2-10 Theory
of
Operation
Introduction
Functional Characteristics and
Communication Protocol
This section describes
the
recording formats
used
in the HP disc memory
and
the HP disc
command
set.
Recording Formats
The HP disc drive supports
both
the HP
Standard
Disc Format (hereafter referred
to
as the
HP Format)
and
the
IBM
(IBM
3740)
Standard
Data Interchange Format (hereafter referred
to
as
the
IBM
Format). Format similarities
and
differences are described in the folloWing
paragraphs.
Format Similarities
The following features
are
common to both the
IBM
Format
and
the
HP
Format.
Media.
A double-sided disc
is
used
as the recording medium. The disc
is
composed of
recording material
and
is
enclosed in a
square
plastic jacket.
An
index hole in the disc
is
used
to provide a rotational position reference.
The
disc drive
will
also accept a Single-sided disc.
Single-sided discs must
be
used for the
IBM
Format
mode
of operation.
Tracks.
There are 77 physical tracks on
each
side of
the
disc, with a spacing of
0.0208
inch
between
track centers (48 tracks
per
inch). The outermost track
is
Track 0
and
the
innermost
track
is
Track 76.
Recording.
Information
is
stored on the disc as a series of magnetic flux reversals. Since a
single
head
is
used
to
read
from
and
write
on
each side of the disc, a self-clocking
code
must
be
used
to store the information.
The portion of the disc
or
the duration of the time used to store a single bit
is
referred to as a
. bit cell. The first
part
of the bit cell
is
called the clock window
and
the remainder of the bit
cell
is
called the data window. A flux reversal in the clock window is called a clock transition
and
a flux reversal in the data window
is
called a data transition. A bit cell that contains a
data transition stores a 1
and
a bit cell with
no
data transition stores a
O.
Track
Format.
Each track
is
divided into sectors,
as
shown in Figure 2-3. The data con-
tained in
one
sector
is
the smallest
amount
of information that can
be
written
at
a time.
The
disc is soft-sectored,
that
is, there
is
no
hardware indication of where
each
sector starts.
Instead, the beginning of each sector
is
indicated by information recorded
on
the disc.
In
order
to allow soft-sectoring, each sector
is
divided into two fields. First,
there
is
an
ID
field which contains information to identify the sector. Next, there
is
a
data
field which
contains the actual data. The
ID
field
is
written only when the disc
is
formatted, never during
actual operation. Thus,
an
ID
field serves as a fixed marker for the beginning of each sector.
The entire data field is re-written each time a write operation occurs to the sector.
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