PC floppy cable twists...

[Allison]

>
>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