HP 7942 Service Manual page 34

Theory of Opera tion
7942 and 7946
aid is provided by a CS/8 0 External Exerciser
which links the drive internal diagnostics and
utility programs to service -trained personnel. The
external exerciser can also be used to check the
HP-IB channel and the interaction of the drive
with the system host.
Refer to the following source in this publication
for a detailed description of the tape drive theory
of operation.
• Tape Drive Service Manual, Section III,
Troubleshooting.
3-2. DISC DRIVE INTRODUCTION
The disc drive assemblies are medium performance,
random access, mass storage devices intended for
use with small and medium sized computers. The
formatted storage capacities of the disc drive as-
semblies are 24 megabytes and S S megabytes,
respectively. In this section, "disc drive" refers to
both disc drive assemblies, unless otherwise
specified.
The head-disc module in the disc drive is sealed
and uses two or four 130-millimetre (S.12-inch)
diameter nonremovable discs. One read/write head
is used for each disc surface. The bottom surface
of the lowest disc in the stack contains continuous
prerecorded servo data which is used to ensure the
precise positioning of the read/write heads.
The disc drive employs a peripheral mass storage
control structure which consists of a host depen-
dent controller (HDC) and a device dependent con-
troller (DDC). See figure 3 - 4. In this structure, the
common functions of typical mass storage devices
(disc drive, tape drive) are consolidated in the HDC
and drive dependent tasks are performed by the
DDC. The common functions of the HDC include:
host interface, direct memory access (DMA), error
correction, control for DDC execution of com-
mands, status monitoring, and diagnostic self -test
routines. Drive dependent tasks performed by the
DDC involve manipulation of the data stream
necessary to read or write data on the disc. These
opera tions incl ude serializa tion/ deserializa tion,
modified frequency modulation (MFM) en-
coding/decoding, DMA handshaking, and error
detection.
3-2
Associated with the DDC is a disc drive assembly
which contains a sealed head-disc module and
electronic control and read/write circuits. These
circuits operate in conjunction with the circuitry
in the DDC to interpret and generate control sig-
nals, position and maintain the read/write heads
over the desired track, maintain precise disc rota-
tional speed, and perform read and write opera-
tions.
The HDC is capable of interfacing with a second
mass storage device, such as a cartridge tape drive,
fitted with a suitable DOC. This facilitates, with a
minimum of additional electronic circuitry, the
connection of the cartridge tape drive as a back-
up device.
The HDC communicates with the host computer
over the Hewlett-Packard Interface Bus (HP-IB)
and with the ODC's over the mass storage struc-
ture's Data/Control Interface Bus (DC-IB). The
DC-IB contains an 8-bit read/write data bus, and
a completely independent 8-bit control/status data
bus with associated 6-bit control/status address
bus. The DDC communicates with the disc drive
assembly via the data and control lines of the
ST-S06 Interface. The ST-506 Interface is an
electrical and mechanical standard for disc drives
established by the Shugart Corporation and adap-
ted as an industry-wide standard.
The disc drive consists of host dependent con troller
printed -circuit assembly PCA - A 5, device depen-
dent controller PCA - A2, disc drive assembly AI,
and power supply assembly A4. Disc drive assemb-
ly A 1 and power supply assembly A4 are mounted
side by side at the bottom of the cabinet and PCA's
A2 and AS are mounted horizontally above them.
The four assemblies are connected together by
cable assemblies.
Circui ts on the host de penden t con troller PCA - AS
include an HP-IB interface integrated circuit (IC),
a microprocessor, firmware in erasable program-
mabIe read -only memory (EPROM), random -access
read/write memory (RAM), a custom -designed
DMA gate array IC, self -test switches and display.
Device dependent controller PCA - A2 includes a
disc controller IC and a phase-locked loop (PLL)
IC. Disc drive assembly A 1 contains a sealed head-
disc module with spindle motor, discs, read/write
(R/W) heads, a servo head, read preamplifier/write
driver IC's, a servo preamplifier IC, a rotary