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7901A
THEORY OF OPERATION
II
4-1.
INTRODUCTION.
4-2.
This section describes the mechanical and electrical
characteristics of the HP 7901A Disc Drive, and contains a
detailed functional description of the disc drive logic.
4-3.
MECHANICAL AND ELECTRICAL CHARAC-
TERISTICS.
4-4.
The Hewlett-Packard 7901A Disc Drive is a ran-
dom access, mass storage device designed for use with small-
to medium-sized computing systems. Through the use of
removable and interchangeable single-disc cartridges, the
disc drive provides the using system with access to large
volume data storage, as well as fast, random access storage
for high-activity programs and data. The disc drive utilizes
interchangeable, front-loading, single-disc cartridges capable
of storing up to 2.5 million bytes of information. Data
storage areas on the disc may be addressed in direct fashion.
A separate magnetic head is used for each disc surface to
retrieve existing data, or to record new data.
4-5.
The removable, single disc is mounted in a protec-
tive cartridge which acts as a guide during loading. When
mounted in the drive, the disc is free to rotate within the
cartridge. At operating speed, the heads are placed in flying
position by a mechanical head-loading mechanism. An
electromagnetically actuated carriage moves the heads to
anyone of 203 positions over the disc. The carriage
7901-22
LINEAR
MOTOR
I
POLE
PIECE
I
L . L L - - - - J . , - I
I
VELOCITY
I
TRANSDUCER
I
CCCAq:
Il\A~'
POLE
PIECE
assembly is supported on ball bearings which are spring
loaded against stainless steel rails. Disc addressing and head
selection are under the control of the using system.
4-6.
The disc drive interfaces directly with the using
system controller and receives all data inputs directly from
the using system processor interface. Similarly, all data and
drive status signals are either sent directly to the using
system, or are transmitted to the using system through the
controller.
4-7.
There are three major functions the disc drive
performs:
a. Provides a medium of data storage.
b. Supplies fast, accurate access to any desired storage area
on the disc.
c. Supplies a method of data retrieval.
4-9.
The data storage medium is a thin layer of mag-
netic material coated on the disc substrate. Access to store
or retrieve data is provided by a carriage assembly that
supports two read/write heads (one read/write head per disc
surface). The carriage assembly (see figure 4-1) consists of a
coil of wire free to move within the combined field of two
permanent magnets. In essence, the CQil and magnet form a
linear motor that moves the carriage assembly.
DISC
SPINDLE
MOTOR
Figure 4-1. Disc Drive Overview
4-1
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