Radio Shack TRS-80 Service Manual page 36

5-Meg Hard Disk
Service Manual
-me*
«-»« ®
Each clock
or data bit on the serial stream triggers the
one-shot.
If
the time between successive triggers
is
less
than the one-shot time constant, the one-shot remains
retriggered.
As the one-shot
is
triggered
by
data stream
bits, so
is
the up/down counter
(U2)
whose count mode
is
controlled by the state of the
one-shot outputs
.
While the
one-shot
is
being retriggered, the counter counts
up.
When
any data bit fails
to
reach the one-shot before
its
time
constant
is
over, the one-shot resets and
in
turn clears the
counter. Only when
16
successive retriggers occur, can the
counter reach
its
terminal count. At this
time, the counter
overflow goes true and sets the DRUN* latch output
(U3,
pin
6)
low which switches read data
in
and reference clock
out*
An AND-OR-INVERT gate
(U4)
performs the switching. DRUN*
is
read through
(U74)
by the 8X300 to determine the condition
of the
MFM data
stream.
VCO
The Hard Disk controller uses
a
single chip VCO
(U32)
which
simplifies circuitry and adjustments. The operating point
of
the
VCO
is
initially set by adjusting the variable capacitor
(C33)
for
a 10 MHZ
output
at TP9
and the frequency control
voltage input
(TP8) to 2.5 V
+/-
.5
V.
It
should
be noted
here that both
of
these adjustments are done using the same
trim cap
(C33
)
.
The output of the error amplifier and filter
is
fed to the
VCO and represents how
far the VCO frequency
is
from that
of
the incoming signal. The error signal, which
is
proportional
to the difference, allows the
VCO to shift from center
frequency and become
the same as the frequency of the
frequency
of the input signal. When the loop
is in
lock, the
difference frequency component will
be DC and
is
passed
by
the low pass filter.
Frequency control
is
actually
a
matter
of
frequency
range.
The difference component may fall outside
of the
band edge
of the low-pass filter and be removed along with the sum
frequency component.
If
this
is
the case, then
no
information
is
transmitted around the loop and the VCO
remains at its initial free running frequency.
As the input
frequency approaches that
of the VCO, the frequency
of the
difference component decreases and approaches
the band edge
of the filter. Now part of this difference component
is
passed which tends
to
drive the VCO
to the frequency
of
the
difference component more and allows more
of it to be passed
through the filter. This
is a
positive feedback, which
causes the VCO
to
snap into "lock" with the input signal.
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