don't use AT-compatible floppies in a 3B2!!!

[Brian Kantor]

In article <1363 at lznv.ATT.COM> psc at lznv.ATT.COM (Paul S. R. Chisholm) writes:
>For some reason, high density floppies (the kind that usually format to
>1.2M on AT compatibles) *don't* work on 3B2s....
>Someone want to explain to me the difference between double sided,
>double density, 96 tracks per inch (3B2), and double sided, high
>density, 96 tracks per inch (AT)?  Does the latter store more sectors
>per track?

Yes.  The actual difference is that the data rate and rotational speed
of the diskette is higher in the "high density" mode.  "double density"
5-1/4" floppies spin at 300 rpm and have a 250KHz data rate; "high
density" spin at 360 RPM and use a 500 KHz data rate.  Note that the
standard IBM 360K floppy drive is a "double density" drive, it just has
half as many tracks.

I am told that the grain structure of the magnetic material is different
between the two types of diskettes to optimize recording for the
different rates.

We have often replaced the worn-out 3B2 floppy drive with an AT drive;
since it's $200 CHEAPER to buy the AT drive than the replacement from
AT&T, we're willing to spend the time moving the jumpers on the AT drive
to make it run at 300 RPM and use the 250KHz data rate.  Note that
floppy drive manufacturers DO make precisely the right drive to replace
the 3B2 drive; it is often sold as an "IBM-compatable 720K drive".  They
are real hard to find (none of the local "Taiwan, Inc." stores stock them,
although they'll cheerfully order them if we want to wait three weeks), 
so we just buy AT drives (which you can find everywhere for about $100) 
and throttle them back.  The technical manual on the drive will show you how.

	Brian Kantor	UCSD Office of Academic Computing
			Academic Network Operations Group  
			UCSD B-028, La Jolla, CA 92093 USA
			brian at ucsd.edu ucsd!brian BRIAN at UCSD