B30D
The number of sector pulses that may be accommodated by a 2315-type cartridge sector
ring is determined by machining and mechanical limitations. Therefore, cartridges are
available
with
8,12,
16,
24,
32,
and
48
sectors as standard items.
In disk drives incorporating a fixed disk
as
well
as
a 2315-type removable disk, the number
of sector pulses for the fixed disk are
also determined by a plug-in array.
In
all cases, the pulses on the
SECTOR
PULSE line correspond only
to
the respective
platter
as
selected by the PLATTER SELECT line. This is accomplished by a multiplexer
internal to the
disk
drive.
The
number
of
sector pulses that may
be
accommodated by the electronically generated
sectoring is listed in Table
1-1.
The duration
of
any sector pulse is 8.0 + 2.0
fJ.sec.
The
SECTOR
PULSE line is low during
the duration of the pulse, and high at
all other times. If a unit is selected at the moment
that a sector pulse is in progress, that pulse duration
will
be shortened, i.e., the pulse will
be
shaved
an
indeterminate amount. This will have the effect
of
introducing a time
uncertainty (instantaneous jitter) just for the shaved pulse.
3.17.4
SECTOR
COUNT LINES
These lines specify the sector count presented
in
binary format. This count indicates the
particular segment
of
the disk surface currently under the read/write heads.
The
signals
on
these lines are, in essence, the states of a binary counter. In all cases, the count
presented corresponds only to the respective platter as selected by the PLATTER
SELECT
line; this is accomplished through the use
of
separate counters multiplexed internal to the
disk drive.
The count is represented by the binary value with a low
Jevel
corresponding to a binary
one.
The
most significant
bit
is bit number
6;
the least significant
bit
is bit number
O.
For convenience of description the sector count may
be
expressed in decimal
as
the sum
of the true
bit
weights
as
follows.
Bit
Number
Decimal
6 5 4
3
210
Bit
64
32
16 8 4 2 1
Weight
The signals on the
SECTOR
COUNT lines change state 2.4 +
1.61J.sec
prior
to
the leading
edge
of
the sector pulse, assuming non-shaved pulses.
The
sector pulse immediately following
an
index pulse defines the beginning
of
sector
zero and, at the time of this sector pulse, the
SECTOR COUNT lines will present a zero
value. Thereafter, the value will
be
incremented until the maximum count is achieved.
The
maximum count that will Occur is
(N
-
1)
where N is the number of sector pulses. This is
determ ined by the particular configuration as described under
SECTOR
PULSE (Paragraph
3.17.3).
3-20
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