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voice-coil head positioning mechanism (actuator),
and air filtration components. Also located in disc
drive assembly A I are a microprocessor and
programmed logic array (PLA), a read/write data
channel, and circuits for actuator servo control and
spindle speed control. Power supply assembly A4 is
a self -contained switch -mode power supply which
supplies de voltages and a power-on reset signal to
the disc drive.
A typical operation of the disc drive, locate and
read, is performed as follows: The locate and read
command from the host computer enters the HnC
HP-IB interface IC, is stored in RAM, and inter-
preted by the Hnc microprocessor. The Hnc mic-
roprocessor executes code from the executive (EX-
Ec) firmware in EPROM to carry out its control
and management functions.
The exec code in
EPROM directs the nnc 0 (disc drive) code in
EPROM to execute commands necessary for com-
mand completion. The nnc 0 EPROM firmware
controls the disc drive assembly via the nC-IB bus
and the nne.
The nnc is given the seek argument, which causes
the disc controller IC in the nnc to drive the ac-
tuator servo control in disc drive assembly A 1 via
its microprocessor and PLA with a stream of pulses
which represents the seek offset argument. Once on
track, the nDC verifies the locality from informa-
tion on the data track. If the head is on the proper
track, the disc rotates until the correct sector is
read. The MFM -encoded serial data stream from
the disc passes from the read/write head through
the read preamplifier/write driver IC and the
read/write data channel to the nnc PLL IC and
disc cQntroller IC. Here the MFM data is decoded
and packed in to bytes. These bytes are sent to the
HnC RAM, previously assigned by the HnC mic-
roprocessor via the nMA gate array IC. As the
data is being assembled, it is checked, one sector at
a time, for errors. Errors are corrected in the Hnc
using information supplied by the disc controller
IC. The Hnc exec firmware coordinates the trans-
fer of data from the RAM to the host computer
via nMA gate array Ie and the HP-IB interface
IC.
The
operation continues
until
the
requested
amount of data has been successfully read, as-
sembled, checked for errors, buffered in RAM, and
sent to the host computer. When the data transfer
is complete, the nnc gives ending unit status to
Theory of Operation
7942 and 7946
the exec code. The ending status is sent to the host
computer.
A locate and write operation is similar to the lo-
cate and read operation previously described except
for the direction in which the user data flows. The
set up and head positioning operations are the
same. However, the data is accepted into the RAM
from the HP-IB interface IC prior to the seek and
verify operation. This is done to allow the data to
start moving from the RAM through the nnc disc
controller IC serializer and formatter, and the disc
drive read/write channel as quickly as possible
once it is determined that the head is in the proper
position.
Included in the Hnc firmware are diagnostic self-
test routines which exercise key functions of the
disc drive and indicate faults on a
l-digit
hexadecimal display which is visible through an
opening in the rear panel of the disc drive. Also, a
front panel FAULT/ON LINE indicator driven by
nnc PCA-Al shows the operating status of the
drive. The disc drive has dedicated maintenance
tracks where the results of some of the self tests
are logged. The internal diagnostics permit off -line
testing of the HnC, nnc, disc drive assembly, and
power supply. This furnishes a quick and easy
means of fault isolation to the unit, assembly, and
subtest level. An additional troubleshooting aid is
provided by a CS/80 External Exerciser which
links the disc drive internal diagnostics and utility
programs to service -trained personnel. The CS/ 8 0
External Exerciser can also be used to check the
HP-IB channel and the interaction of the disc
drive with the system host.
The disc drive circuitry is discussed in this section,
first at a basic block diagram level and then at a
more detailed functional block diagram level. A
description of the disc recording format is also
provided.
In order to facilitate text references to the three
sheets of the functional block diagram (figure 3- 5),
each sheet is identified by a large numeral in the
lower right - hand corner of the page. These
numerals are printed in boxed characters in the
text, for example:
III.
The mnemonics used in the functional block
diagram and accompanying text to identify analog
and digital signals are defined in table 3 -1. In the
"Source" column of table 3-1, the assembly where
3-3
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