Seagate Ultra 160, Ultra 320 manual Task attribute message codes, ACA Auto Contingent Allegiance

When a target does a physical reconnection to an initiator to continue a tagged task, the Simple Queue mes- sage shall be sent following the Identify message to resume the I_T_L_Q nexus for the task. Only one I_T_L_Q nexus may occur during a physical reconnection. If the Simple Tag message is not sent, then only an I_T_L nexus occurs for the task (i.e., an untagged command).

If a target attempts to do a physical reconnection using an invalid tag, then the initiator should create an atten- tion condition. After the corresponding MESSAGE OUT phase, the initiator shall respond with an Abort Task message.

If a target does not implement tagged queuing and a queue tag message is received, the target shall switch to a Message In phase with a MESSAGE REJECT message and accept the task as if it were untagged, provided there are no outstanding untagged tasks from that initiator.

See Section 7.0 of this manual and the ANSI SCSI Architecture Model-2 standard for the task set management rules.

Table 40:	Task attribute message codes
				Support
Code	IU Transfers									Clear attention
	Disabled				IU Transfers Enabled		Message name	Direction		condition
	Iniator	Target			Initiator	Target
24h	O		O		N/A	N/A	ACA		Out	Not required
21h	Q		Q		N/A	N/A	Head of Queue		Out	Not required
22h	Q		Q		N/A	N/A	Ordered		Out	Not required
20h	Q		Q		N/A	N/A	Simple	In	Out	Not required
Key:
M = Mandatory support			O = Optional support				Q = Mandatory if tagged queuing is implemented
In = Target to initiator			Out = Initiator to target

Yes = Initiator shall clear the attention condition before last ACK of the MESSAGE OUT phase.

Not required = Initiator may or may not clear the attention condition before last ACK of the MESSAGE OUT phase (see Section 5.1).

N/A = Not applicable

4.4.2ACA (AUTO CONTINGENT ALLEGIANCE)

See Table 41 for the format of the ACA message.

Table 41: ACA message format

Bit	7	6	5	4	3	2	1	0
Byte
0				MESSAGE CODE (24h)
1				TAG (00h-FFh)

The ACA message specifies that the task shall be placed in the task set as an ACA task. The rules used by the task manager to handle ACA tasks within a task set are defined in Section 7.0 of this manual and the SCSI Architecture Model-2 standard.

Parallel SCSI Interface Product Manual, Rev. A 103