Path: utzoo!utgpu!water!watmath!clyde!rutgers!mailrus!ames!oliveb!intelca!mipos3!cadev4!dbraun
From: dbraun@cadev4.intel.com (Doug Braun ~)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RESET BUTTONS AND HARD DISKS
Message-ID: <1691@mipos3.intel.com>
Date: 11 Feb 88 16:26:08 GMT
References: <3505@batcomputer.tn.cornell.edu> <-497141@cpedev> <662@gethen.UUCP> <19824@bu-cs.BU.EDU>
Sender: nobody@mipos3.intel.com
Reply-To: dbraun@cadev4.UUCP (Doug Braun ~)
Organization: Corporate CAD, INTeL Corporation, Santa Clara, CA
Lines: 50

In article <19824@bu-cs.BU.EDU> madd@bu-it.bu.edu (Jim Frost) writes:
>
>This isn't necessarily true.  In all but the cheapest (and the very
>very expensive) drives, the heads will not touch the surface of the
>disk unless the disk is moves (relatively) violently.  This is why you
>usually need to park the heads only when you move the drive.  Think of
>it this way: drives take several seconds to spin down.  If the heads
>were only electronically suspended, they would lose suspension far
>before the drive stopped spinning.  The heads would scrape across the
>surface of the disk, damaging both the heads and the disk, until the
>disk came to a full stop.  Trust me, you'd hear it.
>
>I recall a time back in the CP/M days when a drive that just dropped
>the heads was turned off before doing a power down sequence.  The
>noise that that thing made was enough to make anyone cry.

This portion of his reply is incorrect.  The heads are suspended
by the air stream caused by the rotating disk.  The heads are literally
"flying" or "planing" above the disk.  This has nothing to
do with anything electronic.  Winchesters usually have a tiny bit of
lubricant on the disks to protect the heads when they do land
as the drive stops spinning.  More expensive drives will actually
physically push the heads away from the disk at power-off,
thus making the drive very shock-resistant when turned off.
Other drives will move the heads to an unused area of the disk
(the "landing zone") at power-off.  The cheaper stepper-motor drives
(which includes almost ALL <=20meg drives used with XT's, including
the popular Seagate ST-238 and its brethren) just die when the power
goes off, and the heads land where they may.  On these you have to
explicitly park the heads before power-off.

The noise mentioned above is undoubtedly due to a mechanical brake
that stops the rotating disks ASAP after power-off.  Some drives
have a brake with a little solenoid pulls the brake pad away
from the motor rotor when power is applied.  Other drives use a relay
to short out the motor coils when power is removed, electrically
braking the motor.

Take apart a few of these beasties, and you'll know what's
REALLY going on.


Doug Braun				Intel Corp CAD
					408 496-5939

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