host without the delay of a media access. However, the controller is optimized
to
return any
requested
data
to
the host as fast as possible.
If a
new
READ command requests
data
not contained in the buffer while
the
drive is
performing a Look Ahead Read, the process is aborted, and the drive will immediately seek
to
the new track with no effect
on
access or transfer performance. Filling the track buffer with
unrequested
data
has a lower priority
than
delivering requested
data,
Other conditions
that
may affect completing a full-track READ before receiving the next READ command are: head
position relative to the requested data, transfer size, and the host
tr~nsfer
rate.
Head
Alignments
The drive executes periodic head alignments
to
maintain proper track following tolerances.
During the
Power-On sequence,
the
drive executes a complete calibration and head alignment
for all heads. For subsequent operation, there are two modes for head alignments: the
automatic mode, and
the
Initiator-controlled mode. In the automatic (default) mode,
the
drive automatically implements single head alignments in Seek, Read, and Write commands.
In the Initiator-controlled mode, head alignments are executed upon receipt
of
a Rezero Unit
command.
Head Alignment States
The drive will enter either a head alignment needed or critical
state
according
to
changes in
temperature since the last head alignment, or the elapsed time since
the
last head alignment.
Temperature
Considerations
Elapsed
Time
Considerations
H a
temperature
change
exceeds
the
first
of
two
factory
Under
normal
conditions,
the
drive
schedules
head
defined
thresholds,
the
drive
enters
the
head
alignment
alignment
calibrations
at
preset
intervals
after
spin-up.
The
needed
state.
H a
head
alignment
occurs
the
drive cancels
drive
enters
the
needed
state
at
these
intervals.
H a
head
the
needed
state.
H
the
temperature
change exceeds
the
alignment
occurs
the
drive
cancels
the
needed
state.
H twice
second (higher)
threshold
before a
head
alignment
is
the
expected
time
interval
passes
without
a
calibration,
the
accomplished,
the
drive
enters
the
critical
state.
drive
enters
the
critical
state.
The
time
periods
between
head
alignments
are
pre-defined
at
the
factory
to
assure
reliable
operation
under
start-up
and
long
term
conditions.
Head Alignment Modes
The Mode Select command (page 09H, byte 8, bits 6 and 7), allows
the
Initiator
to
control
the
head alignment function as follows:
DRAFT
Product Features 3·3
1/18/93
09:58
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