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SI'CTION 4
THEORY OF OPERATION
INTRODUCTION
This
section contains a description on the theory of operation of the
drive.
The discussion begins with a general suwnary of magnetic data
recording,
followed
by
a
description of each major function of the
drive.
Circuit
block
diagrams and schematics are located in the appendices.
They
are
useful
to show the interconnections between the electronic
circuits and the mechanical components.
4.1
DATA RECORDING
Digital
information
is
represented
by
a sequence of bits:
either
o
or 1.
Small areas of the medium in which such binary information is
stored,
for
example
the iron oxide coating of a magnetic disk, must
be put in one state or the other to represent the data.
Recording
of
data
on
a
magnetic medium is based on the principles
of electromagnetics.
When current flows in a coil of wire it produces
a
magnetic
field.
The
field is
confined
in
a
core of magnetic
material
around
which the wire is wound.
A narrow slot, called
the
air
gap,
is
placed
in the core located closest to the media.
It
is
the
magnetic field in the vicinity of the air gap that magnetizes
the
magnetic medium
(Figure 4-1).
When the current is reversed, the
polarity of magnetization is reversed.
Information
can
be
recorded
on
the
diskette
by
using a double-
frequency
code.
The magnetization profiles in each bit cell for the
number sequence shown for FM recording are shown in Figure 4-2.
When
data
data stored
the
first
represents
represents
is
read, the same head that writes the
is a digital bit representing a
a
or 1.
flux
reversal
represents a clock bit.
a
stored
bit
1.
The
absence
pf
a bit O.
data is used.
The
In each bit cell,
A second reversal
a
second reversal
/ " " " - - - - ELECTROMAGNETIC CORE
MAGNETIC FIELD OF HEAD
MAGNETIC FIELD OF MEDIA
ELECTRIC
----~...:::_-4..J
CURRENT
~
•
ROTATION OF MEDIA
FIGURE 4-1
ELECTROMAGNETIC CORE
4-1
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