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3.2.1.1.4
Selection using attention condition timeout procedure
Two optional selection timeout procedures are specified for clearing the SCSI bus if the initiator waits a mini-
mum of one selection timeout delay and there has been no BSY signal response from the target:
• Optionally, the initiator shall assert the RST signal.
• Optionally, the initiator shall continue asserting the SEL signal and shall release the Data Bus,
DB(P_CRCA), and/or DB(P1). If the initiator has not detected the BSY signal to be true after at least one
selection abort time plus two system deskew delays, the initiator shall release the SEL signal allowing the
SCSI bus to go to the BUS FREE phase. SCSI devices shall ensure that, when responding to selection, the
selecting was still valid within one selection abort time of their assertion of the BSY signal. Failure to comply
with this requirement may result in an improper selection (for example, two targets connected to the same
initiator, wrong target connected to a SCSI initiator port or a SCSI target port connected to no initiator).
3.2.1.2
Selection without using attention condition
3.2.1.2.1
Information unit transfers disabled or enabled
The initiator shall set the Data Bus to a value that is the OR of its SCSI ID bit, the target's SCSI ID bit, and the
appropriate parity bit(s) (i.e., DB(P_CRCA), and/or DB(P1)) and it shall clear the attention condition, indicating
that an INFORMATION UNIT OUT phase is to follow the SELECTION phase.
If the arbitration was a normal arbitration, then the initiator shall wait at least two system deskew delays and
release the BSY signal. The initiator shall then wait at least one bus settle delay before attempting to detect an
assertion of the BSY signal from the target.
If QAS was used for arbitration, then the initiator shall wait at least one bus settle delay before attempting to
detect an assertion of the BSY signal from the target.
The target shall detect it is selected when the SEL signal and its SCSI ID bit are true and the BSY and I/O sig-
nals are false for at least one bus settle delay. The selected target may examine the Data Bus in order to deter-
mine the SCSI ID of the selecting initiator. The selected target shall then assert the BSY signal within one
selection abort time of its most recent detection of being selected; this is required for correct operation of the
selection time-out procedure.
The target shall not respond to a selection if bad parity is detected (see sections 3.9.2.1 and 3.9.3.1). Also, if
more or less than two SCSI ID bits are on the Data Bus, the target shall not respond to selection.
The SCSI initiator port shall wait at least two system deskew delays after detecting that the BSY signal is true.
The SCSI initiator port shall then release the SEL signal and may change the Data Bus signals. The target
shall wait until the SEL signal is false before asserting the REQ signal to enter an information transfer phase.
If information unit transfers are enabled (see Section 4.3.12) for the connecting initiator, the target shall follow
the phase sequences defined in Section 3.11.
If information unit transfers are disabled (see Section 4.3.12) for the connecting initiator, the target shall follow
the phase sequences defined in Section 3.12.
If a SCSI initiator port, when selecting without using an attention condition, detects an unexpected COMMAND
phase, it shall set its transfer agreement to the default transfer agreement and set its negotiation required flag
to true, create an attention condition, and on the corresponding MESSAGE OUT phase shall issue an ABORT
TASK message. On the next selection of the SCSI target port that received the ABORT TASK message the
SCSI initiator port shall do a selection using the attention condition and should negotiate to enable information
unit transfers.
3.2.1.2.2
Selection without using attention condition time-out procedure
Two optional selection time-out procedures are specified for clearing the SCSI bus if the initiator waits a mini-
mum of one selection time-out delay and there has been no BSY signal response from the target:
Parallel SCSI Interface Product Manual, Rev. A
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