3.17.5 INDEX
PULSE
This is a pulse which is true once per disk revolution and is utilized to define the sector
reference (sector zero). The index pulse always occurs during the sector just prior to
sector zero, i.e., during sector N - 1, where N is the maximum number
of
sectors. (Refer
to
SECTOR
COUNT lines, Paragraph 3.17.4,
and
SECTOR
PULSE
lines, Paragraph 3.17.3.)
The
duration of the index pulse is 10.4 + 3.0
J.1sec.
The
INDEX
PULSE
line will
be
low for
the duration of the pulse. At
all other times, the line will
be
high.
If
a unit is selectd at the
moment that
an
index purse
is
in progress, that pulse duration will
be
shortened, i.e., the
pulse will
be
shaved
an
indeterminate amount.
In
all cases, the pulse on the
INDEX
PULSE
line corresponds only to the respective platter
as
selected
by
the
PLATTER
SELECT line. This is accomplished by a multiplexer internal
to the disk drive.
3.17.6
READ
CLOCK
This is a pulse which goes true at the beginning
of
a new bit cell during a
read
operation.
The leading edge
of
this pulse defines the beginning of the new bit cell.
During a portion
of
each
bit-cell time, the
READ
CLOCK line will
be
high
and
remain in this
state
until the next bit cell time is to
be
defined by a pulse leading edge.
This
line is utilized to establish a timing reference for purposes
of
interpreting pulses on
the
READ
DATA
line.
3.17.7
READ
DATA
The
signals
on
this line define the content
of
the data
read
during a
Read
operation.
The
signal is a pulse for each logic one bit
read
from the disk. The line will
be
low during
the time
of
the phe pulse. The line remains high for
each
logic zero bit
read
from the disk.
The timing of pulses on this line is relative to the
READ
ClOCK
line (Paragraph 3.17.6).
The
relative timing is such that a logic one bit should
be
interpreted
if
the
READ
DATA
line
is
low at any time when the
READ
CLOCK line is high between consecutive pulses
on
the
READ
CLOCK line.
3.17.8
ILLEGAL CYLINDER
ADDRESS
This is a level which, when low. indicates that
an
illegal cylinder address was detected
during the last CYLINDER
ADDRESS
STROBE.
The
line will go low within 1.0
!J.sec
after the trailing edge of the strobe only when
an
illegal
condition is detected.
The
line will go high within 1.0
f.Lsec
of
the leading edge of the
strobe
if
the line is Is not already high at the time of the strobe. These actions will take
place only
if
the strobe is recognized.
The
unit must
be
selected
and
ready in order to
recognize the strobe.
An
illegal condition will
be
indicated for either or both of the following.
(1)
The cylinder address specified
at
the time
of
the strobe exceeds the range
of
legal
addresses for that particular model of disk drive.
(2)
A strobe is issued to the drive while it is busy seeking.
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