Dear Allison,
   
   You wrote:
> Density does not
> change with different write current but the two different 5.25 medias
> (RX50 and RX33) require different write currents required by the
> different media coercivity and thickness.  Density is how closed the
> bits are packed and that is affected by rotation speed(drive) and data
> rate(controller).
   
   Do you really think I didn't know this? From 1991 to 1995 floppy disks
were my first and foremost area of research, and I probably know more about
them than anyone in the U.S. When I lived in Russia I was working on Floppy
Disk Analyser (FDA), a tool for copying copyprotected diskettes that used
intimate knowledge of the floppy disk format and the floppy disk controller
in IBM PC AT and compatibles and a special artificial intelligence system
to surpass all competitors by many light years.
   
   As for the write current, it determines the intensity of the magnetic
field of the write head (directly proportional to it). It must be 300 Oe
for iron oxide media and 600 Oe for cobalt-containing media. Since "double-
density" data are always written on the former and "high-density" data are
always written on the latter, there is a one-to-one correspondence between
the recording density, the media type, the write field intensity, and the
write current. Since the controller switches densities by changing the data
transfer rate, it can use the contents of the transfer rate select register
to decide when to assert the density select signal to the drive. The drive
then uses this signal to select the write current. Also if the drive is
dual-speed, it uses this same signal to select the rotation speed. That's
why it is usually called "density select", rather than "write current
select". Also note that the jumper in question doesn't control the write
current or the rotation speed directly. It merely tells the drive the
expected POLARITY of the density select signal on the interface (lead 2),
which can be different on different machines, since the SA-400/450 spec
predates high-density recording and doesn't define this signal.
   
   Sincerely,
   Michael Sokolov
   Phone: 440-449-0299
   ARPA Internet SMTP mail: sokolov@alpha.ces.cwru.edu