Specifications
6.3
RULES
OF
ENCODING
6.3.1
FM
Encoding:
a. Writes
data
bits at
the
center of
the
bit cell.
b. Writes clock bits at
the
leading
edge
of
the
bit cell.
6.3.2
MFM
Encoding:
a. Writes
data
bits at the center of
the
bit cell.
b. Writes clock bits at
the
leading
edge
of the bit cell i;:
•
There
is
no
data bit written
in
the
previous bit cell,
and
•
There
will
be
no
data
bit
written
in
the
present
bit
cell.
NOTE
In
MFM,
the
write oscillator frequency
is
doubled while maintaining
the
same
flux
changes
per
inch
as
FM.
Thus, the bit cell
in
MFM
is
half that
in
FM. Data transfer rate
is
also doubled, since a 1
to
1 relationship ex-
ists between flux changes
per
inch
and
data bits
per
inch
(2
to 1
in
FM).
Index
is
the physical detector indicating
one
revolution of the media.
It
is
used
to
initiate format operations,
generate the READY signal
in
the storage device,
and
ensure
that
one
complete revolution of the media
has
been
searched.
It
is
also
used
for a deselect storage device signal after a certain
number
of revolutions.
Some
of
the
more
common
format fields are explained
in
the following paragraphs.
6.4
TRACKS AND
RECORDING
FORMATS
6.4.1
Tracks
The
SA810/860
drive
is
capable of recording
up
to
77/154
tracks of data.
The
tracks are
numbered
0-76
for
each
side. Each track
is
made
available to the
read/write
heads
by
accessing the
head
with a
stepper
motor
and
carriage
assembly,
and
selecting
the
desired side of the diskette.
Basic Track Characteristics:
No. Data Bits/Track Single Density
No. Data Bits/Track Double Density
Index
Pulse Width
Index/Sector
Pulse Width
(Hard
Sectored
Only)
6.4.2
Track
Format
41.664
bits (5208 bytes)
83,328
bits (10,416 bytes)
1.8
±
0.6
ms
0.4
±
0.2
ms
Tracks may be formatted
in
numerous
ways
and
are
dependent
on the using system.
The
SA810/860
use index
and
sector recording formats respectively.
6.4.3
Hard
Sector
Recording
Format
In
this format,
the
using system may record
up
to
32
sectors (records)
per
track. Each track
is
started by a physical
index pulse
and
each
sector
is
started by a physical sector pulse. This type of recording
is
called hard sectoring.
Figure 6-6 shows a typical hard sector recording format for 1 of
32
sectors.
6.4.4
Soft
Sector
Recording
Format
In
this format, the using system may record
one
long record
or
several smaller records. Each track
is
started by a
physical index pulse
and
then
each
record
is
preceded
by a unique recorded identifier. This type of recording
is
call-
ed
soft sectoring.
6.4.5
Typical
Track
Index
Format
Figure 6-7 shows a track format, which
is
IBM
compatible, using index recording format with soft sectoring.