Zenith H-100 - Page 50

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DATA READ/WRITE OPERATIONS
During a data write operation. the controller board is
enabled by the proper address and by pWR*. After the proper
control words are sent to select the proper drive. address
lines AO and A1 are made high and A2 is made low. This
connects the data register of the 1797 to the internal data
bus. As long as AO and A1 are high and A2 and FDWR are
low. the data from the S-100 bus will go to the 1797 data
register and be shifted out serially on pin 31. the write
output line. Also. on pin 31. clock pulses are inserted
between each bit.
The track and sector registers in the 1797 determine where
the data is to be written to on the disk. Whenever a sector
is filled with data. software determines the next sector
to be written to by making the AO and A1 signals equal to
o and 1. A2 equal to O. Software then writes the sector
number to the sector register and the track number to the
track register.
The 1797 translates the track nunbers into the proper step
and direction commands to the drive.
A read operation requires the board to be enabled as
described earlier. However. the I/O address decoder does
not enable the status latch because the address provided
by the CPU is not correct for a status read from the latch.
Instead. the address lines causes the 1797 to dump the bits
in its data register onto the internal data bus of the
H-207. which connects to the U36 buffer and the S-100 bus.
The 1797 fills its data register from the data shift
register. which fills serially from the RAWREAD data stream
at U22-27. See "Data Separation and Precompensation" for
a discussion on RAWREAD data processing.