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Functional Description
7936 and 7937
(7937) data surfaces. The output signals. labeled
Drive ID bits
(DIDO, DID1),
are connected via
read/write PCA-A2 to the input of the servo con-
troller in servo PCA-AI. See figure
III.
5-19. HEAD GAIN REFERENCE PROM AND
DAC. Read-only memory (PROM) and digital-to-
analog converter (DAC) ICs in the HDA supply a
reference current to servo PCA-A2 that represents
the gain characteristics of the read/write heads and
servo head installed in the drive. The PROM is
programmed with this data during manufacture
and checkout of the drive. These gain characteris-
tics are included in the determination of the AGC
levels for the sampled and dedicated AGC
amplifiers in read/write PCA-A2.
During operation of the drive, the read/write con-
troller supplies read/write head address informa-
tion to the PROM together with details of where
the head is located on the disc. The head address
information is supplied by signals Chip 0, I
(CHO-H, CH1-H)
from the head select encoder and
Head Select 1, 2
(HD1 S-H, HD2S-H)
from the
read/write controller. Head position information is
provided by Write Current Select 1, 2
(WCS1,
WSC2),
also from the read/write controller. In ad-
dition, Buffered Start of Sector
(BSOS-L)
indicates
when to look at the gain of the read/write heads
(for sampled servo) or the servo head (for dedi-
cated servo). A 6.25V reference to the DAC,
labeled
DAR,
is supplied from servo PCA-AI.
The DAC converts the 8-bit output from the
PROM into a head gain reference current for the
selected head. The DAC output is labeled Servo
Gain Reference
(SGR).
Signal
SGR
is connected to the dedicated and
sampled AGC circuits in servo PCA-AI where it is
converted to a reference voltage and integrated
with outputs from the position demodulators to
determine the AGC levels for the dedicated and
sampled servo AGC amplifiers. in read/write
PCA-A2.
5-20. ACTUATOR.. The read/write heads and
servo head are mounted on a rotary arm supported
by precision ball bearings. A bobbin-type voice coil,
attached to the rotary arm, and mounted between
two fixed permanent magnets, comprise a head
5-14
positioning mechanism (actuator).
The actuator
provides the driving force needed to move the arm
f or positioning the heads over the discs. The mag-
netic field in the gap between the two magnets al-
lows acceleration of the moving arm to be control-
led by the current in the voice coil. Drive current
for the voice coil is supplied by the servo power
amplifier in servo PCA-AI. Crash stops are incor-
porated in the actuator to prevent damage to the
heads in the event that the servo system loses con-
trol of the actuator.
When the drive is powered down, the rotary arm is
driven to a nondata head landing zone at the inner
diameter of the discs. A mechanical latch for the
arm prevent.s damage to the heads during shipment
of the drive by immobilizing them in the landing
zone.
5-21. SPEED SENSOR.
The speed sensor is a
Hall-effect device mounted on the head-disc as-
sembly near the spindle pulley.
A magnet,
embedded in the pulley, triggers the sensor once
per revolution of the spindle. The sensor output,
labeled Speed Sense
(SPDSNS-L),
is connected to
the servo controller in PCA-AI. See figure
m .
This output is used by the controller at power on
to determine when the spindle is rotating fast
enough to allow the actuator to move the heads
away from the landing zone. This action occurs
when the speed reaches approximately 3000 rpm.
5-22.
READ/WRITE PCA-A2
The servo system components in read/write
PCA-A2
include
the
dedicated
servo
AGC
amplifier, sampled servo switch, and sampled servo
AGC amplifier. See figure
(iDJ .
5-23. DEDICATED SERVO AGC AMPLIFIER.
Differential Servo Data
(SDX, SDY),
from the ser-
vo read preamplifier/write driver
Ie
in head-disc
assembly A3, is buffered and gain controlled by
the dedicated servo AGC amplifier. The input to
the preamplifier section of the IC is the servo code
read from the dedicated servo surface. The dif-
ferential output from the AGC amplifier, labeled
Dedicated Servo
(DEDN, DEDP),
is passed to the
input of the dedicated servo filter in servo
PCA-AI. Dedicated AGC
(DAGC),
developed by
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