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Chapter 4 SCSI Interface

General SCSI Bus Operation

64 Quantum|ATL PowerStor L500 Series Tape Library User’s Guide

Attention Signal

Response

The library responds to an ATN condition at least at every phase

transition, as long as the initiator sets the ATN bit before the target

un-asserts the REQ for the last byte of the previous phase.

Generally ATN is recognized immediately by the library, and it

changes the bus phase to Message Out.

Status Phase

The library enters the status phase just once per command, unless

the initiator requests a retry. The only exception is during error

cases when the device goes immediately to bus free, as defined in

the SCSI-2 standard.

Possible status bytes that the library may return are as follows:

• Good (00): This status indicates the library successfully

completed the command.

• Check Condition (02): A contingent allegiance condition

occurred. The Request Sense command should be sent

following this status to determine the nature of the event.

• Busy (08): The target is busy. This status is returned whenever

the device is unable to accept a command from an otherwise

acceptable initiator. The initiator should reissue the command

at a later time.

• Intermediate Good (10h): This status is returned instead of

Good status for commands issued with the LINK bit set.

Following the return of this status, the library proceeds to the

Command phase for the transfer of the next linked command.

• Reservation Conflict (18h): This status is returned by the

library whenever a SCSI device attempts to access the library

when it has been reserved for another initiator with a Reserve

Unit command.

• Command Terminated (22h): This status is returned for a

command that was terminated by a “Terminate I/O Process”

message. This status also indicates that a contingent allegiance

condition has occurred.

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