FLOPPY DISK
A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5??-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable. Floppy disk are the media used to read and write data is a floppy drive. A floppy is made up of a circular thin Mylar plastic material coated with magnetic particles. Every magnetic particles holds one data bit. The polarity of magnetic orientation will be opposite for 0 or 1,it is difficult to demagnetize a magnetic particle. However, the polarity can be reversed. The medium is permeable having high retentivity. Higher the retentivity, longer the life of information stored. The Mylar disk is enclosed in a plastic jacket to protect its surface. While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have been superseded by data storage methods with much greater capacity, such as USB flash drives, portable external hard disk drives, optical discs, memory cards, and computer networks.
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Fig: A Floppy Disk.
Construction and Operation:
While floppy disk drives vary in terms of size and the format of data that they hold, they are all internally quite similar. In terms of construction and operation, floppy drives are similar to hard disk drives, only simpler. Of course, unlike hard disks, floppy disk drives use removable floppy media instead of integrated storage platters. This section takes a look at the basic components and physical operation of a floppy disk drive.
Read/Write Heads:
The read/write heads on the floppy disk are used to convert binary data to electromagnetic pulses, when writing to the disk, or the reverse, when reading. This is similar to what the heads on a hard disk do. There are several important differences between floppy disk and hard disk read/write heads. One is that floppy disk heads are larger and much less precise than hard disk heads, because the track density of a floppy disk is much lower than that of a hard disk. The tracks are laid down with much less precision; in general, the technology is more "primitive". Hard disks have a track density of thousands of tracks per inch, while floppy disks have a track density of 135 tracks per inch or less.
Head Actuator:
The head actuator is the device that physically positions the read/write heads over the correct track on the surface of the disk. Floppy disks generally contain 80 tracks per side. The actuator is driven by a stepper motor.
Spindle Motor:
The spindle motor on the floppy is what spins the floppy disk when it is in the drive. When the disk is inserted, clamps come down on the middle of the disk to physically grasp it. These clamps are attached to the spindle motor, which turns the disk as it spins.
Logic Board:
The floppy disk contains an integrated logic board that acts as the drive controller. Like the rest of the floppy disk this is a relatively simple affair, containing the electronics that control the read/write heads, the spindle motor, head actuator and other components. The circuits on this board also talk to the floppy disk controller over the floppy interface.
Classification of floppy disks
According to the size, floppy disks are generally classified into two:
• 5 1⁄4-inch diskette
• 3 1⁄2-inch diskette
Separate drives are used for these 2 types of diskettes.
5 1⁄4-inch floppy disks:
This disk has their apertures; the central circular aperture is to allow the diskette to be located on the hub of the spindle motor, which spins the floppy disk. The small circular hole called the index hole lines up with the small hole in the medium when it is rotated.
Index hole:
This hole is used to locate the starting sector(the 1st sector) of a track on the medium. The elongated aperture will allow the drive to read or write from or to the surface. A notch at the edged of the jacket, called write protect_ notch is to protect the disk from writing on to it, if the notch is closed, the disk is protected from being written over it.
3 1⁄2-inch floppy disk:
The 3 1⁄2-inch drives use a more rigid plastic case that helps in protecting the disk having high density of data. The metal shutter protects the medium access hole it is spring loaded. The closed shutter will be opened by the drive mechanism. The 3.5-inch (8.9 cm) disk replaced the 5.25-inch (13.34 cm) disk in the early 1990s, capable of holding 1.44 megabytes of information. For a while there were two floppy disk drive types often included with computers.
Fig: Floppy Disks and It’s parts.
Recording format
Data can be stored on a disk in different ways. There are two types of recording format.
• FM(frequency modulation)
• MFM(modified F M)
Frequency modulation:
In this format, every data bit cell starts with a flux transaction. Then if the current data is ‘1’ a flux transaction will be recorded at the middle of the bit cell.
Modified F.M:
Here there will not be any clock at the start of every bit cell. But as in the case of F.M, a flux transaction will be recorded in the middle of a bit cell if the current data is ‘1’ if the data is ‘0’ there will not be any flux transaction in the bit cell. However, if both the current data & the previous data are 0s,there will be a flux reversal at the starting of the current data bit cell. We will be able to store double the amount of data on the same area by using the MFM format instead of FM format. The recording format will be decided by the operating system.
Hard sector and Soft sector of Disks
Some disks may have multiple index, one index hole for each sector. So each sector will be identified by an index hole. Such disks are called Hard sectored diskette:. The IBM PC identify sector-1. The sectors are identified by the information recorded at the beginning of each sector. So such diskettes are called soft sector diskettes.
Clusters: DOS rarely deals with individual sectors. Instead, data storage is done in-group of sectors called clusters. The cluster size varies depending on the diskette type its format and the version of DOS.
The Floppy disk interfaces :
The interface consists of three basic components
• The Floppy drive controller
• The power cable
• The Control/Data cable
The Floppy drive controller:
The Floppy Disk drive controller is an electronic circuit board that does interface b/w the system and floppy disk driver. The major functions of the floppy disk controller are
• To translate the logic commands from the computer in to the exact electrical signal that control the disk drive
• To translate the stream of pulses generated by the floppy drive head into data in the form the computer can deal with
The major function blocks are as follows:
• System interface logic
• Floppy disk controller logic
• Read write logic
• Drive select logic
• Control logic
System interface logic:
The system interface logic communicates with the system and the controller electronics. All the signals to and from FDC are buffered through I/O slot.
Floppy disk control logic:
The PC can support to floppy drives, the FDC can be programmed to select only of the two drive at any time. The drive select logic energies the respective drive. For a read or write operation after decoding the commands received from the CPU. Before any read/write request is processed the read/write heads to locate the required track. Necessary calculations made setting the current track number and the required sector number. The control logic issues direction and step commands to head assembly to locate it over the desired address. The control logic also analysis the interface signals from write protect index hole and track 00 sensors and inform the CPU about the status of the floppy disk drive.
Decoding the command from CPU either read or write operation is executed by read/write logic. The data received in bytes are serialized and encoded in MFM format by adding necessary clock pulses before writing on to the disk. During reading, the data bit stream is read from the disk, data bits are separated and deserialised. Then the data bytes are sent to CPU. The DMA request for the data transfer is made by the FDC. At the Completion of the data transfer, Interrupt is raised to inform the CPU about the status by the FDC. The complete activities are sequenced and controlled by the timing and control unit.
How a floppy drive works?
• When you push 3.5 inch floppy disks into the drive, the floppy presses against a system of leavers. One leaver open’s the floppy’s shutter to expose the cookie-(a thin Mylar disk coated on either side with a magnetic material similar to the coating on a cassette tape that can record data.)
• Other leavers and gears move two read/write heads until they touch the cookie on either side. The heads, using tiny electromagnets generate magnetic pulses that change the polarity of metallic particles embedded in the disk’s coating.
• The drives circuit board receives signals including data and instructions for writing the data to disk, from the floppy drive’s controller board. The circuit board translates the instructions into electrical signals that control the movement of the disk and the read write heads.
• If the signal include instructions to write data to the disk, the circuit board first checks to make sure that no light is visible through a small window in one corner of the disk’s housing. But if the window is open and a beam from a light emitting diode can be detected by a photo sensitive diode on the opposite side of the disk, the drive knows the disk is write protected and refuses to record new data.
• A motor located beneath the disk spins a shaft, a notch on the hub of the disk causing the disk to spin.
• A stepper motor-which can turn a specific distance in either directions according to signals from the circuit board moves a second shaft that has a spiral groove cut into it. An arm attached to the read/write headrests inside the shaft’s groove. As the shaft turns, the arm moves back and forth positioning the read/write heads over the disk.
• When the heads are in the correct position, electrical impulses create a magnetic field in one of the heads to record data to either the top or bottom surface of the disk.
• When the heads are reading data they react to magnetic fields generated by the metallic particles on the disk by sending electrical signals to the computer.
