Radio Shack TRS-80 Service Manual page 45

5-Meg Hard Disk Service Manual
the CRC device during read operations
.
The NRZ data with CRC
appended
is
then sent
to the
MFM generator device
(U5)„
MFM Generation
The conversion from NRZ write data to MFM write data takes
place
in
the MFM/Precompensation device
(U5).
This device
accepts NRZ data and
a
complimentary WCLK and produces
MFM
data and clocks by sending the data through circuitry which
decides when and where
to
write clocks on the data stream
under the
MFM encoding
rules. The proper encoding of the
data into MFM requires the device
to
apply three rules
to
the data*
1.
If
the current data cell contains
a
data
bit, no
clock bit will
be generated.
2.
If
the previous data cell contained
a
data bit, no
clock bit will
be generated.
3.
If
the previous data cell and the present data cell
are vacant,
a
clock bit will
be
produced in the
current clock
cello
The terms "data cell" and "clock cell" are defined by the
state
of WCLK.
While WCLK
is low, it is a
data cell and
while
high, it
is a
clock
cell* It
can be seen then that
both clock and data cells are one-half the period of WCLK or
100 nanoseconds. Also note that by the rules started above,
a
clock and data bit can never occur within the same WCLK
period and legal spacings
for bits can be one, one and
a
half, or two times the WCLK period
only. The rules are
implemented within the device by shift registers that hold
the next two, last, and present data bits and combinational
logic. The state
of
WCLK
is
considered and
the
appropriate
bit cells are filled and combined
on the
MFMW output line of
the device. This line
is
subject
to
decoding slivers, so it
is
run through
a
re-timing latch
(U16)
to clean
it up.
Write Precompensation
The
MFM data stream
is
now totally compatible with the
recording rules and may be sent
to
suitable line drivers
for
transmission to the drive except for one modification. Due
to the
decreasing radius on the physical surface
of
the
disk, the inside tracks have less circumference and
therefore exhibit
an
increase
in
recording flux density over
the outside tracks. This increase
in
flux density aggravates
a
problem
in
magnetic recording known
as
dynamic bit
shift.
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