6300
Additionally, failure of the lamp in the Position Transducer is detected by the Lamp Failure
Detector. A signal derived from this detector is fed back to the Position Control Logic for
purposes of determining when
an
emergency condition exists. This signal is Position
Transducer Failure (SPTFG).
.
4.5
REAOIWRITE OPERATIONS
A double frequency recording method is used in the 03000 Disk Drive. Read/Write
operations
are
accomplished by a read/write head. During a write operation, a bit is
recorded
on
the disk whenever the coils
of
the read/write
head
are
switched
by
the Write
Driver circuits. During a
read
operation, a clock
or
data bit is sensed on the disk whenever
the flux direction induced in the coil winding is reversed
as
a result of a change
in
polarity
of the
flux pattern presently passing under the
head
gap.
The recording
head
Is a split-ring core containing coli windings so that a magnetic field
with a given flux direction prevails at the core gap while the coil is energized. When current
flows through the coil, the flux induced in the core establishes a fringe flux at the gap. As
a magnetic recording surface passes by the gap, the fringe flux magnetizes the surface of
the disk.
During a write operation, a bit is recorded when the flux direction in the core is reversed by
switching between coils of
the read/write head.
The
fringe flux is reversed at the
gap
and,
hence, the portion of the flux flowing through the recording medium is reversed.
If the flux
reversal is considered instantaneous in comparison to the motion of the recording surface,
and
the
gap
is observed at the moment of reversal, it
can
be
seen
that the portion of the
surface that just passed the gap is magnetized in one horizontal direction while the portion
directly under the
gap
Is magnetized in the
oppOSite
direction. Between these two areas,
the flux must reverse
180
degrees; this recorded flux reversal represents a bit.
During a
read
operation, the
gap
first passes over
an
area
that is magnetized
in
one
horizontal direction, and a constant flux is induced into the core and the coil.
The
coil
provides
no
instantaneous output voltage for this condition. However, when the recorded
bit
(180
degrees horizontal flux reversal) passes the gap, the flux induced into the core
and
coil must go through a 180-degree reversal. This reversal means that the coil
sees
a change
in
flux which results in a voltage output pulse.
A basic clock frequency signal is encoded
In
the data pulses
to
produce a single
composite signal at the read/write head. The composite signal represents either a logic
zero bit condition or a logic one bit condition for
each
bit-cell time defined by the clock.
Figure
4-3 illustrates the use
of
a clock frequency to establish the basic bit-cell timing
cycle.
The
insertion of a data pulse between clock pulses in a bit-cell period produces a
composite read/write signal which uses only
clock pulses for a logic zero bit indication,
and
data pulses for a logic one bit indication. A zero bit-cell (clock pulses only) produces a
single change in direction
of
the flux pattern. A one bit-cell (data pulse located between
two clock pulses) produces a double change in direction of the flux pattern.
In
either
case,
the clock signal causes a change in direction
of
magnetism from plus to minus or minus to
plus polarity, thus causing the storage of a bit. Because both clock and data information
are
synchronized on a composite signal, double frequency recording is sometimes
referred to
as
self-clocking.
In
double-frequency recording, a clock bit is always inserted at the beginning of
each
bit-cell time
to
establish the basic recording frequency. A data bit is inserted between
clock bits (at twice the frequency) so that the data bit results in two flux reversals within a
single bit-cell time.
If
the data bit is not present, a single flux reversal occurs in a bit-cell
time.
4-10
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