Seagate Ultra 320 User Manual download pdf (Page 72)

58 Parallel SCSI Interface Product Manual, Rev. A

(a) Optionally, the initiator shall assert the RST signal (see Section 5.3);

(b) Optionally, the initiator shall continue asserting the SEL signal and shall release the Data Bus,

DB(P_CRCA), 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 selection was still valid within one selection abort time of their assertion of the BSY sig-

nal. Failure to comply with this requirement may result in an improper selection (for example, two tar-

gets connected to the same initiator, wrong target connected to a SCSI initiator port, or a SCSI target

port connected to no initiator).

3.3 RESELECTION phase

3.3.1 RESELECTION phase overview

The RESELECTION phase allows a SCSI target port to physically reconnect to a SCSI initiator port for the pur-

pose of continuing some operation that was previously started by the initiator but was suspended by the target

(i.e., the target physically disconnected by allowing a BUS FREE phase to occur or issued a QAS REQUEST

message before the operation was complete). During the RESELECTION phase, the I/O signal is asserted to

distinguish this phase from the SELECTION phase.

Refer to Section 3.4 for a description of the fairness algorithm which applies during SELECTION and RESE-

LECTION phases.

3.3.2 Physical reconnection

The SCSI device that won a normal arbitration has both the BSY and SEL signals asserted and has delayed at

least a bus clear delay plus one bus settle delay before ending the normal ARBITRATION phase.

The SCSI device that won a QAS has the SEL signal asserted and has delayed at least a QAS arbitration

delay before ending the QAS phase.

The SCSI device that won the arbitration identifies itself as a SCSI target port by asserting the I/O signal.

The winning SCSI device shall also set the Data Bus to a value that is the logical OR of its SCSI ID bit and the

initiator’s SCSI ID bit and the appropriate parity bit(s) [i.e., DB(P_CRCA), and/or DB(P1)].

If the arbitration was a normal arbitration, then the target shall wait at least two system deskew delays and

release the BSY signal. The target shall then wait at least one bus settle delay before attempting to detect an

assertion of the BSY signal by the initiator.

If QAS was used for arbitration, then the target shall wait at least a bus settle delay before attempting to detect

an assertion of the BSY signal from the initiator.

The initiator shall be physically reconnected when the SEL and I/O signals and its SCSI ID bit are true and the

BSY signal is false for at least one bus settle delay. The physically reconnected initiator may examine the Data

Bus in order to determine the SCSI ID of the physically reconnected target. The physical reconnected initiator

shall then assert the BSY signal within one selection abort time of its most recent detection of being physically

reconnected; this is required for correct operation of the timeout procedure.

The initiator shall not respond to a physical reconnection if bad parity is detected (see sections 3.9.2.1 and

3.9.3.1). Also, if more than or less than two SCSI ID bits are on the Data Bus, the initiator shall not respond to

a physical reconnection.

After the target detects the assertion of the BSY signal, it shall also assert the BSY signal and wait at least two

system deskew delays and then release the SEL signal. The target may then change the I/O signal and the

Data Bus. After the physically reconnected initiator detects the SEL signal is false, it shall release the BSY sig

nal. The target shall continue asserting the BSY signal until it relinquishes the SCSI bus.

Note. When the target is asserting the BSY signal, a transmission line phenomenon known as a wired-OR

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