Specifications

1.4.8
Read
Operation
Reading
data
from the
SA810/860
drive
is
accomplished by:
a. Activating DRIVE SELECT line.
b. Selecting
head
(SA860 only).
c. WRITE GATE being inactive.
The
timing relationships required to initiate a read
sequence
are
shown
in figure 1-5. These timing specifications are
required
in
order to guarantee that the position of the
read/write
head
has stabilized prior to reading.
The
timing of
READ DATA
(FM)
is
shown
in
figure 1-6.
READ DATA
SEP DATA
SEP CLOCK
A
I
LJ
I
B
I
A
I
4.00
itS
NOM
U I
~
l.--200
± 50 ns
I
U
~I
~200
±
50ns
A
B
A
I
U
I I
U U
A = LEADING EDGE OF BIT MAY
BE
± 400 ns FROM ITS
NOMINAL
POSITION.
B = LEADING EDGE OF BIT MAY
BE
± 200 ns FROM ITS
NOMINAL
POSITION
..
FIGURE 1·6. READ
SIGNAL
TIMING
(FM ENCODING)
39216·05
The
encoding scheme of the recorded data
can
be
FM
or MFM. The first of these, FM, provides single-density
recording.
The
superior efficiency of
MFM
permits the bit cell period to be half that of the
FM
code,
thereby pro-
viding double-density recording. Differences
among
FM
and
MFM
encoding are concerned with
the
use of clock
bits
in
the
write data stream.
FM
encoding rules specify a clock bit at the start of every
bit
cell.
MFM
encoding rules allow clock bits to
be
omitted
from
some
bit cells, when either the preceding bit cell
or
the current bit cell contains a data bit.
See
figure 1-7.
a
o
o o
BIT
CELLS
D
D D
D
D
FM
--..
2F
~
MFM
FIGURE 1·7. FM
AND
MFM
CODE
COMPARISONS
1-8