8300
The sector number count is represented by the binary value contained in the sector number
counter. The QA output
of
U144 is the least significant bit and the
QC
output of
U143
is the
most significant bit
of
the number. The counter is clocked, or counted up, by the raw
sector pulse obtained from the Sectoring Selection Programming array plug at J125 (zone
C,
D,
E,
F-14). For each pulse the counter
wi"
be
incremented by one count. The Index
Pulse which occurs once per revolution
of
the disk, is defined
as
occurring during the
sector just prior
to
sector zero; i.e., during sector N minus one where N is the maximum
number of sectors. Therefore, at the time
of
occurrence
of
the index pulse,
U83,
the Upper
Count Control Flip-Flop (zone
E8)
will
be
pre-set by the occurrence
of
the Index Pulse.
This
will cause the Q output
of
U83, pin 10,
to
go low, enabling the load inputs (LD) of
U144
and U143. However, the counter will not load zeros until the next sector pulse. At
that time, instead
of
incrementing the count,
it
will be clocked
to
load all zeros. At the
time that this clocking occurs, the Upper Sector Count
Flip-Flop will
be
zero-set since the
K-input
of
the
flip-flop
is enabled at all times. This will remove the enable from the load
inputs (LD)
of
the Sector Number Counter and allow
it
to
be
incremented or clocked by the
raw sector pulse. The Sector Pulse immediately following
an
Index Pulse defines the
beginning
of
sector zero. At the
time
of
that Sector Pulse, the Sector Count lines will
present a zero value
as
the result
of
having loaded the Sector Number Counter with all
zeros. Thereafter, the value
wi"
be incremented until the maximum count is achieved. The
maximum count that
will occur is N minus one, where N is the number
of
sector pulses.
Recall from the previous discussion that the lower platter is always sectored electronically
in
dual-disk machines. Recall also that the removable cartridge in either single-disk
machines or dual-disk machines may
be
sectored electronically. The exception to this is if
the cartridge has
an
index-only notch, then
it
must be sectored electronically.
Electronic sectoring is selected by the Sectoring
Selection Programming array installed in
J125. When electronic sectoring is selected, the output
of
the electronic sectoring
electronics is connected
to
the Upper Sector Pulse Former, the Upper Sector Number
Counter, and the Upper Count Control Flip-Flop.
NOTE
The
Lower Sector Number Counter, Lower Count
Control,
and
Lower
Sector
Pulse
Former
are
connected to the electronic sectoring electronics
at
all
times since the lower platter is always sectored
electronically.
Electronic sectoring requires that the
disk
drive generate pulses electronically for
sectoring
in
lieu
of
the mechanical slots normally used for sectoring. Since the lower disk
is sectored electronically regardless
of
the mechanical configuration, pulses must always
be
generated for sectoring the lower disk. These pulses are generated by counting down,
with
an
electronic counter, the output of a high frequency oscillator. Because the sector
pulses must
be
synchronous with the instantaneous speed
of
rotation
of
the spindle, it is
necessary to phase lock
this
high-frequency oscillator
to
the spindle.
The high-frequency oscillator is a voltage controlled oscillator which is a part
of
the Sector
Phase Lock Loop. The function and purpose
of
the Phase Lock Loop is to phase-lock the
voltage controlled oscillator frequency
to
the phase lock pulses obtained from the Phase
Lock Ring through the Lower Time Demultiplexer. The sector phase lock loop is shown in
zones
12
through
20
at the bottom
of
the schematic. The input to the Sector Phase Lock
Loop is from
U183,
pin 13, (zone
D15),
which is the phase lock pulse output from the
Lower Time Multiplexer.
It should be recalled that the phase lock pulses
are
separated
from the lower index pulse by the Lower Time Demultiplexer.
.
5-40
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