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the AGC integrator in the dedicated servo circuit,
is fed back to the AGC amplifier to control its
gain.
5-24. SAMPLED
SERVO
SWITCH.
The
sampled servo switch gates differential Read/Write
Data (OX, OY), allowing only the sampled servo
code prerecorded between data sectors to reach
the input of the sampled servo AGC amplifier.
The gating action of the circuit is controlled by
differential Sampled Servo Enable
(SSEN-H,
SSEN-L),
from the sampled servo timing circuit in
servo PCA-At. See figure
II.
5-25. SAMPLED SERVO AGC AMPLIFIER.
The differential output from the sampled servo
switch is buffered and gain controlled by the
sampled servo AGC amplifier.
The differential
output from the amplifier, labeled Sampled Servo
(SMPN, SMPP),
is passed to the input of the
sampled servo filter in servo PCA-AI. Sampled
AGC
(SAGe),
developed by the AGC integrator in
the sampled servo circuit, is fed back to the AGC
amplifier to control its gain.
5-26.
SERVO PCA-At
The servo system components in servo PCA-AI in-
clude the servo controller, AGC circuits, sampled
servo circuit, dedicated servo circuit, servo power
amplifier, and amplifier control and fault indica-
tion logic.
See
figures
II
and
iI.
5-27. SERVO CONTROLLER.
The servo con-
troller consists of an 8-bit control-oriented micro-
computer IC. Included on the single chip are an
8-bit central processing unit (CPU), 32 input/out-
put (I/O) lines, internal memory of 8k by 8 ROM
(program) and 128 by 8 RAM (data), two 16-bit
timer/counters, and a full duplex serial port. The
Ie
is clocked by an on-chip clock circuit and ex-
ternal crystal oscillator.
The servo controller receives commands from the
microprocessor in controller PCA-A6, provides
Functional Description
7936 and 7937
control for the drive servo system to complete
these commands, and returns servo status informa-
tion to the microprocessor. All communication
with controller PCA-A6 is via the Servo Control
Bus.
5-28. Controller Intercommunication. Either con-
troller (the microprocessor in PCA-A6 or the servo
controller) is capable of initiating communication
to the other at any time. To arbitrate such an in-
terface, a dual first-in-first-out (FIFO) register in
PCA-A6 is used between the two controllers.
The FIFO contains two bytes for each controller
and is clocked separately by each controller.
When data is written to the FIFO, an interrupt line
to the other controller is asserted. Servo Buffer
Full
(SBF-L)
is used to determine if the controller
has read the previous message from the FIFO,
making the interface ready for another message
from the servo controller.
A transaction in either direction is initiated by the
transmitting controller writing two bytes to its
write FIFO. (Dual FIFOs allow both controllers to
write at the same time.) This write causes the inter-
face interrupt of the receiving controller to be as-
serted. When the receiver reads both data bytes in
its read FIFO, the interrupt is deasserted, and
pointer of that FIFO is reset. This allows the
transmitter to write again into this FIFO, when
necessary.
Typically, controller PCA-A6 initiates transactions
by sending the servo controller a 2-byte command,
upper byte first.
Controller PCA-A6 initiated
request servo function commands include seek,
recalibrate, and spin up.
Request servo status
commands initiated by controller PCA-A6 include
report HDA type, report status, report seek time,
and report present track. It is also possible for the
servo controller to initiate a status message to con-
troller PCA-A6.
The servo controller always
returns a 2-byte immediate status message in
response to all servo function commands and a
2-byte detailed status message in response to a con-
troller Report Status command.
5-15
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