CONTROL COMMANDS
The Recalibrate command does not have a result phase.
The Sense Interrupt Status command must be issued
after the Recalibrate command to effectively terminate it
and to provide verification of the head position (PCN).
During the command phase of the recalibrate operation,
the FDC is in the BUSY state, but during the execution
phase it is in a NON-BUSY state. At this time, another
Recalibrate command may be issued, and in this manner
parallel Recalibrate operations may be done on up to four
drives at once.
Control commands differ from the other commands in that
no data transfer takes place. Three commands generate
an interrupt when complete: Read ID, Recalibrate, and
Seek. The other control commands do not generate an
interrupt.
Read ID
The Read ID command is used to find the present
position of the recording heads. The FDC stores the
values from the first ID field it is able to read into its
registers. If the FDC does not find an ID address mark
on the diskette after the second occurrence of a pulse on
the nINDEX pin, it then sets the IC code in Status
Register 0 to "01" (abnormal termination), sets the MA bit
in Status Register 1 to "1", and terminates the command.
Upon power up, the software must issue a Recalibrate
command to properly initialize all drives and the
controller.
Seek
The read/write head within the drive is moved from track
to track under the control of the Seek command. The
FDC compares the PCN, which is the current head
position, with the NCN and performs the following
operation if there is a difference:
The following commands will generate an interrupt upon
completion. They do not return any result bytes. It is
highly recommended that control commands be followed
by the Sense Interrupt Status command. Otherwise,
valuable interrupt status information will be lost.
PCN < NCN: Direction signal to drive set to "1" (step
in) and issues step pulses.
Recalibrate
PCN > NCN: Direction signal to drive set to "0" (step
out) and issues step pulses.
This command causes the read/write head within the
FDC to retract to the track 0 position. The FDC clears the
contents of the PCN counter and checks the status of the
nTR0 pin from the FDD. As long as the nTR0 pin is low,
the DIR pin remains 0 and step pulses are issued. When
the nTR0 pin goes high, the SE bit in Status Register 0 is
set to "1" and the command is terminated. If the nTR0
pin is still low after 79 step pulses have been issued, the
FDC sets the SE and the EC bits of Status Register 0 to
"1" and terminates the command. Disks capable of
handling more than 80 tracks per side may require more
than one Recalibrate command to return the head back to
physical Track 0.
The rate at which step pulses are issued is controlled by
SRT (Stepping Rate Time) in the Specify command.
After each step pulse is issued, NCN is compared against
PCN, and when NCN = PCN the SE bit in Status Register
0 is set to "1" and the command is terminated.
During the command phase of the seek or recalibrate
operation, the FDC is in the BUSY state, but during the
execution phase it is in the NON-BUSY state. At this
time, another Seek or Recalibrate command may be
issued, and in this manner, parallel seek operations may
be done on up to four drives at once.
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SMSC [ SMSC CORPORATION ]
