Hardware Reference
In-Depth Information
The disk-sweep function is designed to prevent the head from remaining stationary above
one cylinder in the drive for too long, where friction between the head and platter eventu-
ally would dig a trench in the medium. Although the heads are not in direct contact with
themedium,theyaresoclosethattheconstantairpressurefromtheheadfloatingabovea
single cylinder could cause friction and excessive wear.
Figure 9.12
shows both a wedge
and an embedded servo.
Figure 9.12
A wedge and an embedded servo.
Wedge Servo
Early servo-controlled drives used a technique called a
wedge servo
. In these drives, the
gray-code guidance information is contained in a “wedge” slice of the drive in each cyl-
inder immediately preceding the index mark. The index mark indicates the beginning of
each track, so the wedge-servo information was written in the PRE-INDEX GAP, which
is at the end of each track. This area is provided for speed tolerance and normally is not
used by the controller.
Some controllers had to be notified that the drive was using a wedge servo so they could
shortenthesectortimingtoallowforthewedge-servoarea.Iftheywerenotcorrectlycon-
figured, these controllers would not work properly with the drive.
Another problem was that the servo information appears only one time every revolution,
which means that the drive often needed several revolutions before it could accurately de-
