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Functional Description
7936 and 7937
Error 1
(TERR1-L)
and Timing Error 2
(TERR2-L).
The fa ult detection circuit monitors
SM DT
-H and
GAP-L
to ensure that the sampled servo timing is
correct relative to the dedicated servo timing. If
either signal does not occur at the proper time,
TERR1-L
is asserted. If three consecutive servo
sync pulse are missing, or do not occur within a
predefined window,
TERR2-L
is asserted.
5-44. Position Demodulator.
The sampled servo
position demodulator monitors the Sampled Servo
(SSRVN)
signal and generates a Sampled Position
(SPOS)
analog signal which represents the position
of the currently selected data head relative to the
sampled servo code. This signal is used to control
actuator positioning when track following on a
data track.
The
SSRVN
signal from the sampled servo
amplifier is input to a dibit integrator, which is en-
abled at the proper time by
SSGT-L.
The in-
tegrator processes the sampled servo code by
measuring the energy contained in the first half of
each dibit, holding it for sampling, then discharging
it to a constant value before processing the next
dibit.
The output of the dibit integrator is fed to a pair
of sample-and-hold circuits, which demultiplex the
odd and even dibits into separate channels. The
timing of the sample-and-hold circuits is controlled
by the
SSDEMOD-H
signal from the sampled servo
timing circuit.
A second signal, Sampled Dump
(SDMP-H),
discharges the sample-and-hold circuits
in preparation for the next sampling cycle.
The outputs of the two sample-and-hold circuits
are input to a position discriminator, which sub-
tracts the two signals. The resultant
SPOS
signal
represents the position of the data head relative to
the sampled servo code.
SPOS
is used to position
the actuator when reading and writing data.
The sample-and-hold circuit outputs are also used
as the input to a level discriminator circuit. This
summing amplifier generates a Sampled Level
(SLVL)
signal proportional to the sum of the two
inputs.
SLVL
represents the level of the sampled
servo signal and is used by the sampled servo AGC
integrator to generate the proper AGC level.
5-20
5-45. SAMPLED/DEDICATED
SELECr
SWITCH.
The sampled/dedicated select switch
selects either Sampled Servo Position Error signal
(SPOS)
or Dedicated Servo Position Error signal
(DPOS)
for connection to the input of the servo
power amplifier via the slew rate limit switch. The
selection of the desired input signal is controlled
by Sampled Servo Select
(SSSL-H)
from the servo
controller. The switch output is labeled Position
Error
(PES).
5-46. SLEW RATE LIMITER. The function of
the slew rate limiter is to prevent single sector
dropouts of the sampled servo from disturbing the
operation of the sampled servo loop. When a large
step in
SPOS
occurs, caused possibly by a bad sec-
tor, the slew rate limiter limits the rate at which
SPOS
can change, reducing the possibility of the
head being driven off track. The limiter output is
labeled Slew Position Error
(SLPES).
5-47. SLEW RATE LIMIT SWITCH. The pur-
pose of the slew rate limit switch is to disconnect
the slew rate limiter from the sampled servo loop
when the servo system is in a settling (off-track)
mode. The switch
is operated
by
Offtrack
Unlatched
(OFTU-L)
from the off-track detector.
5-48. OFF-TRACK DETECTOR.
The function
of the off-track detector is to process Position
Error
(PES)
into a digital indication of whether
the heads are off track or on track. The output of
the
detector,
labeled
Offtrack
Unlatched
(OFTU-L),
is used by the servo controller, together
with a track verify, to report to controller PCA-A6
that a head settle on track has occurred.
5-49. TRACK CROSSING DETECTOR.
The
track crossing detector is a voltage comparator
that detects when Dedicated Servo Position Error
signal
(DPOS)
falls below a certain level. At this
time,
the
detector
output
Track
Crossing
(TRKeR-H)
is enabled to indicate that a dedicated
servo track has been crossed.
Also included in the track crossing detector cir-
cuitry is a divide-by-five circuit that the servo con-
troller activates at certain times to divide the track
crossings by five. This is necessary when the
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