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Theory of Operation
4-10.
The head support arms are attached to the carriage
assembly. Accurate positioning of the heads is accom-
plished by moving the carriage with signals generated by an
extremely accurate optical transducer. High-speed damping
is provided by the output of a velocity transducer also
connected in the carriage servo control circuit.
4-11.
There are 203 "cylinders" per disc, with each
cylinder representing one position of the carriage assembly.
The term cylinder encompasses both surfaces of the disc,
therefore, there are 203 "tracks" per disc surface. Moving
the carriage assembly to a particular cylinder places both
heads over that cylinder. Selection of a head determines
which track is to be used in an operation. An encoder,
which is attached to the carriage assembly, distinguishes
each cylinder position. One part of the encoder is used to
detect the "home" position, which is defined as cylinder
000. Two other channels of the encoder provide coarse and
fine carriage positioning information to center the heads
over a desired cylinder. The encoder also updates a current
cylinder address register as the carriage moves the heads
from one cylinder to the next.
4-12.
The disc is rotated by means of two pulleys, a belt,
and a spindle motor. The spindle motor control circuits
apply ac power for motor operation and the removal of
power when exchanging disc cartridges or when the drive is
not ready for use. Disc rotation is at a nominal speed of
2400 r/min which develops air pressure that forces the
heads away from the disc surface. Supporting arms for the
heads provide spring action that forces the heads toward
the disc surface. At about 100 microinches from the disc
surface the two forces are equal so the heads "fly" at this
altitude.
4-13.
A short, thin-walled, slotted skirt is attached to
the disc hub. The skirt has 24 equally spaced slots, with a
light source on one side and a photo cell on the other.
These units are located on a sector transducer that provides
the means for detecting angular (sector) position. If radial
lines were drawn through the slots, the disc would be
divided into 24 equal, pie-shaped segments (see figure 4-2).
The area of a track between two of these radial lines is
defined as a sector; thus each track has 24 sectors. The
sectors are numbered from 00 through 23.
4-14.
The addressing structure of the disc drive is shown
in figure 4-2, which defines a controller operation with a
four-unit disc drive installation. When a disc drive is com-
bined with two or more (four units maximum) HP 7900A
or HP 7901A Disc Drives, the controller selects an on-line
unit by addressing a unit over two lines that are dedicated
to this function. Operator-determined configuring in each
disc drive provides the proper logic arrangement for
controller-to-disc drive identification. A disc cylinder is
selected when the controller issues a command to the disc
drive and places the cylinder address on eight parallel out-
bus lines from the controller to the disc drive. A head and
sector select function also uses the same eight outbus lines,
accompanied by a separate enabling command from the
controller to disc drive. A further function of the outbus
lines is to carry write or read select commands. Specific
4-2
7901A
functions of an active disc drive are monitored by the
controller over a set of five parallel inbus lines and four
separate status lines.
4-15.
DETAILED FUNCTIONAL DESCRIPTION.
4-16.
The disc drive consists of a spindle motor and
control circuits to rotate the disc, a carriage and control
circuits to position the heads, sector detection circuits to
provide rotational position information, and read-write data
control circuits. The following discussion describes drive
enable and motor control, carriage position control, sector
detection control, data control, and the disc drive power
supply. An overall block diagram (figure 4-3) is used to
support this discussion. Schematic diagrams located in sec-
tion V also can be used as reference for discussion of signal
development within an assembly. Rather than repeated
reference to figure numbers, each assembly is identified by
its alpha-numeric designation, eg, assembly A6, for refer-
ence.
4-17.
DRIVE EN ABLE AND MOTOR CONTROL.
4-18.
The disc drive is placed in operation by setting
controls on the unit to the desired position as detailed in
section III. When the drive is to be used as one unit in a
multiple (four maximum) disc drive installation, a switch
that is part of unit select assembly A17 is set (position 0
through 3) to enable selection by the controller. The switch
position lights an associated lamp on the front panel, and
provides a logic output on the Select A and Select
B
lines to
I/O multiplex assembly A12. The Select A and
B
logic is
applied to a circuit that is used by the controller to select a
disc drive for operation. When a match occurs between the
Select A and B logic from assembly A17 and the Select 1
and 2 logic from the controller, and the Select Enable signal
is high, I/O multiplex assembly A12 logic is enabled for
interface operation between the disc drive and controller.
Development of the Select Enable signal is described later
in this discussion.
4-19.
Select A and
B
logic in assembly A12 also is
applied to the address decode circuit which provides unit
identification to the controller at the request of the con-
troller. During disc drive operation, all outputs from the
decode circuit are high, except when the Attention signal is
high and the controller pulses the Control Outbus
6
line.
The decoder responds to this request from the controller by
providing a low output to the inbus circuit associated with
the disc drive unit identifier. For example, if the Select A
and Select
B
logic established the disc drive as unit 1, the
decoder responds to a controller request by providing a low
to the control inbus 1 circuit. In this manner the controller
can identify each disc drive in an installation.
4-20.
With power applied to a disc drive and the spindle
motor not operating, the DOOR UNLOCKED lamp on
indicator assembly A1 is lighted by a low Latch Drive signal
from drive control assembly A10. The low Latch Drive
signal, in tum, is developed by a low Speed Down output
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