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[ The PC Guide | Systems and Components Reference Guide | Floppy Disk Drives | Floppy Disk Drive Construction and Operation ]

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.

In terms of technology, floppy disks still use the old ferrite style of head that was used on the oldest hard disks. In essence, this head is an iron core with wire wrapped around it to form a controllable electromagnet . The floppy drive, however, is a contact recording technology. This means that the heads directly contact the disk media, instead of using floating heads that skim over the surface the way hard disks do. Using direct contact results in more reliable data transfer with this more simplistic technology; it is impossible to maintain a consistent floating head gap at any rate when you are using flexible media like floppies.

Since floppy disks spin at a much slower speed than hard disks--typically 300 to 360 RPM instead of the 3600 RPM or more of hard disks--they are able to contact the media without causing wearout of the media's magnetic material. Over time, however, some wear does occur, and magnetic oxide and dirt builds up on the heads, which is why floppy disk heads must be periodically cleaned. Contact recording also makes the floppy disk system more sensitive to dirt-induced errors, cause by the media getting scratched or pitted. For this reason, floppy disks are much less reliable, overall, than hard disks.

The floppy disk also uses a special design that incorporates two erase heads in addition to the read/write head. These are called tunnel-erase heads. They are positioned behind and to each side of the read/write head. Their function is to erase any stray magnetic information that the read/write head might have recorded outside the defined track it is writing. They are necessary to keep each track on the floppy well-defined and separate from the others. Otherwise interference might result between the tracks.

An analogy to demonstrate how these work would be as follows. Imagine a man laying a gravel walkway on a narrow dirt path, by walking a wheelbarrow along it that has a slit in the bottom cut to the width of the path, through which the gravel falls. Most of the gravel will fall on the dirt path but some will fall on the grass to either side of the path. The tunnel-erase heads would be like two helpers that follow behind the man with the wheelbarrow and "pick up" any gravel that has fallen on the grass instead of the path. (Of course gravel can't cause "interference" the way magnetic patterns can.)

All modern--and even not-so-modern--floppy disks are double-sided. Very, very old floppy disks originally were single-sided only. Since the disks are double-sided, there are two heads, one per side, on the drive. The heads contact the media on each side by basically squeezing the media between them when the disk is inserted. The heads for different drives vary slightly based on the drive format and density.

Next: Head Actuator


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