Specifications
6.1.2
Byte
A byte, when referring
to
serial data (being written
onto
or
read
from
the
disk
drive),
is
defined
as
eight consecutive
bit cells.
The
most significant bit cell
is
defined as
bit
cell 0
and
the least significant bit cell
is
defined
as
bit cell 7.
When
reference
is
made
to a specific
data
bit (Le., data
bit
3),
it
is
with respect to
the
corresponding bit cell
(bit
cell
3).
During a write operation,
bit
cell 0 of
each
byte
is
transferred
to
the disk
drive
first with
bit
cell 7 being transferred
last. Correspondingly, the most significant byte of data
is
transferred to the disk first
and
the least significant byte
is
transferred last.
When
data
is
being
read
back from the
drive,
bit cell 0 of
each
byte
will
be transferred first with bit cell 7 last. As with
writing, the most significant byte
will
be
transferred first from
the
drive
to
the
user.
Figure
6-3
illustrates the relationship of the bits within a byte. Figure
6-4
illustrates the relationship of
the
bytes for
read
and
write data.
BYTE
I 0 I
2 3
1
BIT CELL 0 OF BYTE 0
IS
FIRST DATA
TO
BE
SENT
TO
THE DRIVE WHEN
WRITING AND FROM THE
DRIVE WHEN READING
4
5
6
7.
8 9 10
11
12 13 14 15 1
16
r~:E
1
t
BIT CELL 7
OF
BYTE
17
IS
LAST DATA TO BE SENT TO
THE DRIVE WHEN WRITING
AND FROM THE DRIVE
WHEN READING
50574·29
FIGURE 6·4. DATA BYTES
6.2
RECORDING
FORMAT
(DOUBLE
DENSITY)
Double capacity can be obtained by use of
MFM
(modified frequency modulation) rather
than
FM
(frequency
modulation) which
is
the standard
method
of
encoding data on the diskette.
The
differences between
FM
and
MFM
encoding are shown
in
figure 6-5. Note
that
MFM
results
in
a 1
to
1 relation-
ship between the flux chan.ges
per
inch
and
the data bits
per
inch recorded
on
the diskette. This also results
in
a
doubling of the data transfer rate, from
250
to
500
k bytes, when
compared
to
FM.
Data error rate performance equal to standard capacity diskettes using
FM
encoding can be achieved by using:
a.
The
SA810/860
diskette
drive
with
its
proprietary ceramic/ferrite
read/write
head.
b.
Phase
locked loop (VFO)
data
separator.
c. Write precompensation.
Provision of
the
phase
locked loop
data
separator
and
write precompensation circuitry
is
the responsibility of the
user of
the
SA810/860
diskette drive.
Shugart
will
provide
design information, as required, to
SA810/860
users who desire to incorporate double
capacity diskette
drives
in
their products.
The
bit cell for
MFM
encoded
data
is
one
half the duration of the bit cell for
FM
encoded
data. Also, unlike FM,
MFM
bit cells
do
not always contain a clock bit at the leading
edge.
This lack of a clock
bit
makes
data
separation
more
complex. Also, the window size
is
half
the
FM
window size, which results
in
less tolerance
to
bit shift. The only
reliable
method
to separate
MFM
encoded
data
is
through use of a
phase
locked loop (VFO) type of
data
separator.
The
VFO,
once
synchronized, tracks the data
and
generates clock
and
data windows. This
improves
the
bit shift
tolerance
over
the
conventional "hard"
data
separators commonly
used
in
FM
recording which use windows of
fix-
ed
timing.
6-2