Purpose of using tape drive

  • Backup unit – take copies of disk contents
  • Transport files from one site to another conveniently
  • But CD with large capacity, high reliability, easy portability & flash ROM

Basic Principle

  • Magnetic medium causes magnetization by passing current through a coil in read/write head.
  • Head is stationary & media moves during R/W.

Standard format for recording on magnetic disk

Frequency Modulation (FM)

  • Single density format
  • Clock pulse written at beginning of each bit
  • Data pulse at center 
  • 1 – Data pulse present
  • 0 – Data pulse absent
  • Each bit cell is of 4 micro sec for floppy disk
  • Two flux changes per bit 

Modified Frequency Modulation (MFM)

  • Double density format

  • No Clock pulse at beginning

Data Pulse

Meaning

1

No clock pulse, data pulse at centre

0 following 1

Neither clock not data

0 following 0

Clock pulse written at beginning & no data pulse
  • Each bit cell is of 2 micro sec for floppy disk
  • So, disk capacity doubled
  • Only one flux change (since, no clock pulse)

Recording on magnetic medium

  • Info stored in flux reversals on medium 
    • not in amplitude / direction of magnetization
  • Each data bit is recorded in form of flux change

Head configuration

  • R/W head – core with air gap & set of coils
  • For writing data on medium:
  • Data converted into current
  • Passed through R/W Head coils 
  • Current generates magnetic field (flux) in air gap

  • Current direction controlled to produce opposite polarity mag fields
  • Series of flux reversals on medium
  • During reading, when flux transition pass under head gap, voltage introduced in R/W coils
  • Voltage converted into data pulses
  • Magnetization due to current in head during write operation & induced emf in head during read operation

Recording on magnetic medium

Disk Drive Types

1. Hard disk drive (HDD)

  • Medium is rigid circular platter – disk

2. Floppy Disk Drive (FDD)

  • Medium is flexible circular diskette

Both surfaces, top & bottom can be used for storing data

HDD provides better performance, cos,

  • Higher capacity of data storage
  • Faster access time of data
  • Higher data transfer rate
  • Better reliability of operations
  • Less data errors or data loss

Disk Drive Block Diagram

Interfacing Hard disk drives - ST 506

Interfacing floppy disk drives - ST 506

Common Concepts between FDD & HDD

  • Data is written bit by bit on disk
  • Clock bit is also written on medium
  • Data recorded on concentric circular tracks
  • Disk rotates at fixed speed
  • Moving head positioned on desired track by positioning mechanism

Comparison/difference between FDD and HDD

FDD

HDD

Head touches media – on R/W

Does not touch – flying height

Max 2 R/W head (:2 surface)

Multiple R/W head (many platters on single spindle)

Head Positioning Mechanism:

Uses stepper motor

Head Positioning Mechanism:

Uses:

Stepper motor (open-loop disk drive)

Voice Coil  Servo Mechanism (closed loop disk drive)

Diskette rotation in low speed

300 rpm or 360 rpm

High speed

2400rpm, 3600rpm or 7200rpm

Lesser no. of tracks – 40/80

Track density – 48/96 TPI

No. depends on hard disk size – 14”, 18”, 51/4” , 31/2”, etc. 

Low recording density (DPI)

High since rotates fast

 

Floppy Diskette

  • Ultra thin plastic (Mylar) Piece in circular shape 
  • Thickness – few thousand of an inch
  • Coated with magnetic material enclosed in protective jacket
  • Oval access hole on jacket – provide contact b/n R/W head & diskette.

Sizes of Diskettes

1. 8 inch disk

  • Old standard diskette
  • Now Obsolete

2. 51/4 inch disk – Mini-floppy

  • Widely used recent computer – PC, PC-XT, PC-AT
  • Vinyl cover to protect disk surface

3. 31/2 inch disk – Micro floppy

  • New industry standard diskette
  • has rigid plastic cover with metal/plastic slide – protect disk when not in use

Single sided disk

  • Earlier floppy diskettes
  • Only one side of diskette was used to store info

Double sided disk

  • Currently manufactured
  • Both sides of diskette are used for storing info
  • Diskette Classification
  • Single density diskette
  • Double density diskette
  • Uses MFM to store info. Stores twice amount of info that stored on single density diskette of same size.

Components

Track

  • Diskette –logically divided to fixed no. of tracks-Concentric circles
  • Reading/Writing only on specified tracks & not in between.

Inch of diskette

No. of tracks

8

 77

51/4 double density

 40

51/4 high density 

 80

Sectors

  • Tracks divided into no. of sectors
  • depends on size & recording method used
  • Fixed no. of data bytes are written, like, 128,256,512 or 1024

Head

  • Read/Write heads are mounted on common assembly in FDD.
  • Head Moves to & fro between outermost & inner mot track in both (forward & backward) direction.
  • Tracks are sequentially numbered, starting from outermost track as track 0.

Index hole

  • Small hole punched on diskette near center
  • Indicates the beginning of track
  • Initial writing on any track is only after sensing index hole
  • Index sensor senses the index hole once on each revolution

Write protect feature

Allows to read & Denies to write attempt

1. Technique 1:

  • In 51/4” disk – small notch punched at outer edge of jacket – Write Protect notch
  • If notch opened – Permit writing
  • If notch closed – Deny writing

2. Technique 2: 

  • In 31/2” disk, write-protect window with plastic tab to open /close
  • Write Protection provided when window is opened

Classification based on sector organization

Hard Sectoring

  • No. of Sectors on each track is physically fixed on manufacturing
  • Beginning identified by sector hole punched on plastic disk

Soft Sectoring

  • No. of Sectors per track is chosen by software – varies 
  • No physical hole for sector information

Hard sector

Soft sector

Fixed sector size

Sector size chosen by software

Obsolete

Currently used
 

Several standards, differing in like no. of sectors per track, no. of data bytes per sec & length of gaps on diskette

 

Floppy disk format

Widely used – IBM system 34 & IBM 3740

Each track has many fields having specific bit patterns & gaps for 

  • data field
  • synchronization
  • error detection & identification

Data recording 

  • Data is written bit by bit on track
  • Read/Write heads use electro magnetic recording techs
  • One head on top & another on bottom surface
  • Write operation – WRITE DATA line carries Clock & Data Pulse
  • Write circuits pass current to R/W head – flux transition
  • Read – produce emf on diskette rotation
  • Pre-amplifier circuits in FDD amplifies shape pulse

Need for writing clock pulses in diskette

  • If data bits – 0 – no flux transition
  • While reading, will appear there is no data
  • Clock bits differentiate between no data & zero data