psc@lznv.ATT.COM (Paul S. R. Chisholm) (04/20/88)
< If you lined all the news readers up end-to-end, they'd be easier to shoot. >
For some reason, high density floppies (the kind that usually format to
1.2M on AT compatibles) *don't* work on 3B2s. They seem to format fine,
but they fail verification, often in the very first track. Regular DSDD
disks (which usually format to 360K on a PC clone) work better, though
you're asking them to perform above their specs. Disks especially made
for 720K work best of all.
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?
-Paul S. R. Chisholm, {ihnp4,cbosgd,allegra,rutgers}!mtune!lznv!psc
AT&T Mail !psrchisholm, Internet psc@lznv.att.com
This is *not* an official announcement by AT&T; I'm not speaking for my
employer, I'm just speaking my mind.brian@ucsd.EDU (Brian Kantor) (04/21/88)
In article <1363@lznv.ATT.COM> psc@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@ucsd.edu ucsd!brian BRIAN@UCSD
wtm@neoucom.UUCP (Bill Mayhew) (04/23/88)
High density (like "AT" 1.2 megabyte disks) are made from a different sort of magnetic material, similar to chromium dioxide cassette tape. If you look at the surface of an HD disk it looks sort of black, while standard floppies have a brownish color. The material in HD disks has a larger valued coercivity, which means that it is more difficult to (re)magnetize. Regular disk drives', even DSDD drives', recording heads don't putout enough magnetic field to properly write the HD medium. The reason for failure, in this case, is not related to bit density. Ironically (iron .. a pun), one is more likely to have success attempting to format a regular disk in an HD drive than vice - versa. I would not reccomend it, however. --Bill