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The SELECTION phase allows an initiator to select a drive to initiate a drive
function.
The SCSI device that won the arbitration has both the BSY and SEL signals
asserted and has delayed at least one bus clear delay plus one bus settle delay
before ending the ARBITRATION phase. The SCSI device that won the arbitration
becomes an initiator by not asserting the I/O signal.
During SELECTION, the I/O signal is negated so that this phase can be
distinguished from the RESELECTION phase.
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1. Sets the DATA BUS to the OR of its SCSI ID bit and the drive’s SCSI
ID bit.
2. Asserts the ATN signal (signaling that a MESSAGE OUT phase is to
follow the SELECTION phase).
3. Waits at least two deskew delays.
4. Releases the BSY signal.
5. Waits at least one bus settle delay.
6. Looks for a response from the drive.
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7. Determines that it is selected when the SEL signal and its SCSI ID bit
are true and the BSY and I/O signals are false for at least one bus settle
delay.
8. Can examine the DATA BUS to determine the SCSI ID of the
selecting initiator.
9. Asserts the BSY signal within a selection abort time of its most recent
detection of being selected (this is required for correct operation of the
selection time-out procedure).
• The drive does not respond to a selection if bad parity is
detected. Also, if more than two SCSI ID bits are on the DATA
BUS, the drive does not respond to selection.