Seagate Ultra 320 ユーザーズマニュアル

102  

                                                   Parallel SCSI Interface Product Manual, Rev. A 

get causing an unexpected bus free. The initiator shall accept such action as aborting the WDTR negotiation, 
and both SCSI devices shall go to eight-bit data transfer mode for data transfers between the two SCSI 
devices. Any prior synchronous data transfer agreement shall remain intact.

If the initiator recognizes that WDTR negotiation is required, it creates an attention condition and sends a 
WDTR message to begin the negotiating process. After successfully completing the MESSAGE OUT phase, 
the target shall respond with a WDTR message or a MESSAGE REJECT message.

If an abnormal condition prevents the target from responding with a WDTR message or with a MESSAGE 
REJECT message, then both SCSI devices shall go to eight-bit transfer mode for data transfers between the 
two SCSI devices.

Following a target's responding WDTR message, an implied agreement for wide data transfers shall not be 
considered to exist until:
a. the initiator receives the last byte of the WDTR message and parity is valid; and
b. the target does not detect an attention condition before the ACK signal is released on the last byte of the 

WDTR message.

If the initiator does not support the target’s responding Transfer Width Exponent,

 

the initiator shall create an 

attention condition and the first message shall be a MESSAGE REJECT message. If during the WDTR mes-
sage the initiator creates an attention condition and the first message of the MESSAGE OUT phase is either a 
Message Parity Error or MESSAGE REJECT message, the wide data transfers shall be considered to be 
negated by both SCSI devices. In this case, both SCSI devices shall use the eight-bit data transfer mode for 
data transfers between the two devices.

Two-byte task attribute messages are used to specify an identifier, called a tag, for a task that establishes the 
I_T_L_Q nexus. The Task Attribute messages are listed in Table 40. The Tag

 

field is an 8-bit unsigned integer 

assigned by the application client and sent to the initiator in the send SCSI command request (see ANSI SPI-4, 
T10/1365D Rev. 8,  Section 19.3.2). 

The tag for every task for each I_T_L nexus shall be uniquely assigned by the application client. There is no 
requirement for the task manager to check whether a tag is currently in use for another I_T_L nexus. If the task 
manager checks the tag value and receives a tag that is currently in use for the I_T_L nexus, then it shall abort 
all tasks for the initiator and the associated logical unit and shall return Check Condition status for the task that 
caused the overlapped tag. The sense key shall be set to Aborted Command and the additional sense code 
shall be set to Overlapped Commands Attempted with the additional sense code qualifier set to the value of 
the duplicate tag (see Section 7.3). A tag becomes available for reassignment when the task ends. The 
numeric value of a tag is arbitrary, providing there are no outstanding duplicates, and shall not affect the order 
of execution.

For each logical unit on each target, each application client has up to 256 tags to assign to tasks. Thus a target 
with eight logical units may have up to 14,336 tasks concurrently in existence if there were seven initiators on 
the bus. 

Whenever an initiator does a physical connection to a target, the appropriate task attribute message shall be 
sent following the Identify message to establish the I_T_L_Q nexus for the task. Only one I_T_L_Q nexus may 
be established during an initial connection or physical reconnection. If a task attribute message is not sent, 
then only an I_T_L nexus is established for the task (i.e., an untagged command).