w8sdz%brl@sri-unix.UUCP (11/30/83)
From: Keith Petersen <w8sdz@brl>
REVERSING MEDIA ON SINGLE HEAD FLEXIBLE DISK DRIVES
typed by Keith Petersen, W8SDZ,
from information supplied by Dysan Corporation
There has been a tendency by some end users to economize by
attempting to use the media on both sides in a single head disk
drive. We must not lose sight of the fact that the value of the
data stored on diskettes exceeds the cost of the media by a wide
margin. Loss of data on either read or write means time delays,
reconstruction of lost data, and customer dissatisfaction with
the system, drive and/or media manufacturer. All of this can be
avoided in advance if the end user is made aware of the whys and
why nots.
HEAD SHOE AND PAD OPERATION
The relationship of the head to the media is such that when the
jacket is properly inserted, and all interlocks are satisfied,
the head is loaded onto the media on the recording side, and a
felt loading pad is applied to the non-recorded side. In normal
operation, a gradual build-up of oxide will accumulate on the
pressure pad. There might even be some wear on the non-recorded
side due to a scouring action of the oxide impregnated pad.
If the media is reversed, the scouring action will now occur on
the prime recorded side, and the previously scoured side is now
presented to an abrasive wearing by the contaminated load pad.
Since this data is not being read, there is not any means of
detecting the amount of wear or the loss of data. While a
catastrophic failure might not occur, it is possible that some
drop-out or other read error might go undetected. Worse yet, is
the possibility that the error condition might be intermittent,
which makes the entire operating system suspect. Another adverse
effect of reversing the media, is caused by reversing the
direction of rotation of the media against the pressure pad.
This reversal of direction is apt to "break off" any build-up of
oxide particles. This presents a potential loose contaminent
situation.
The net effect of this reversing (or flipping) action over a
period of time is to reduce performance and increase the
probability of drop outs and errors.
DISKETTE TENSIONING
On most Floppy Disk Drives, when the diskette is properly
inserted and operation has begun, pressure is applied to the
jacket on both sides so that proper tension is created on the
flexible media prior to the recording head. This also provides a
wiping action of the liner material against the flexible media.
When the jacket is reversed (or flipped), the direction of
rotation is reversed, breaking loose any extraneous particles
built-up by prior wiping. Thus, reversing the media increases
the probability of extraneous contamination and again increases
the possibility of errors.
TWO HEAD DRIVES
The above problem areas do not occur on two head drives that are
designed for two sided applications. On a two head drive, the
pressure pad has been replaced by a second head mounted in a
ceramic shoe. The operation now consists of a head-media-head
relationship. The soft pressure pad with possible oxide build-up
has been eliminated.
The diskette tensioning apparatus is the same on one and two head
drives. Since media spin direction is not reversed by flipping,
the oxide break-off problem does not occur.
SUMMARY
The foregoing summarizes the reasoning why Dysan and major OEM
suppliers of diskette drives do not recommend two sided media for
one head drive application. Dysan feels that the potential
operating problems would make an unwarranted reflection on our
reputation by using media in an unsuitable fashion. When IBM
introduced the 3740 diskette, they intentionally interlocked
reversal possibilities by offsetting the index hole from the
centerline. IBM does not make a reversable diskette.
Dysan does test and supply two sided media for operation in two
head (two sides) disk drives.graham@parsec.UUCP (12/15/83)
#R:sri-arpa:-1417400:parsec:48000005:000:1 parsec!graham Dec 14 20:50:00 1983