Hardware Reference
In-Depth Information
Note
ManypeopledonotrealizethatHead0,orthefirstheadonafloppydiskdrive,isthebottom
one. Single-sided drives, in fact, used only the bottom head; the top head has been replaced
byafeltpressurepad.Anotherbitofdisktriviaisthatthetophead(Head1)isnotpositioned
directly over the bottom head (Head 0). The top head is instead offset by either four or eight
tracks inward from the bottom head, depending on the drive type.
A motor called a
head actuator
moves the head mechanism. The heads can move in and
outoverthesurfaceofthediskinastraightlinetopositionthemselvesovervarioustracks.
On a floppy drive, the heads move in and out tangentially to the tracks they record on the
disk. This is different from hard disks, where the heads move on a rotating arm similar
to the tone-arm of a record player. Because the top and bottom heads are mounted on the
samerack,ormechanism,theymoveinunisonandcan'tmoveindependentlyofeachoth-
er.Theupperandlowerheadeachdefinestracksonitsrespectivesideofthediskmedium.
At any given head position, the tracks under the top and bottom head simultaneously are
called a
cylinder
. Most floppy disks are recorded with 80 tracks on each side (160 tracks
total), which is 80 cylinders.
The heads themselves are made of soft ferrous (iron) compounds with electromagnetic
coils.Eachheadisacompositedesign,witharead/writeheadcenteredwithintwotunnel-
erase heads in the same physical assembly (see
Figure 10.6
).
Figure 10.6
Composite construction of a typical floppy disk drive head.
Floppy disk drives use a recording method called
tunnel erasure
. As the drive writes to a
track,thetrailingtunnel-eraseheadserasetheouterbandsofthetrack,trimmingitcleanly
on the disk. The heads force the data into a specified narrow “tunnel” on each track. This
process prevents the signal from one track from being confused with the signals from ad-
jacent tracks, which would happen if the signal were allowed to naturally “taper off” to
each side.
