Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–33
B.8.15 NO OPERATION Message (08h)The NO OPERATION message allows an initiator to tell the drive to continue with
the operation it was performing before the drive communicated a need for a
MESSAGE OUT phase.
For example, if the drive does not respond to an ATN condition until a later phase,
and at that time the original message is no longer valid, the initiator can send the
NO OPERATION message when the drive enters the MESSAGE OUT phase.
B.8.16 Queue Tag Messages (20h, 21h or 22h)Queue Tag messages are a part of SCSI command queuing. They establish the
I_T_L_Q nexus with the drive. If an IDENTIFY message does not immediately
precede the Queue Tag message, the drive discards the message and goes to the BUS
FREE phase.
If a valid IDENTIFY message precedes the Queue Tag message but does not grant
the disconnect privilege, the drive fetches the Command Descriptor Block and
immediately returns BUSY status.
Queue Tag messages are two-byte SCSI messages. The message format is shown in
Figure B-3.
Bit 76543210
Byte
0Message Code (20h, 21h, or 22h)
1Queue Tag
Figure B–3 Queue Tag Message — Data Format
Command queuing is enabled or disabled by the DQue bit in the MODE SELECT
Control Mode page. If that bit has queuing disabled, the drive returns a MESSAGE
REJECT message in response to a QUEUE TAG message. Since an I_T_L_Q nexus
was not established, the I_T_L nexus remains and any commands received are
treated as untagged.
The drive supports all three SCSI-defined queue tags:
•HEAD OF QUEUE TAG
•ORDERED QUEUE TAG
•SIMPLE (SIMPLE QUEUE TAG)
The type of queue tag specified affects the execution order of the commands. The
tags specify the placement of a task in a task queue. The execution orders apply to
commands from all initiators.