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Functional Description
7936 and 7937
operation, and the decoder is enabled to look for
the framing word in the bit stream. The framing
word is a 100010 I 00 I bit sequence, decoded such
that no single bit error will impact the proper
framing of the following data string. When fram-
ing occurs, the bit-serial VLFM data going into the
decoder will be mapped to byte-parallel data. A
clock generated in the decoder will transfer this to
the DMA/ECC electronics in PCA-A6.
Portions of the read/write system circuitry are lo-
cated
in
head-disc
assembly
A3,
read/write
PCA-A2, servo PCA-A 1, and controller PCA-A6.
See figure
[iJII.
Details of these circuits, with the
exception of those in PCA-A6, are provided in the
following paragraphs. Refer to Appendix
A
at the
rear of the manual for a description of the
PCA-A6 circuitry.
5-87.
HEAD-DISC ASSEMBLY A3
The parts of the read/write system in head-disc as-
sembly
A3
include
the
recording
medium,
read/write heads, and read preamplifier/write
driver ICs.
5-88. RECORDING MEDIUM.
The recording
medium in the drive is a sputtered thin-film metal
magnetic coating on either side of a circular
2IO-millimeter (8.3-inch) diameter aluminum sub-
strate. In the HP 7936, there are seven of these
surfaces for data and one surface for dedicated
servo code. The dedicated servo code is recorded
on the lower surface of the bottom disc in the
stack. In the HP 7937, there are 13 surfaces for
data and one surface for dedicated servo code.
Again, the dedicated servo code is recorded on the
lower surface of the bottom disc in the stack.
5-89. READ/WRITE
HEADS.
The
drive
employs Winchester technology read/write heads,
one for each data surface.
When the disc is
powered down, the actuator moves the heads to a
"landing zone" at the inner diameter of the discs
where the heads come to rest on the disc surfaces.
Each head consists of a gapped ferrite core
mounted in a ceramic shoe. There are two wind-
ings wound around the ferrite core and the
windings are connected at a common point and
5-28
phased in such a way that the common point acts
as a center tap. These windings are used for both
reading and writing by detecting or producing a
magnetic field at the gap in the ferrite core.
In a write operation, data is written by passing a
current through the windings of the selected head.
The current generates a flux field across the gap
and aligns the magnetic particles contained in the
surface of the disc. The writing process orients the
poles of each magnetized particle to permanently
store the direction of the field as the particles pass
beneath the head. The direction of the flux field is
a function of the write current direction. Erasing is
accomplished by writing over data which may
have been previously recorded on the disc.
In the read operation, as the data surface passes
beneath a head, the magnetically stored flux fields
intersect the gap in the ferrite core. Gap motion
through the flux field causes a voltage to be in-
duced into the windings wound around the core.
This induced voltage is analyzed by the read chain
to define the da ta recorded on the surf ace of the
disc. Each flux reversal, caused by a write current
polarity change, generates a readback voltage pulse.
5-90. READ PREAMPLIFIER/WRITE DRIVER
ICS. The read preamplifier/write driver ICs are
off-the-shelf units designed for disc drive head
control. Each IC can select I of 4 heads, read from
or write to the selected head, and supply a
read/write fault signal.
Since the drive has a total of seven (HP 7936) or
13 (HP 7937) read/write heads, four of the ICs are
used for head control. The ICs are multiplexed
with Chip Enable
(CEO-L
through
CE3-L)
from
the head select encoder in read/write PCA-A2.
Binary Head Select
(HS1-H, HS2-H),
also from the
head select encoder, are decoded by the chosen IC
to select the desired head.
Read and write select is controlled by Write Select
A
(WSA)
and Write Select B
(WSB)
from the write
chain in read/write PCA-A2.
(Both signals
originate at a common point within the write
chain.)
When
WSA
and
WSB
are low, read is
selected: and when
WSA
and
WSB
are above 3.5V,
write is selected.
When the IC is in the write mode, differential
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