If you have spent any time at all
working with a computer, then chances are good that you have
used a floppy disk at some point. The floppy disk
drive (FDD) was the primary means of adding data to a
computer until the CD-ROM drive became popular. In fact, FDDs
have been an key component of most personal computers for
more than 20 years.
Basically, a floppy disk
drive reads and writes data to a small, circular piece of
metal-coated plastic similar to audio cassette tape. In this
edition of , you will
learn more about what is inside a floppy disk drive and how
it works. You will also find out some cool facts about FDDs.
History of the Floppy Disk Drive
The floppy disk drive (FDD) was invented
at IBM by Alan Shugart in 1967. The first floppy drives used
an 8-inch disk (later called a "diskette" as it got
smaller), which evolved into the 5.25-inch disk that was used
on the first IBM Personal Computer in August 1981. The 5.25-inch
disk held 360 kilobytes compared to the 1.44 megabyte capacity
of today’s 3.5-inch diskette.
The 5.25-inch disks were dubbed
"floppy" because the diskette packaging was a very
flexible plastic envelope, unlike the rigid case used
to hold today’s 3.5-inch diskettes.
By the mid-1980s, the improved
designs of the read/write heads, along with improvements in
the magnetic recording media, led to the less-flexible, 3.5-inch,
1.44-megabyte (MB) capacity FDD in use today. For a few years,
computers had both FDD sizes (3.5-inch and 5.25-inch). But
by the mid-1990s, the 5.25-inch version had fallen out of
popularity, partly because the diskette’s recording surface
could easily become contaminated by fingerprints through the
open access area.
Parts of a Floppy Disk Drive
Floppy Disk
Drive Terminology
- Floppy disk
- Also called diskette. The common size is 3.5 inches.
- Floppy disk
drive - The electromechanical device that reads
and writes floppy disks.
- Track -
Concentric ring of data on a side of a disk.
- Sector -
A subset of a track, similar to wedge or a slice of
pie.
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The Disk
A floppy disk is a lot like a cassette
tape:
- Both use a thin plastic base
material coated with iron oxide. This oxide is a ferromagnetic
material, meaning that if you expose it to a magnetic field
it is permanently magnetized by the field.
- Both can record information
instantly.
- Both can be erased and reused
many times.
- Both are very inexpensive and
easy to use.
If you have ever used an audio
cassette, you know that it has one big disadvantage -- it
is a sequential device. The tape has a beginning and
an end, and to move the tape to another song later in the
sequence of songs on the tape you have to use the fast forward
and rewind buttons to find the start of the song, since the
tape heads are stationary. For a long audio cassette tape
it can take a minute or two to rewind the whole tape, making
it hard to find a song in the middle of the tape.
A floppy disk, like a cassette
tape, is made from a thin piece of plastic coated with a magnetic
material on both sides. However, it is shaped like a disk
rather than a long thin ribbon. The tracks are arranged in
concentric rings so that the software can jump from
"file 1" to "file 19" without having to fast forward through
files 2-18. The diskette spins like a record and the heads
move to the correct track, providing what is known as direct
access storage.
In the illustration above, you can see how the disk
is divided into tracks (brown) and sectors (yellow).
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The Drive
The major parts of a FDD include:
- Read/Write Heads:
Located on both sides of a diskette, they move together
on the same assembly. The heads are not directly opposite
each other in an effort to prevent interaction between write
operations on each of the two media surfaces. The same head
is used for reading and writing, while a second, wider head
is used for erasing a track just prior to it being written.
This allows the data to be written on a wider "clean slate,"
without interfering with the analog data on an adjacent
track.
- Drive Motor:
A very small spindle motor engages the metal hub at the
center of the diskette, spinning it at either 300 or 360
rotations per minute (RPM).
- Stepper Motor:
This motor makes a precise number of stepped revolutions
to move the read/write head assembly to the proper track
position. The read/write head assembly is fastened to the
stepper motor shaft.
- Mechanical Frame:
A system of levers that opens the little protective window
on the diskette to allow the read/write heads to touch the
dual-sided diskette media. An external button allows the
diskette to be ejected, at which point the spring-loaded
protective window on the diskette closes.
- Circuit Board:
Contains all of the electronics to handle the data read
from or written to the diskette. It also controls the stepper-motor
control circuits used to move the read/write heads to each
track, as well as the movement of the read/write heads toward
the diskette surface.
The read/write heads do not touch
the diskette media when the heads are traveling between tracks.
Electronic optics check for the presence of an opening in
the lower corner of a 3.5-inch diskette (or a notch in the
side of a 5.25-inch diskette) to see if the user wants to
prevent data from being written on it.
Click on the picture to see a brief video of a diskette
being inserted. Look for the silver, sliding door
opening up and the read/write heads being lowered
to the diskette surface.
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Read/write heads for each side of the diskette
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Writing Data on a Floppy Disk
The following is an overview of how a floppy
disk drive writes data to a floppy disk. Reading data is very
similar. Here’s what happens:
- The computer program passes
an instruction to the computer hardware to write
a data file on a floppy disk, which is very similar to a
single platter in a hard disk drive except that it is spinning
much slower, with far less capacity and slower access time.
- The computer hardware and the
floppy-disk-drive controller start the motor in the
diskette drive to spin the floppy disk.
The disk has many concentric
tracks on each side. Each track is divided into smaller
segments called sectors, like slices of a pie.
- A second motor, called a stepper
motor, rotates a worm-gear shaft (a miniature
version of the worm gear in a bench-top vise) in minute
increments that match the spacing between tracks.
The time it takes to get to
the correct track is called "access time." This stepping
action (partial revolutions) of the stepper motor moves
the read/write heads like the jaws of a bench-top vise.
The floppy-disk-drive electronics know how may steps the
motor has to turn to move the read/write heads to the
correct track.
- The read/write heads stop
at the track. The read head checks the prewritten
address on the formatted diskette to be sure it is using
the correct side of the diskette and is at the proper track.
This operation is very similar to the way a record player
automatically goes to a certain groove on a vinyl record.
- Before the data from the program
is written to the diskette, an erase coil (on the
same read/write head assembly) is energized to "clear" a
wide, "clean slate" sector prior to writing the sector data
with the write head. The erased sector is wider than the
written sector -- this way, no signals from sectors in adjacent
tracks will interfere with the sector in the track being
written.
- The energized write head
puts data on the diskette by magnetizing minute, iron,
bar-magnet particles embedded in the diskette surface, very
similar to the technology used in the mag stripe on the
back of a credit card. The magnetized particles have their
north and south poles oriented in such a way that their
pattern may be detected and read on a subsequent read operation.
- The diskette stops spinning.
The floppy disk drive waits for the next command.
On a typical floppy disk drive,
the small indicator light stays on during all of the above
operations.
Floppy Disk Drive Facts
Here are some interesting things to
note about FDDs:
- Two floppy disks do not get
corrupted if they are stored together, due to the low level
of magnetism in each one.
- In your PC, there is a twist
in the FDD data-ribbon cable -- this twist tells the computer
whether the drive is an A-drive or a B-drive.
- Like many household appliances,
there are really no serviceable parts in today’s FDDs. This
is because the cost of a new drive is considerably less
than the hourly rate typically charged to disassemble and
repair a drive.
- If you wish to redisplay the
data on a diskette drive after changing a diskette, you
can simply tap the F5 key (in most Windows applications).
- In the corner of every 3.5-inch
diskette, there is a small slider. If you uncover the hole
by moving the slider, you have protected the data on the
diskette from being written over or erased.
- Floppy disks, while rarely used
to distribute software (as in the past), are still used
in these applications:
- in some Sony digital cameras
- for software recovery after
a system crash or a virus attack
- when data from one computer
is needed on a second computer and the two computers
are not networked
- in bootable diskettes used
for updating the BIOS on a personal computer
- in high-density form, used
in the popular Zip drive