20 Oct
2005
20 Oct
'05
4:42 p.m.
Subject: Re: PC floppy cable twists...
From: "Chuck Guzis" <cclist at sydex.com>
Date: Thu, 20 Oct 2005 16:02:43 -0700
To: cctalk at classiccmp.org
On 10/20/2005 at 6:23 PM Allison wrote:
I think we're saying the same thing in different ways. If you don't have a
head load signal, the head remains in contact with the media the entire
time the media's in the drive. If the spindle is rotating, that leads to
head and media wear the entire time (althought he drive needs to spin a bit
to seat the media correctly when first inserted). If the motor's turned
off, no media wear, even though the head's still in contact.
Nope not talking about media wear at all. I'm talking about the
SA400/TM100 generation drives that used a brush type motor with
belt drive that have a far shorter life than most of the halfheights
that use a brushless direct drive pancake motor.
The media wear is less an issue than the other problem with double sided
drives. Clapping, those with head load solinoids (SA450 comes to mind)
the head load/unload would not be gentle on the media. Thats why most
later drives continiously load the head even when not spinning.
And yes, motors with brushes will fail with time.
When I wrote my first
8085 diskette driver, I just turned the drive motors on at boot, having
come off an 8" floppy environment. I was admonished by a Micropolis
engineer that this was not a good idea and that the motor should be turned
on only when needed. So media was inserted into drives with the spindle at
a dead stop, leading to serious problems with seating the media. So drives
were modified to include a little spin-up when the drive door was being
closed. Not everyone did this gracefully, however, leading to wrinkled
areas around the media hub and so reinforcing rings were introduced.
Actually testing showed that freqquent start/stops were harder on the
motor to a certain point. It was the high current that would eventually
wear the commutator. If you ran the motors the problem was they were not
ball bearing and the bushing would wear and the brushes as well.
The interesting thing is the chicken-and-egg nature of
this. Initially,
you didn't want to spin the media all of the time on a 5.25" drive because
the brushes in the DC motor would wear. So a motor control line was
incorporated. But then, if you could stop the media from spinning, you
didn't have to have a separate head-load mechanism to guard the meda and
the head against wear while not in use.
there was a balancing act. Wearout for mechanical reasons and surge
startup. The trick was keeping the drive running once accessed as
it was likely you'd be back soon, if not time out. That lowered the
start stop cycle wear and got you closes to the mechanical wearout limit.
The later drives with direct drive motors and the like eliminated those
concerns. Then powering down was power/heat savings.
Media wear, I've had disks spinning for years in clean environments and
several dead drives that were still fine.
Allison