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6 ST43401N/ND, ST43402ND User’s Manual, Rev. B
When the drive is not powered on, the heads rest on the disc surface in a
preassigned landing zone beyond the data zone. The actuator automatically
latches in this position at shutdown for moving or shipping protection. When
the drive is activated to bring the discs up to speed, the heads fly on a cushion
of air close to the disc surface.
After arriving at the data track and selecting a head, the controller locates the
portion of the track where the data is to be read or written. This is called track
orientation. Signals from the drive indicate the beginning of each track. The
controller uses these signals to determine the position of the head on the track.
When the head is above the desired track location, the controller commands
the drive to actually read or write the data. During a read operation, the drive
retrieves data from the discs, processes it, and transmits it to the controller.
During a write operation, the drive receives data from the controller, processes
it, and stores it on the discs.
The drive can recognize certain errors that may occur during its operation.
When an error is detected, it is indicated to the initiator by the SCSI controller.
Initiator
Unit
Selection
Logic
Read/Write
Logic
Head
Selection
Logic
Seek
Logic
Track
Orientation
Logic
Error
Detection
Logic
I/0 Lines
SCSI
Controller
Power Supply
Discs
Heads
Data recorded in
concentric tracks
Figure 3. Drive functional block diagram