 1.2 mege
machine, but not reliably vice versa.

An HD disk is laid out thus:  80 tracks * 2 sides * 15 sectors
per track * 1/2 Kbyte per sector = 1,200 Kbytes per disk.  To
get decent MOL, the RPM is also increased on a 1.2 meg drive from
300 pm up to 360 rpm.  This gives a data transfer rate of 500,000
bits per second of raw data to/from the head.

A 360K disk is liad out thus:  40 tracks * 2 sides * 9 sectors per
track * 1/2 Kbyte per sector = 360 Kbytes per disk.  The drive runs
at 300 rpm, which yields 250,000 bits per second measured at the
head.  (IBM type MFM encoding assumed.)  The Unix PC can also be
programmed to format at 10 sectors per track.

A 720K quad density drive is much like a 360K drive, except that
there are 80 tracks of 1/2 the normal width.  The oxide used is
approximately (if not exactly) the same as that used in 360K media.

To the best of my knowledge there aren't any readily available HD
1.2 meg drives that can read the oddball 720K format.

The best bet would be to attempt to teach cpio (if it doesn't
already know) how to read the 3b1 diskettes on the 3b2.  You might
be able to go 3b2 --> 3b1, if you use virgin (or bulk erased) media
on the 3b2 and then only read, but never wrtie to it on the 3b1.
The key is to only ever write on the disk (3b1 or 3b1) with the
machine that did the actual formatting.

There is a company that makes an oddball 1.2 meg drive that runs at
300 rpm for use in XT compatible machines that have FDC chips that
are fixed at 250,000 bps.  Priority One Electronics either does or
did stock such drives.  I have never used one of those beasts, so I
don't know how well it works.  I can't vouch for the quality of
Priority One either, as I have never bought anything from them.
Priority One usually advertises somewhere in Byte each month.


--Bill