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
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.
Upon power up, the software must issue a
Recalibrate command to properly initialize all
drives and the controller.
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.
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:
PCN < NCN: Direction signal to drive set to
PCN > NCN: Direction signal to drive set to
"1" (step in) and is
"0" (step out) and i
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.
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.
Recalibrate
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.
Note that if implied seek is not enabled, the read
and write commands should be preceded by:
1) Seek command - Step to the proper track
2) Sense Interrupt Status command - Terminate
the Seek command
3) Read ID - Verify head is on proper track
4) Issue Read/Write command.
The Recalibrate command does not have a result
phase.
The Sense Interrupt Status command
The Seek command does not have a result phase.
Therefore, it is highly recommended that the
must be issued after the Recalibrate command to
effectively terminate it and to provide verification of
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