Servo System - HP 7936 Support Manual

Both of the controllers interface to the drive
electronics via a standard interface (ESI). This in-
terface consists of a Disc Data Bus, a Data Path
Control Bus, and a Servo Control Bus. Each bus is
a bidirectional parallel 8-bit bus with associated
control and handshake lines. The mnemonics
which identify the bus signals are listed below.
Refer to table 5-1 for a description of the signal
functions.
DDO-DD7
DDS-H
SEF-L
SOS-L
WHO-L
DISC DATA BUS
Disc Data Bus
Disc Data Strobe
Start ECC Field
Start of Sector
Write Hold Off
DATA PATH CONTROL BUS
DPCO-DPC7
DPAO-H
DPCS-L
DPIRQ-L
DPRHW-L
Data Path Control Bus
Da ta Path Address Zero
Data Path Control Select
Data Path Interrupt Request
Data Path Read High, Write Low
SERVO CONTROL BUS
SDO-SD7
SBF-L
SDAV-L
SPR-L
SRD-L
SWR-L
Servo Data Bus
Servo Buffer Full
Servo Data Available
Servo Processor Reset
Servo Read
Servo Write
The controller also supplies drive signals for the
three LEDs
in
LED PCA-A 7 that indicate the
operating status of the drive. These signals are
labeled Red LED (RLED), Green LED (GLED), and
Yellow LED (YLED). The signals reach PCA-A 7 via
read/write peA-A2, signal jumper cable W7,
power distribution PCA-A5, and power supply
output cable
\\"4.
See figure
~.
The +5V supply for the LEDs, labeled LED Power
(LEDP), originates in read/write PCA-A2, and is
connected to PCA-A
7
via the same path as the
LED drive signals.
5-12.
Functional Description
7936 and 7937
SERVO SYSTEM
Control of the servo system is based on dedicated
servo code (prerecorded on the single servo sur-
face) and embedded servo code (prerecorded on all
of the data surfaces between data sectors). The
servo system follows the specified track while
reading or writing and when needed, initiates and
controls seeks between cylinders. In addition, the
servo system is responsible for determining the
type of HDA being addressed (7936 or 7937),
monitoring the spindle speed, reporting current
servo system status, and interpreting incoming
commands from controller PCA-A6.
The servo system makes use of an off -the-shelf
microcomputer integrated circuit (servo controller)
to provide an interface between the servo system
and controller PCA-A6. See figure 5-16. In
response to commands passed over the Servo
Control Bus, the servo controller interprets incom-
ing commands from PCA-A6 (report HDA type,
spin up, recalibrate, seek, report status, report seek
time, report present track) and reports servo system
information. The servo controller also reports
faults in the servo system.
The dedicated servo circuit takes the differential
output signal from the servo preamplifier IC, as
processed by the dedicated servo AGC amplifier in
read/write PCA-A2, and filters the signal to
eliminate high-frequency noise. The dedicated ser-
vo circuit provides the input to the servo system
phase-locked loop (PLL) which supplies the
reference timing inf orma tion needed for writing.
The PLL and associated timing circuits also supply
timing information needed for position discrimina-
tion to allow for the determination of track seek-
ing information during seeking. The dedicated
servo circuit also generates an automatic gain con-
trol (AGC) voltage which is fed back to the dedi-
cated servo AGC amplifier in read/write PCA-A2.
The AGC reference for the dedicated track fol-
lower is adjusted by a read-only memory (PROM)
and digital-to-analog converter (DAC) on the HDA
to compensate for servo gain variations across the
disc surface.
The sampled servo circuit takes the differential
embedded servo code, as read from the selected
read/write head and amplified by the sampled ser-
vo AGC amplifier in read/write PCA-A2, to keep
the head on track. The sampled servo amplifier is
5-11