Com 226 comp trouble shooting II theory book

Week8
This Week Learning Outcome
To Understand


The Cause of Floppy Drive Failure
Introduction (FLOPPY) The ability to interchange programs and data between various compatible computers is a fundamental requirement of almost every computer system. This kind of file-exchange compatibility helped rocket IBM PC/XTs into everyday use and spur the personal computer industry into the early s. A standardized operating system, file structure, and recording media also breathed life into the fledgling software industry. With the floppy disk, software developers could finally distribute programs and data to a mass-market of compatible computer users. The mechanism that allowed this quantum leap incompatibility is the
floppy-disk drive (Fig. 16-1). A floppy-disk drive (FDD) is one of the least expensive and most reliable forms of massstorage ever used in computer systems. Virtually everyone of the millions of personal computers sold each year incorporates at least one floppy drive. Most notebook and laptop computers also offer a single floppy drive. Not only are FDDs useful for transferring files and data between various systems, but the advantage of removable media—the floppy disk itself—make floppy drives an almost intuitive backup system for data files. Although floppy drives have evolved through a number of iterations from 8" to 5.25" to 3.5", their basic components and operating principles have changed very little.
Magnetic-Storage Concepts
Magnetic-storage media has been attractive to computer designs for many years—long before the personal computer had established itself in homes and offices. This popularity is primarily because magnetic media is nonvolatile. Unlike system RAM, no electrical energy is needed to maintain the information once it is stored on magnetic media. Although electrical energy is used to read and write magnetic data, magnetic fields do not change on their own, so data remains intact until other forces act upon it (such as another floppy drive. It is this smooth, straightforward translation from electricity to magnetism and back again that has made magnetic storage such a natural choice. To understand how a floppy drive works and why it fails, you should have an understanding of magnetic storage. This part of the chapter shows you the basic storage concepts used for floppy drives.
Media
For the purposes of this book, media is the physical material that actually holds recorded information. Ina floppy disk, the media is a small mylar disk coated on both sides with a precisely formulated magnetic material, often referred to as the oxide layer. Every disk manufacturer uses their own particular formula for magnetic coatings, but most coatings are based on a naturally magnetic element (such as iron, nickel, or cobalt) that has been

alloyed with nonmagnetic materials or rare earth. This magnetic material is then compounded with plastic, bonding chemicals, and lubricant to form the actual disk media. The fascinating aspect of these magnetic layers is that each and every particle media acts as a microscopic magnet. Each magnetic particle can be aligned in one orientation or another under the influence of an external magnetic field. If you have ever magnetized a screwdriver’s steel shaft by running a permanent magnet along its length, you have already seen this magnetizing process inaction. Fora floppy disk, microscopic points along the disk’s surfaces are magnetized in one alignment or another by the precise forces applied by read/write (R/W) heads. The shifting of alignment polarities would indicate a logic 1, but no change in polarity would indicate a logic 0 (you will see more about data recording and organization later in this chapter. In analog recording (such as audiotapes, the magnetic field generated by read/write heads varies indirect proportion to the signal being recorded. Such linear variations infield strength cause varying amounts of magnetic particles to align as the media moves. On the other hand, digital recordings, such as floppy disks, save binary sands by applying an overwhelming amount of field strength. Very strong magnetic fields saturate the media—that is, so much field strength is applied that any further increase infield strength will not cause abetter alignment of magnetic particles at that point on the media. The advantage to operating in saturation is that sands are remarkably resistant to the degrading effects of noise that can sometimes appear in analog magnetic recordings. Although the orientation of magnetic particles on a disk’s media can be reversed by using an external magnetic field, particles tend to resist the reversal of polarity. Coercivity is the strength with which magnetic particles resist change. More highly coercive material has a greater resistance to change, so a stronger external field will be needed to cause changes. High coercivity is generally considered to be desirable (up to a point) because signals standout much better against background noise and signals will resist natural degradation because of age, temperature, and random magnetic influences. As you might expect, a highly coercive media requires a more powerful field to record new information. Another advantage of increased coercivity is greater information density for media. The greater strength of each media particle allows more bits to be packed into less area. The move from 5.25" to 3.5" floppy disks was possible largely because of a superior (more coercive) magnetic layer. This coercivity principle also holds true for hard drives. To pack more information onto ever-smaller platters, the media must be more coercive. Coercivity is a common magnetic measurement with units in oersteds (pronounced “or-steds”). The coercivity of atypical floppy disk can range anywhere from 300 to 750 oersteds. By comparison, hard-drive and magneto-optical (MO) media usually offer coercivities up to
6000 oersteds or higher. The main premise of magnetic storage is that it is static (once recorded, information is retained without any electrical energy. Such stored information is presumed to last forever, but in actuality, magnetic information begins to degrade as soon as it is recorded. A good magnetic media will reliably remember (or retain) the alignment of its particles over along

period of time. The ability of a media to retain its magnetic information is known as
retentivity. Even the finest, best-formulated floppy disks degrades eventually (although it could take many years before an actual data error materializes. Ultimately, the ideal answer to media degradation is to refresh (or write over) the data and sector ID information. Data is rewritten normally each time a file is saved, but sector IDs are only written once when the disk is formatted. If a sector ID should fail, you will seethe dreaded Sector Not Found disk error and any data stored in the sector cannot be accessed. This failure mode also occurs in hard drives. Little can be done to ensure the integrity of floppy disks, aside from maintaining one or more backups on freshly formatted disks. However, some commercial software is available for restoring disk data (especially hard drives.
Troubleshooting Floppy Disk Systems This section of the chapter is concerned with drive problems that cannot be corrected with cleaning or mechanical adjustments. To perform some of the following tests, you should have a known-good diskette that has been properly formatted. The disk might contain files, but be certain that any such files are backed up properly on a hard drive or another floppy disk—if you can’t afford to lose the files on a disk, don’t use the disk.

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