Upgrading and Repairing PCs

Analyzing 3 1/2'' Floppy Disk Construction

3 1/2'' disks differ from the older 5 1/4'' disks in both construction and physical properties. The flexible (or floppy) disk is contained within a plastic jacket. The 3 1/2'' disks are covered by a more rigid jacket than are the 5 1/4'' disks. The disks within the jackets, however, are virtually identical except, of course, for size.

For more information about the construction of a 5 1/4'' floppy disk, see the Technical Reference section on the DVD packaged with this book.

The 3 1/2'' disks use a much more rigid plastic case than 5 1/4'' disks, which helps stabilize the magnetic medium inside. Therefore, the disks can store data at track and data densities greater than the 5 1/4'' disks (see Figure 11.7). A metal shutter protects the media-access hole. The drive manipulates the shutter, leaving it closed whenever the disk is not in a drive. The medium is then completely insulated from the environment and from your fingers. The shutter also obviates the need for a disk jacket.

Because the shutter is not necessary for the disk to work, you can remove it from the plastic case if it becomes bent or damaged. Pry it off the disk case; it will pop off with a snap. You also should remove the spring that pushes it closed. Additionally, after removing the damaged shutter, you should copy the data from the damaged disk to a new one.

Rather than an index hole in the disk, the 3 1/2'' disks use a metal center hub with an alignment hole. The drive "grasps" the metal hub, and the hole in the hub enables the drive to position the disk properly.

On the lower-left part of the disk is a hole with a plastic slider—the write-protect/enable hole. When the slider is positioned so the hole is visible, the disk is write-protected, meaning the drive is prevented from recording on the disk. When the slider is positioned to cover the hole, writing is enabled, and you can save data to the disk. For more permanent write-protection, some commercial software programs are supplied on disks with the slider removed so you can't easily enable recording on the disk. This is exactly opposite of a 5 1/4'' floppy, in which covered means write-protected, not write-enabled.

On the other (right) side of the disk from the write-protect hole is usually another hole called the media-density-selector hole. If this hole is present, the disk is constructed of a special medium and is therefore an HD or ED disk. If the media-sensor hole is exactly opposite the write-protect hole, it indicates a 1.44MB HD disk. If the media-sensor hole is located more toward the top of the disk (the metal shutter is at the top of the disk), it indicates an ED disk. No hole on the right side means that the disk is a low-density disk. Most 3 1/2'' drives have a media sensor that controls recording capability based on the absence or presence of these holes.

The actual magnetic medium in both the 3 1/2'' and 5 1/4'' disks is constructed of the same basic materials. They use a plastic base (usually Mylar) coated with a magnetic compound. High-density disks use a cobalt-ferric compound; extended-density disks use a barium-ferric media compound. The rigid jacket material on the 3 1/2'' disks has occasionally caused people to believe incorrectly that these disks are some sort of "hard disk" and not really a floppy disk. The disk cookie inside the 3 1/2'' case is just as floppy as the 5 1/4'' variety.

Floppy Disk Media Types and Specifications

This section examines the types of disks that have been available to PC owners over the years. Especially interesting are the technical specifications that can separate one type of disk from another, as Table 11.8 shows. The following sections define all the specifications used to describe a typical disk.

Table 11.8. Floppy Disk Media Specifications

	5 1/4''			3 1/2''
Media Parameters	Double-Density (DD)	Quad-Density (QD)	High-Density (HD)	Double-Density (DD)	High-Density (HD)	Extra High-Density (ED)
Tracks per inch (TPI)	48	96	96	135	135	135
Bits per inch (BPI)	5,876	5,876	9,646	8,717	17,434	34,868
Media formulation	Ferrite	Ferrite	Cobalt	Cobalt	Cobalt	Barium
Coercivity (oersteds)	300	300	600	600	720	750
Thickness (micro-in.)	100	100	50	70	40	100
Recording polarity	Horiz.	Horiz.	Horiz.	Horiz.	Horiz.	Vert.

Density

Density, in simplest terms, is a measure of the amount of information that can be reliably packed into a specific area of a recording surface. The keyword here is reliably.

Disks have two types of densities: longitudinal density and linear density. Longitudinal density is indicated by how many tracks can be recorded on the disk and is often expressed as a number of tracks per inch (TPI). Linear density is the capability of an individual track to store data and is often indicated as a number of bits per inch (BPI). Unfortunately, these types of densities are often confused when discussing different disks and drives.

Media Coercivity and Thickness

The coercivity specification of a disk refers to the magnetic-field strength required to make a proper recording. Coercivity, measured in oersteds, is a value indicating magnetic strength. A disk with a higher coercivity rating requires a stronger magnetic field to make a recording on that disk. With lower ratings, the disk can be recorded with a weaker magnetic field. In other words, the lower the coercivity rating, the more sensitive the disk.

HD media demands higher coercivity ratings so the adjacent magnetic domains don't interfere with each other. For this reason, HD media is actually less sensitive and requires a stronger recording signal strength.

Another factor is the thickness of the disk. The thinner the disk, the less influence a region of the disk has on another adjacent region. The thinner disks, therefore, can accept many more bits per inch without eventually degrading the recording.

Caring for and Handling Floppy Disks and Drives

Most computer users know the basics of disk care. Disks can be damaged or destroyed easily by the following:

• Touching the recording surface with your fingers or anything else

• Writing on a disk label (which has been affixed to a disk) with a ball-point pen or pencil

• Bending the disk

• Spilling coffee or other substances on the disk

• Overheating a disk (leaving it in the hot sun or near a radiator, for example)

• Exposing a disk to stray magnetic fields

Despite all these cautions, disks are rather hardy storage devices; I can't say that I have ever destroyed one by just writing on it with a pen because I do so all the time. I am careful, however, not to press too hard, so I don't put a crease in the disk. Also, touching a disk does not necessarily ruin a disk, but rather makes the disk and your drive head dirty with oil and dust. The real danger to your disks comes from magnetic fields that, because they are unseen, can sometimes be found in places you never imagined.

For example, all color monitors (and color TV sets) that use cathode-ray tube (CRT) technology have a degaussing coil around the face of the tube that demagnetizes the shadow mask when you turn on the monitor. If you keep your disks anywhere near (within 1' of) the front of a color monitor, you expose them to a strong magnetic field every time you turn on the monitor. Keeping disks in this area is not a good idea because the field is designed to demagnetize objects, and it indeed works well for demagnetizing disks. The effect is cumulative and irreversible. Note that LCD or plasma displays don't have degaussing coils and therefore do not affect magnetic media.

Another source of powerful magnetic fields is an electric motor found in vacuum cleaners, heaters, air conditioners, fans, electric pencil sharpeners, and so on. Do not place these devices near areas where you store disks. Audio speakers also contain magnets, but most of the speakers sold for use with PCs are shielded to minimize disk corruption.

Store 3 1/2'' disks between 40° and 127° Fahrenheit, and store 5 1/4'' disks between 40° and 140° Fahrenheit. In both cases, humidity should not exceed 90%.

Airport X-Ray Machines and Metal Detectors

One of my favorite myths to dispel is that the airport X-ray machine somehow damages disks. I have a great deal of experience in this area from having traveled around the country for the past 20 years or so with disks and portable computers in hand. I fly about 150,000 miles per year, and my portable computer equipment and disks have been through X-ray machines hundreds of times.

X-rays are essentially just a form of light, and disks and computers are not affected by X-rays at anywhere near the levels found in these machines.

What could potentially damage your magnetic media is the metal detector. Metal detectors work by monitoring disruptions in a weak magnetic field. A metal object inserted in the field area causes the field's shape to change, which the detector observes. This principle, which is the reason the detectors are sensitive to metal objects, can be dangerous to your disks; the X-ray machine, however, is the safest area through which to pass either your disk or your computer.

The X-ray machine is not dangerous to magnetic media because it merely exposes the media to electromagnetic radiation at a particular (very high) frequency. Blue light is an example of electromagnetic radiation of a different frequency. The only difference between X-rays and blue light is in the frequency, or wavelength, of the emission.

Some people worry about the effect of X-ray radiation on their system's EPROM (erasable programmable read-only memory) chips. This concern might actually be more valid than worrying about disk damage because EPROMs are erased by certain forms of electromagnetic radiation. In reality, however, you do not need to worry about this effect either. EPROMs are erased by direct exposure to very intense ultraviolet light. Specifically, to be erased, an EPROM must be exposed to a 12,000uw/cm² UV light source with a wavelength of 2,537 angstroms for 15–20 minutes, and at a distance of 1''. Increasing the power of the light source or decreasing the distance from the source can shorten the erasure time to a few minutes.

The airport X-ray machine is different by a factor of 10,000 in wavelength. The field strength, duration, and distance from the emitter source are nowhere near what is necessary for EPROM erasure. Many circuit-board manufacturers even use X-ray inspection on circuit boards (with components including EPROMs installed) to test and check quality control during manufacture.

Now, you might not want to take my word for it, but scientific research has been published that corroborates what I have stated. A study was published by two scientists—one of whom actually designs X-ray tubes for a major manufacturer. Their study was titled "Airport X-rays and Floppy Disks: No Cause for Concern" and was published in 1993 in the journal Computer Methods and Programs in Biomedicine. According to the abstract,

A controlled study was done to test the possible effects of X-rays on the integrity of data stored on common sizes of floppy disks. Disks were exposed to doses of X-rays up to seven times that to be expected during airport examination of baggage. The readability of nearly 14 megabytes of data was unaltered by X-irradiation, indicating that floppy disks need not be given special handling during X-ray inspection of baggage.

In fact, the disks were retested after two years of storage, and there still has been no measurable degradation since the exposure.