secondary storage devices

I.T. for Management

Chapter 4Secondary Storage Devices

Secondary Storage Devices

• Most business information and particularly transactions require semi permanent storage

• The primary storage is Volatile in nature• I.e all information is lost when power is

switched off• Hence the need of secondary devices


• Some of commonly used secondary devices– Floppy Disks & drive– Hard disks– CD/DVD Drive– Pen drive– Magnetic tapes


Floppy DisksThe first floppy diskettes were introduced in the year 1971

The floppy disk is packaged in a 8 inch square soft plastic envelopeThey had a capacity of 1 M.BLater in 1976 5.25 inch diskettes were introduced

………Cont.


Floppy DisksBasically, a floppy disk drive reads and writes data to a small, circular piece of metal-coated plastic similar to audio cassette tape. In this article, 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 FDD’ s.

…. cont


Floppy DisksParts 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. The Diskfloppy disk is a lot like a cassette tape:

•

http://www.howstuffworks.com/cassette.htm


Floppy DisksParts of a Floppy Disk Drive

• 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.

Secondary Storage DevicesFloppy Disks

Parts of a Floppy Disk DriveIf 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.

•


Floppy Disks• 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 ringsso 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 , you can see how the disk is divided into tracks (brown) and sectors (yellow).

Secondary Storage DevicesFloppy Drive

• The DriveThe 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.

http://www.howstuffworks.com/cassette.htm


• 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 motorengages 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.

http://www.howstuffworks.com/motor.htm


• 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.

http://www.howstuffworks.com/sc-electronics-communication.htm


• 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.

http://static.howstuffworks.com/mpeg/floppy-disk-drive.mpg



Merits:•In a floppy disk the files can be arranged Sequentially• or in a random manner The floppies being Removable makes it very easy to transportRequests for information can be answered quickly and at randomDe Merits :The Floppy disks tend to get corrupted very easily because of which of these factors Floppy have lesser storage spaceThey are sources of virus


Secondary Storage DevicesHard Disk Drive

• IBM developed the idea of the Winchester or the hard disk

The hard disks are enclosed in a hermetically enclosed container

Capacities of the order of 20 GB,40 GB,80 GB are typical


• How hard disk works:A hard disk works on the same principle of Floppy drive

The entire assembly is permanently installed in the pcThe hard disk has one or more platter(disks) each with both

sides which are recordableAll platters rotate on a central spindle at continuous speed

are of the order of 7500 rpmSeveral read write heads are position on top of each

rotating surfaceWhen the hard disk drive receives the instruction to read or

write ,it uses the available data to locate the correct surface track & sector & reads or writes on it


• How hard disk works:Hard disk is fast since it has to at most wait for the

correct track or sector to come below read or write track,unlike floppy drive where the drive works in addition to searching for track

Response time is of the order of 20 to 25 Millisecond is common


• Here is a typical hard-disk drive: Front side


• Here is a typical hard-disk drive: Backside


In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads:

Many drives use a "voice coil" approach -- the same technique used to move the cone of a speaker on your stereo is used to move the arm.

http://www.howstuffworks.com/speaker.htm


• Merits:– Hard drive provides on line information & very

fast.– Data access randomly & not sequentially– Extra hard disk can be added increasing

storage capacity– Hard disks are hermetically sealed reducing

chances of damage

Secondary Storage DevicesPen Drive

What is pen driveA pen drive is a small removable Flash memory drive

usually connected to the USB portIt provides storage ranging from 16 MB to several Giga

bytesData can be stored as long as it is neededIt is plug & play deviceOS Xp automatically senses pen drive when inserted to

USB portTypical dimensions are 77*25*9 mm

Secondary Storage DevicesPen Drive(Inside)

Internals of a typical flash drive(Saitek brand USB1.1 pictured)

1USB connector

2 USB mass storage controller device

3 Test points

4 Flash memory chip

5 Crystal oscillator

6 LED

7 Write-protect switch

8 Space for second flash memory chip

http://en.wikipedia.org/wiki/Image:Usbkey_internals.jpg

Secondary Storage DevicesPen Drive

• Merits: – Massive storage capacity– Portable & compact– Solid state(no moving parts) hence less chance of

corruption– USB compliant making easy to connect externally– Does not require external power source– Low cost (Rs 800-1000)

Secondary Storage DevicesDVD

• DVDs are of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc.

• A DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data.

http://www.howstuffworks.com/cd.htm


• Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers.

• After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the non readable side. Double-sided discs are printed only on the non readable area near the hole in the middle. Cross sections of the various types of completed DVDs (not to scale) look like this:

http://www.howstuffworks.com/light2.htm




DVD formats

Secondary Storage DevicesHow DVD works

• Each writable layer of a DVD has a spiral track of data. On single-layer DVD's, the track always circles from the inside of the disc to the outside. That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired.

• What the image to the right cannot impress upon you is how incredibly tiny the data track is -- just 740 nanometers separate one track from the next (a nanometer is a billionth of a meter). And the elongated bumps that make up the track are each 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high. The following figure illustrates looking through the polycarbonate layer at the bumps.

Secondary Storage DevicesData storage DVD/CD

• DVDs can store more data than CDs for a few reasons: • Higher-density data storage • Less overhead, more area • Multi-layer storage

• Higher Density Data StorageSingle-sided, single-layer DVDs can store about seven times more data than CDs. A large part of this increase comes from the pits and tracks being smaller on DVDs.

• Specification CD DVD• Track Pitch

1600 nanometers 740 nanometers

• Minimum Pit Length(single-layer DVD) 830 nanometers 400 nanometers

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• Minimum Pit Length(double-layer DVD) 830 nanometers 440 nanometers


• Merits• Superior quality• Interactivity• Flexibility• Durability• Low cost• Compatibility• Can handle all kinds of data including sound

& multimedia files• Many drives allow read/write of both CDS &

DVD

I.T. for Management

End of Chapter 4

secondary storage devices

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