Radio Shack TRS-80 Service Manual page 43

5-Meg Hard Disk
Service Manual
Parallel-to-Serial device
(U7)
will have its
tri-state
drivers disabled*
Serial Data Generation
The Controller records data on the disk
in
MFM
format* In
order to produce the proper data format, the Controller uses
several specialized devices
to
process the parallel data
supplied by the host into
a
serial MFM data
stream* The data
supplied by the host
is
temporarily stored
in
the buffer
RAM
until the correct sector
is
located for the
data
to be
written*
The process of writing
is
essentially the opposite of
reading except that the data separator circuitry
is
not
required and the generation of the
MFM data stream
is
produced
by synchronous clocking techniques*
The functional sections of
the serial data
generation
section are listed below:
1*
Parallel-to-Serial conversion
(U7)
2*
CRC generation
(U6)
3*
MFM
and precompensation
(U5)
Parallel
to
Serial Conversion
Parallel data
is
converted into
a
serial NRZ data stream by
the
parallel-to-serial device
(U7)*
The processor enables
this
conversion
by lowering the WRITE* signal on MAC
CNTRL
(U29)*
WRITE* causes the tri-state buffers present on the
parallel-to-serial device
to
become active, supplying the
CRC
device with data, clocks,
and BDONE strobes*
The
processor presents parallel data on the
10
bus along
with the WR4* write strobe which latches the data into the
parallel port on the BDONE* Inside the parallel-to-serial
device, the parallel latches are loaded into
a
serial shift
register on every eighth WCLK transition*
As the data
is
transferred
to the
shift registers, the BDONE status flag
is
set*
The processor reads this flag through U61 to determine
when
to
write the next parallel byte
to the device* The
timing of the
parallel accesses is at
a
rate one-eighth that
the bit rate of the NRZ data stream*
The
output
of
the last register in the shift string
is
brought out
of
the
device
as a
NRZ serial data stream* The
shifting clock
is
also brought out
as
SHFTCLK
to be used as
the clock for the CRC device*
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