• 1st PC, single sided drive, one head, one surface to store data
  • capacity – 160K & 8 sectors per track
  • Then, double sided drives with 9 sectors per track – 360K capacity
  • Early Mini FDD
    • 1.6 Mb FDD, 360 rpm, 8” FDD compatible
    • 1Mb FDD, 300 rpm
    • 1Mb/ 1.6 Mb, 300/360 rpm dual speed
  • Current FDD : 31/2 inch FDD – micro floppy drive
  • Two types
    • Double density FDD with 720 K storage & 9 sectors
    • High density FDD with 1.4 M storage & 18 sectors
  • Latest 31/2 inch FDD offers 2.88M storage & 36 sectors

Different FDD format

SlNo Item/feature 5 1/4 inch 360kb 5 1/4 inch 1.2mb 5 1/4 inch 720kb 3 1/2 inch 1.44mb 3 1/2 inch 2.88mb
1 Capacity(Formatted) 360kb 1.2mb 720kb 1.44mb 2.88mb
2 Tracks/side 40 80 80 80 80
3 Sectors/track 9 15 9 18 36
4 Bytes/sector 512 512 512 512 512
5 Data transfer rate 250kbs 500kbs 500kbs 500kbs 500kbs
6 Recording format MFM MFM MFM MFM MFM
7 Disk speed 300rpm 300rpm or 360rpm 360rpm 360rpm 360rpm
8 Total sectors 708 2371 1426 2847 5726
9 Common terminology DSDD DSHD DSDD DSHD DSQD

Head movement

  • One R/W head for each surface – mounted on common assembly – move together
  • Head movement – forward (in) or backward (out)
  • Actuator mechanism with stepper motor & metal belt
  • Stepper motor: rotates in discrete steps & both direction
  • Coupled to head assembly by metal belt(coiled split steel band)
  • Rotational movement to linear movement of head by steel band which winds or unwinds around spindle of stepper motor

Access time

  • 360K FDD uses 4 phase stepper motor
  • Step pulse makes motor rotate by 3.6 degrees & R/W head move by one track
  • Time taken by heads to move from one track to next is access time – 5ms for 360K FDDs. 
  • Heads are spring loaded by head load mechanism containing solenoid
  • Head load time is order of 50ms.

Head Coils

  • R/W head consists of two R/W coils & one erase coil
  • Erase winding is energized on writing to reduce width of track
  • Erase coil only trims the track & creates gaps called guard bands & does not erase the track. 

Types of Erase operation

Straddle erase

Tunnel erase

Both erase & write operations done simultaneously

Erase operation done after write

 

Spindle motor

Used to rotate diskette at 300 or 360rpm

1. Belt driven Spindle Motor

  • Earlier FDD used this
  •  A belt & pulley couple spindle motor to disk spindle
  • Manual speed adjustment

2. Direct Drive Spindle Motor

  • Modern FDD uses this
  • Shaft drives  head assembly
  • More reliable & smaller
  • Automatic Tachometer for speed adjustment

PCB

Logic Board

  • Performs controller interface, R/W circuit , head positioning circuits & sensor electronics

Servo board

  • Controls spindle motor & make it rotate at constant speed 
  • Recent FDD – single PCB with custom built gate arrays & SMD components. 
  • SA-450 interface is used