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.