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Floppy Disks

Since the first computer booted up, the floppy disk has been a blessing and a curse, subject of the same old saying usually reserved for kids, spouses, and governments: "You can't live with them, and you can't live without them." They were the first storage system for personal computers, and they've remained relatively unchanged ever since. Microprocessors might be 10,000 times faster than they used to be. Hard disks might hold 10,000 times more bytes. But today's basic floppy disk hardly holds about the same amount of data as the very first. Although the floppy disk has shrunk from eight inches to three-and-a-half and grown a hard turtle-like shell, the basic disk still holds little more than a megabyte, truly puny in a world of giga-everything.

Despite the best efforts of computer-makers, floppy disks aren't dead yet. You can still depend on them in the vilest of emergencies, when nothing else works to bring your computer to life. You (or a technician) can still troubleshoot your computer using its floppy disk drive. And you can still exchange files with friends on the cheap, even disposable, floppy disk medium. The floppy disk still has a role, although a modest one, in the life of your computer.

The floppy disk provides a recording medium that has several positive qualities. The flat disk surface allows an approximation of random access. As with hard disks, data is arranged in tracks and sectors. The disk rotates the sectors under a read/write head, which travels radially across the disk to mark off tracks. More importantly, the floppy disk is a removable medium. You can shuffle dozens of floppies in and out of drives to extend your storage capacity. The floppy disk in the drive provides online storage. Offline, you can keep as many floppy disks as you want.

The term floppy disk is one of those amazingly descriptive terms that abound in this age of genericisms. Inside its protective shell, the floppy disk medium is both a floppy (flexible) and wide, flat disk. To protect it, the floppy resides in a shell. The first floppies had a truly floppy protective shell, one made out of thicker but still flexible Mylar. Today, the floppy fits into a hard case and overall is not very floppy. The disk inside, the one made from the media, remains floppy so the name remains the same—uniquely accurate in a world of computers inhabited by spinning fixed disks and recordable read-only memory.


When the floppy was first conceived, personal computers didn't exist. Its creator, IBM, first used the floppy disk to store diagnostic programs and microcode for its large computer system—instead of tape (too cumbersome, and many applications didn't require such a large capacity) or memory chips (too expensive). It was the Dark Ages equivalent of a CD-ROM. It was also big, about eight inches square, but for the time had a large capacity—about 100KB—and random-access capabilities.

By 1973, engineers had adapted the eight-inch floppy to a convenient read/write medium suitable for both the original application and for storage for data-entry systems, such as IBM's DisplayWriter word processing system. The 8-inch floppy disk had a number of features going for it that made it desirable as a computer data-storage medium. The floppy disk was compact (at least compared to the ream of paper that could hold the same amount of information), convenient, and standardized. Above all, it was inexpensive to produce and reliable enough to depend on.

In 1976, Shugart Associates introduced the 5.25-inch floppy disk, a timely creation that exactly complimented the first commercial personal computers, introduced at about the same time. (Both Apple Computer and Microsoft Corporation were founded in 1976, although fledgling Microsoft offered a BASIC interpreter as its first product—its operating system for floppy disks did not arrive until 1981.) Because these were smaller than the older eight-inch variety, these 5.25-inch floppies were called diskettes by some. The irregularly used name later spread to even smaller sizes of the floppy disk.

In 1980, Sony Corporation introduced the 3.5-inch floppy disk of the same mechanical construction that we know it today. After a lukewarm reception, the little disks gained a foothold when Apple adopted them for its initial Macintosh in 1984. Portable computers added more incentive for the move to 3.5-inch disks, as did IBM's adoption of the format in 1987. The standard floppy disk has remained unchanged since then, despite further attempts to improve it (such as the "quad-density" disks with double the normal capacity).

The first appearance of Mount Rainier technology in CD-RW drives (see Chapter 18, "Optical Storage (CD and DVD)") in late 2002 likely marked the end-of-the-line for the floppy disk. This new industry initiative endows the CD with all the capabilities of a floppy disk, in addition to its much greater capacity. As Mount Rainier drives become widespread, floppy disk drives will become truly obsolete.


The traditional floppy disk medium itself is the thin, flexible disk inside the protective shell. This disk is actually a three-layer sandwich, the meat of which is a polyester substrate that measures about 3.15 mils (thousandths of an inch), or 80 micrometers thick. The bread is the magnetic recording medium itself, a coating less than one-thousandth of an inch thick on each side of the substrate.

The floppy disk medium starts out as vast rolls of the substrate that are coated in a continuous process at high speed. The stamping machine cuts individual disks from the resulting roll, or web, of medium, like a cookie cutter. After some further mechanical preparation (for example, a metal hub is attached to the cookies of 3.5-inch disks), another machine slides the disks into their protective shells.

Although retailers sell some floppy disks as "single sided," a mixture of magnetic oxide and binder coats both sides of the substrate of all floppy disks. Those sold as single sided are only tested on one side, and their manufacturers only certify that one side for storing data. By convention, the bottom surface of the disk is used in single-sided floppy disk drives.

Magnetic Properties

The thickness of the magnetic coating on the floppy disk substrate varies with the disk type and storage density. In the most common disk types, it measures from 0.035 mil to 0.1 mil (that is, 0.9 to 2.5 micrometers). In general, the higher the storage density of the disk, the thinner the magnetic coating. The individual particles are also finer grained. Table 17.3 lists the coating thicknesses for both current and obsolete floppy disk types.

Table 17.3. Floppy Disk Media Characteristics
Disk Type Coating Thickness Coercivity
5.25-inch double-density 2.5 micrometers 290 oersteds
5.25-inch high-density 1.3 micrometers 660 oersteds
3.5-inch double-density 1.9 micrometers 650 oersteds
3.5-inch high-density 0.9 micrometers 720 oersteds

Although all common floppy disk coatings use ferric oxide magnetic media, engineers have tailored the magnetic particles in the mix to the storage density at which the disks are used. The higher storage density media generally have higher coercivities, as shown in Table 17.3.

Despite the names used by various floppy disk types, all floppy disk drives use Modified Frequency Modulation (MFM) recording, which produces double-density recording. In other words, both normal and high-density disks are double-density, even though many manufacturers reserve the double-density term for lower-capacity disks.

Extra-high-density disks (that is, the 2.88MB floppies still occasionally sold) use a novel technology called perpendicular recording. In conventional floppy disks, the grains of magnetic medium are aligned flat against the substrate, and the write process aligns them along a radius of the disk. The particles of the medium used in perpendicular recording are arranged vertically so that one end of each particle points toward the substrate. A special read/write head in the disk drive changed the vertical orientation of the magnetic field of the media particles. In addition, extra-high-density disks use a high-coercivity barium-ferrite recording medium.


The chief difference between a modern 3.5-inch floppy disk and the earliest disks is the hard plastic shell. Magnetically and logically, the disks work the same as the first 8-inch floppies. Drives, too, are basically the same, only miniaturized to suit the medium and modern computers.

The shell protects the magnetic medium inside from whatever terrors lurk in your world, be they teething infants, churning chair casters, or simple carelessness. The tough shell allows you to put a label on the disk and then write whatever identification you want on it, even with a ballpoint pen. The thinner shells of earlier disks could not protect the media within from pen points, so writing on a label already affixed to an early floppy could crease the medium and make it unreadable. Figure 17.2 shows the shell of a 3.5-inch floppy disk and its notable external features.

Figure 17.2. Layout of 3.5-inch floppy disk.


The shell of a 3.5-inch floppy disk measures 3.7 inches (94 millimeters) front to back, and just over 3.5 inches (90 millimeters) wide. Each disk is a little more than one-eighth inch (3.3 millimeters) thick. A single disk weighs just over three-quarters of an ounce, about 22 grams. Despite its 3.5-inch name, the media disk inside the shell actually measures about 3.4 inches (nominally 86 millimeters) across.

To protect the medium inside from dirt, dust, and fingerprints, the head access opening on a 3.5-inch floppy disk is covered with a spring-loaded sliding metal shield or shutter. It opens automatically only when you insert a disk into a drive and slides closed when you pop the disk from the drive. The shutter is an effective dust shield that prevents contaminants from collecting on the medium surface whenever the disk is out of its drive. This protection means that 3.5-inch disks include all the protection they need and don't need an additional sleeve or shuck that older floppy disks required.


As protective as the hard shell of 3.5-inch floppies may be, the medium needs protection from it as well. A thin sheet of unwoven cloth akin a loose-woven paper towel cushions each side of the medium from rubbing against the shell. The interlocked threads of the liner serve as a slip-sheet and dust collector. Light contact with the strands of the liner let the medium slide with little friction. At the same time, the material sweeps dust and particles from the surface of the medium so that it doesn't scrape against the read/write head of the drive.


The creators of the 3.5-inch floppy disk learned that the one area of older floppies that sustained the most damage was the hub hole. Every time you slide a disk into a drive, drive hubs clamp onto the disk to hold and spin it. The hub entered the drive hole and forced the entire disk into the proper position for proper reading and writing. At times, the hub clamping down damaged the edges of the hole. After a disk suffered enough damage, it could become unreliable or unusable. The only protection to the hub hole offered by older floppy disks was an optional reinforcing ring some manufacturers elected to add to the perimeter of the hub hole.

The 3.5-inch floppy eliminates such problems by using a sturdy metal hub with a square center spindle hole, which mates with the mechanism of the disk drive. The stamped steel hub resists damage. The single rectangular cutout in the hub allows the drive mechanism to unambiguously identify the radial alignment of the disk. The cutout serves as a mechanical reference for the position of data on the disk. The hub itself is glued to the disk media.

Insertion Key

A 3.5-inch floppy disk has four edges and two sides, giving you eight possible ways to try to slide it into your disk drive, only one of which is correct. Although the over-square design of the floppy shell prevents you from sliding a disk in sideways, you can still shove one in backward or upside down. To ensure that you don't damage your disk drive by improperly inserting a disk, the shell of 3.5-inch floppies is keyed by a notch in one corner. A tab in the drive blocks this corner, so if the notch is not present you cannot slide the disk all the way into the drive. The last few millimeters of sliding in the disk open the shutter and load the heads against the disk, so the notch prevents the heads from ramming against the floppy's shell instead of the access area.

Molded into the plastic of the shell is a small arrow that serves as a visual reminder to you. It points to the edge of the disk that you should slide into your disk drive, just in case the shutter is not enough guidance.

Write Protection

The fundamental design of the floppy disk is to serve as a read/write medium so that you can store information on it and read it back. Sometimes, however, you might want to protect the data on a floppy disk from change. For example, you might back up your archives to floppy disk. Software vendors prefer protection against writing to the distribution disks holding their programs so that you don't accidentally erase the code and become a support problem for them.

The 3.5-inch floppy design incorporates a write-protect tab that allows you to make any floppy disk a read-only medium. The design uses a hole and a plastic slider. When the slider blocks the hole, you can read, write, and format the disk. When the slider is moved back to reveal the hole, an interlock on the drive prevents you from writing to the disk. You can move the slider back and forth to write-protect a disk and then make it writable again as often as you want.

Software vendors often remove the slider entirely. Without the slider, the write-protected hole cannot be covered and the disk is permanently write-protected. You can, however, circumvent even this "permanent" form of write-protection by blocking off the write-protect hole. The easiest way is to cover both sides of the hole with opaque tape to make a distribution disk writable. This method is not without its dangers, however. If the tape does not stick tightly to the disk, it can jam the drive mechanism, likely preventing you from popping or pulling the disk out of the drive.

Density Key

In order for your disk drive to determine the type of magnetic medium on your disk so that its electronics can be adjusted to match the disk's coercivity, 3.5-inch floppy disks incorporate a density key. This key is actually the presence or absence of a hole in one corner of the disk shell. Double-density disks lack a hole. High-density disks have a hole. An extra-high-density notch marks disks with 2.88MB capacity. Higher-density disks also have a visual indication of their capacity—for example, the stylized "HD" silk-screened near the shutter of high-density disks.

The connections between many floppy disk drives and the host computer often do not properly relay the density key information to the computer and its operating system. When the density key information is not available, you can format a double-density disk as high density. In years gone by, some enterprising manufacturers also offered hole-punches designed to add a density-key hole to double-density disks so that you could format them as high density. Although the difference in coercivities between double- and high-density media is modest, there are other differences in the formulation of the media that make double-density disks unreliable at high-density capacities. Moreover, the hole-punches often left residue in the form of small particles of plastic to contaminate the disks (at the factory the density key is molded rather than punched). These contaminants can shorten the life of the medium or damage your disk drive.


Four formats are commonly used for 3.5-inch floppy disks, three of which are supported on computers. (Computers do not support single-sided 3.5-inch floppy disk formats.) Your disks drives and operating system automatically adjust to the format of the disks you attempt to read, providing your drive is capable of reading the format. All higher-capacity drives can read formats of lower capacity. Table 17.4 summarizes the essential characteristics of these four formats for 3.5-inch floppy disks.

Table 17.4. The Characteristics of a 3.5-inch Floppy Disk
Capacity Units 360KB 720KB 1.44MB 2.88MB
Sides Number 1 2 2 2
Tracks Number 80 80 80 80
Sectors per track Number 9 9 18 36
Sector size Bits 512 512 512 512
Rotation rate RPM 300 300 300 300
Data-transfer rate Kbps 500 500 500 1000
Bit density (max) BPI 8717 8717 17,434 34,868
Track density TPI 135 135 135 135
Coercivity Oersteds 650 650 720 1200

The capacity of a floppy disk is set when the disk is formatted. Using the Format option from the Windows menu associated with your floppy disk drive, you can select the capacity of new floppy disks (or reformat floppies to change their capacity). Preformatted disks relieve you of the chore of formatting floppies yourself, although you can always format over the factory format to change the capacity of a disk.

Double density is the starting format for 3.5-inch floppy disks. It uses 80 tracks with nine sectors per track. The tracks are spaced 135 to the inch (about 5.3 to the millimeter). The small diameter disk only allows a swath about 0.6 inch (15 millimeters) wide around the disk for reading and writing. The high-density format merely doubles the number of sectors per track, packing the data in twice as tightly.

Because 3.5-inch floppy disks spin at a fixed speed of 300 RPM, doubling the density of the data on each track also boosts the speed at which the information is read from the disk. The basic reading speed of 250Kbps for double-density is doubled to 500Kbps with high-density disks.

Extra-high-density disks again double the sector count on each track, to 36 sectors per track, without changing the number of tracks or their spacing. Again, this increase in storage density also increases the reading and writing speed to 1000Kbps.

Note that the capacities of all floppy disks are given with the format in place. The formatting data steals away some of the usable storage area of the disk. Floppy disk–makers occasionally list the unformatted capacities of their products. The unformatted capacity of a double-density (720KB formatted) disk is 1MB; that of a high-density (1.44MB formatted) disk is 2MB; that of an extra-high-density (2.88MB formatted) disk is 4MB.

Before CD-ROM media took over the job of program distribution, software publishers sought and developed ways of shoehorning more data onto every floppy disk. The leading extra-capacity alternative was Microsoft's Distribution Media Format (DMF). This variation in the high-density design allowed Microsoft to fit 1,720,320 bytes on a standard high-density 3.5-inch floppy disk in place of the more normal 1,474,560 bytes (nominal 1.44MB). The DMF format differed from the standard format in that it used 21 sectors per track instead of the normal 18. DMF squeezed more sectors on each track by reducing the interrecord gap (the space between sectors) down to nine bytes.

The differences went deeper, however. Each track used a 2:1 interleave factor so that sectors did not appear in order. This interleaving resulted in slower reading because the disk had to spin around twice for each track to be read. The DMF format also skewed the sectors on adjacent tracks by three sectors so that Sector 1 on Track 4 sat next to Sector 1 on Track 2.

The small interrecord gap made DMF disks difficult to write to with normal floppy disk drives. In fact, Microsoft called DMF a read-only format. In any case, you cannot write to DMF disks using ordinary software. However, several special utilities are available for copying and even creating DMF disks. Note that Microsoft enforces a limit of 16 entries in the root directory of a DMF disk by only allocating a single cluster to service as the root, so DMF-formatted disks usually use a subdirectory structure for their contents.

Microsoft operating systems cannot ordinarily read DMF floppies. Consequently, DMF floppies were used only when software required multiple floppy disks for its installation. The first disk of the installation package—usually called the setup disk—loaded software to reprogram your floppy disk controller to read the DMF disks.

Zip Disks

The Zip disk, developed and first marketed by Iomega Corporation in 1995, was the first floppy disk system with a capacity larger than 100MB. Although initially a proprietary system, Iomega has licensed Zip to other companies. Both drives and media are now available from multiple sources, although the format is totally under the control of Iomega.

To gain its large capacity, the Zip system uses optical technology that was first applied to a little-remembered 20MB floppy disk system called Floptical. The Zip medium uses an optically read servo track to allow repeatable head positioning in fine increments.

The first generation of Zip disks had listed nominal capacities of either 25MB (25,107,968 actual bytes) or 100MB (actually 100,431,872 bytes) per disk.

By increasing the density of storage on slightly modified media, in 1998 Iomega was able to boost the capacity of the Zip system by a factor of two and a half without major modifications to the drive mechanism. As a result, Iomega was able to offer an improved system with 250MB capacity at the same initial price as the original Zip drive.

Thanks to the increased lineal density of data on the 250MB Zip tracks, the data throughput of the new system is more than two times greater than the old, although access time remains comparable. Actual data throughput often is limited by the drive interface. High-capacity Zip drives use the same interfaces as the old, and performance through parallel ports is compromised.

The shell of the Zip disk makes it a true cartridge. To achieve speeds in the hard disk range, the medium must spin at a high rate, and the friction from rubbing against a liner is an anathema to speed. The thicker cartridge gives the Zip disk spinning room and enables it to rotate at 2968 RPM. In addition, the Zip disk drive has hard disk–like access speeds, with the first generation of drives having a 26 millisecond average access time. The disk requires about three seconds to spin up to speed or spin down, which becomes a factor only when you're exchanging cartridges.

The actual Zip media disk inside a cartridge measures true 3.5-inches across. Consequently, the cartridge must be larger than conventional 3.5-inch floppies and MO cartridges, measuring 3.7 inches (94 millimeters) square and a quarter-inch (6.35 millimeters) thick. These dimensions alone make the Zip disk incompatible with traditional floppy disks.

Instead of using a mechanical write-protect mechanism on the cartridge shell, Zip disks are write-protected electronically. The Iomega system provides three protection modes with optional password access limits as part of its ZipTools software. The three modes include conventional write protection that prevents the inadvertent alteration of data on the disk; read/write protection, which requires a password to access data on the disk; and unprotect until eject, which lets you work with the data on the disk but protects the disk when you remove it from the drive. The same software that adds write protection is required to remove it. Passwords, however, are not recoverable—even by Iomega.

Zip disk drives read only Zip disks. Iomega has manufactured products using any of four interfaces or ports—AT Attachment (IDE), parallel port, SCSI, and USB. IDE is now favored for internal drives, and USB 2.0 is favored for external drives.

The move to 250MB required changes to both the medium and drives, although superficially high-capacity Zip disks look almost identical to their older, low-capacity forebears. The drive electronics were adapted to accommodate the higher read and write rates required by the new media, and the magnetic medium has a higher coercivity to improve high-density storage. As a consequence, older drives cannot be upgraded, and disks with 250MB capacity are rejected outright by older drives as incompatible.

Because of the laser/mechanical formatting of the disks and the differing media, low-capacity Zip disks cannot be reformatted to higher capacities. In fact, the servo tracks cannot be erased by any means that would not destroy the cartridge.

On the other hand, high-capacity Zip drives are fully backward compatible with old media. New 250MB drives can read and write 100MB disks. Old drives, however, can neither read nor write 250MB media.

In 2002, Iomega further refined the Zip drive to increase its capacity to 750MB per disk. The chief trick was simply to increase the storage density, resulting in 4,780 tracks per inch with data squeezed into each track at 137Kb per inch. The disk itself spins at 3676 RPM. The drives use the USB 2.0 interface and feature a data-transfer rate that peaks at 7.5Mbps. Although compatible with USB 1.0 connections, using the older interface will severely degrade the performance of the drive.

The new 750MB disks can only be read and written by matching 750MB drives. However, the drives are backward compatible with earlier Zip disk formats. A 750MB drive can both read and write 250MB disks and read 100MB disks. It cannot write 100MB disks.


Jointly developed by Compaq Computer Corporation, the storage products division of 3M Company (now Imation Corporation), Matsushita-Kotobuki Electronics Industries, Ltd., and O. R. Technology, parent company of Optics Research, Inc., the SuperDisk was supposed to be the next generation of floppy disk drive. First marketed in March, 1996 as the LS-120 system, the first drives were made by MKE, installed in Compaq computers, and used media manufactured by 3M. The optical technology used by the drives was developed by ORT.

Outwardly a SuperDisk resembles an ordinary 3.5-inch floppy diskette. The only obvious difference is the shape of the shield over the head slot. Instead of rectangular, the SuperDisk wears a roughly triangular shield. Figure 17.3 shows an LS-120 diskette.

Figure 17.3. An LS-120 diskette.


Inside, however, the SuperDisk system uses a thinner substrate (2.5 mils) than traditional floppies, one that is cut from a different plastic, polyethylene terathalate (PET). This substrate is more flexible than the traditional polyester to give the SuperDisk medium better bend around the head for more reliable contact.


Each SuperDisk diskette can store up to 125,829,120 bytes, or 120MB, accounting for the designation of the drive. Not all of that capacity is usable. As with conventional floppy disks, part of the total capacity must be devoted to the FAT and directory data.

The SuperDisk drive combines two technologies to increase its capacity: an opto-mechanical laser servo system and zoned recording.

The major increase comes from increasing the track density to 2490 per inch, achieved using the laser servo system. The small diameter of the disk only allows 1736 tracks per side at this density. Even so, it requires a special medium, a two-layer metal particle compound.

For compatibility with computer hardware and software, the track layout of the SuperDisk medium is mapped to a logical format (the way the drive appears to your computer), having 960 tracks per side. This mapping results in the drive—which has two heads, one for each disk side—appearing to your system as if it has eight heads.

The SuperDisk system uses zoned recording with 55 different zones across the radius of each disk. The number of sectors per track varies from 51 on the innermost tracks to 92 at the outer edge. Table 17.5 lists the physical and logical formats of an LS-120/SuperDisk diskette.

Table 17.5. LS-120 Disk Format
Format Type Cylinders Heads Sectors
Physical 1736 2 51 to 92 (zoned recording)
Logical 960 8 32

The SuperDisk medium spins at a constant 720 revolutions per minute. As a result of the constant spin and varying sector count, a SuperDisk drive reads data from the disk at a rate that changes with the track position of the read/write head. Near the center of the disk, the transfer rate is lowest, about 400KBps. At the outer edge, the transfer rate reaches 665KBps. Initial SuperDisk drives had an average access time of about 70 milliseconds. Table 17.6 lists the specifications of the SuperDisk medium and its format.

Table 17.6. LS-120 Specifications
Feature Specification
Media diameter 3.5 inches
Recording medium Metal particle
Capacity (formatted) 120MB
Bytes per sector 512
Sectors per track 51–92 (ZBR)
Track data capacity 26,112–47,616KB
Tracks per surface 1736
Track density 2490 tracks per inch
Areal density 33,660 flux changes/inch
Recording surfaces (sides) 2
Track pitch 10.2 µm
Peak transfer rate 3.20 to 5.33MBps
Throughput 313–571
Estimated life 5 million passes


The stepper-motor-based open-loop mechanisms used by conventional floppy disk drives cannot correctly position a read/write head with sufficient precision to reliably locate the fine tracks used by the SuperDisk system. To achieve the necessary track density, the SuperDisk system used an embedded-servo system. Servo data is etched into the disk during factory preparation. The SuperDisk drive then uses a laser to detect and read the etched servo information from the disk and align the read/write head properly on each track. In fact, the "LS" designation is an abbreviation for laser servo.

The read/write head has two gaps—one used for the fine tracks used in high-density recording and one used for working with conventional double-density and high-density floppy disk media. The laser servo design does not increase the capacity of conventional floppy disk media.


All SuperDisk drives are backward compatible with standard 1.44MB floppy disks. The drives can both read and write standard floppies, and the 1.44MB floppies written on a SuperDisk drive are readable by conventional floppy disk drives. Because of the higher spin speed of the SuperDisk drive, however, it can achieve a higher transfer rate with a 1.44MB floppy—approximately 2.5 times higher—than a conventional floppy disk drive.

The SuperDisk media are designed to appear as write-protected to conventional 1.44MB floppy disk drives by virtue of a cutout that corresponds to the write-protect hole in a conventional floppy. Consequently, there is no risk of damage to data by inadvertently sliding an LS-120 disk into a conventional floppy disk drive.

The SuperDisk media are beyond the control capabilities of the chip on the controllers for conventional floppy disk drives. The initial SuperDisk drives instead used an IDE interface.


As computer equipment goes, floppy disk drives are simple devices. The essential components are a spindle motor, which spins the disk, and a stepper-motor, which drives a metal band in and out to position the read/write heads, and an assembly that is collectively called, as with hard disks, the head actuator. A spring-loaded latch locks the disk in place and ejects it when you press the button on the front of the drive.

After more than two decades of development, about the only refinement made to the conventional floppy disk system has been miniaturization. Drives now can be less than one-third the height of the first. No matter their size, however, all conventional floppy disk drives work in essentially the same way.

To carry out their design purpose, all floppy disk drive mechanisms must be able to carry out a few basic tasks. They have to spin the disks at a uniform speed. They must also move their read/write heads with sufficient precision to locate each and every data track on a given disk. And the open-loop head-positioner design requires a known starting place, an index, which the drive must be able to locate reliably.

Speed Control

All the electronics packed onto the one or more circuit boards attached to the drive unit merely control those simple disk drive operations. A servo system keeps the disk spinning at the correct speed. Usually an optical sensor looks at a stroboscopic pattern of black dots on a white disk on the spindle assembly. The electronics count the dots that pass the sensor in a given period to determine the speed at which it turns, adjusting it as necessary. Some drives use similar sensors based on magnetism rather than optics, but they work in essentially the same way—counting the number of passing magnetic pulses in a given period to determine the speed of the drive.

Head Control

Other electronics control the radial position of the head assembly to the disk. The stepper-motor that moves the head reacts to voltage pulses by moving one or more discrete steps of a few degrees (hence, the descriptive name of this type of motor). Signals from the floppy disk controller card in the host computer tell the disk drive which track of the disk to move its head to. The electronics on the drive then send the appropriate number of pulses to the stepper-motor to move the head to the designated track.

The basic floppy disk mechanism receives no feedback on where the head is on the disk. It merely assumes it gets to the right place because of the number of steps the actuator makes. Because the drive does its best to remember the position of the head, hard reality can leave the head other than in its expected place. For instance, you can reach in and manually jostle the head mechanism. Or you might switch off your computer with the head halfway across the disk. Once the power is off, all the circuitry forgets, and the location of the head becomes an unknown.

Head Indexing

So that the head can be put in the right place with assurance, the floppy disk drive resorts to a process called indexing. That is, it moves the head as far as it will go toward the edge of the disk. Once the head reaches this index position, it can travel no farther, no matter how hard the actuator tries to move it. The drive electronics make sure that the actuator moves the head a sufficient number of steps (a number greater than the width of the disk) to ensure that the head will stop at the index position. After the head has reached the index position, the control electronics can move it a given number of actuator steps and know exactly where on the radius of the disk the head is located.

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