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Functional Description
5-1.
INTRODUCTION
The HP 7936 and HP 7937 Disc Drives are high
performance, random access, data storage devices
designed for use in medium-and large-sized com-
puter systems.
In this chapter, unless otherwise
specified, "driv,e" refers to all models of the HP
7936 and HP 7937.
The drive is housed in a rigid enclosure designed to
be mounted on rack slides in a cabinet. Contained
within the enclosure is a sealed head-disc assembly
(HDA), with spindle and disc stack, read/write
heads, servo head, electromagnetic head positioning
mechanism (actuator), read preamplifiers, write
drivers, head gain reference PROM and DAC, and
air filtration system. See figure 5-16. The spindle is
belt driven by an ac induction motor mounted ad-
jacent to the sealed HOA. The motor speed is regu-
lated by the line frequency. This motor also drives
a squirrel-cage type cooling fan.
The disc stack is composed of seven 210-millimetre
(S.3-inch) diameter nonremovable discs. These dis-
cs have seven (HP 7936) or 13 (HP 7937) thin-film
magnetic surfaces for data storage.
(The same
number of discs are used in both drives in order to
maintain the dynamic design parameters of the
head-disc
assembly.)
The
formatted
storage
capacity of
th(~
drive is 307 megabytes (HP 7936)
or 571 megabytes (HP 7937). One movable head is
used to service each surface. The bottom surface
of
the
lowest
disc
in
the
stack
contains
prerecorded dedicated servo code which is llsed to
generate timing and seeking information. When on
track, embedded servo code prerecorded between
data sectors on all of the data tracks is sampled to
keep the heads on track.
In addition to head-disc assembly A3. a number of
printed-circuit assemblies (PCAs) are contained
within the enclosure. These include servo PCA-AI,
read/write PCA-A2, power supply PCA-A4, power
distribution peA-AS, controller PCA-A6, LED
PCA-A 7, and primary power PCA-AS.
Servo
PCA-AI
contains
a
servo
controller,
phase-locked loop (PLL) and servo timing counters.
5
dedicated servo circuit, sampled (embedded) servo
circuit, and servo power amplifier. These circuits
provide sector timing information, control seeks,
and maintain the heads on track.
Read/write PCA-A2 includes a custom-designed
read/write controller that, together with associated
read chain, ·write chain, and head select electronics,
transfers data to and from the disc media. The
PCA also maintains data communication with con-
troller PCA-A6, and performs amplification of the
sampled and dedicated servo codes.
Power supply PCA-A4, in addition to supplying
DC
voltages
for
the
controller
and
drive
electronics, provides power-on reset and power fail
signals.
Power distribution PCA-A5 filters and distributes
the outputs from power supply PCA-A4 to the
controller and drive electronics.
Controller
PCA-A6
is
a
factory-installed
microprocessor-based controller which provides an
interface to the host computer.
Two different
HP-IB controllers are available for this purpose. In
the HP 7936H and HP 7937H Disc Drives, inter-
face to the host is via an Hewlett-Packard
Interface Bus (HP-IB) controller.
In the HP
7936XP and HP 7937XP, the controller has HP-IB
Cache added. (In this manual, the controllers are
identified collectively as PCA-A6.) Both controll-
ers interface with the drive electronics via an
HP-developed standard interface (ESI).
Self -test diagnostics and fault-finding firmware
contained within the controller PCA-A6 exercise
key functions of drive operation. Self test is per-
formed automatically at power on and can also be
initiated by the host. Drive status is indicated by
three LEOs mounted on the front panel. In the
event of a self-test failure, this LED display iden-
tifies the most likely failed assembly within the
drive.
LED PCA-A 7 contains three light-emitting diodes
(LEOs) that signal to the user the operating status
of the drive.
5-1
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