Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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The first message sent by the initiator after the SELECTION phase shall be an IDENTIFY, ABORT, or BUS
DEVICE RESET message. If a target receives any other message it shall go to BUS FREE phase (unex-
pected BUS FREE).
If the first message is an IDENTIFY message, then it may be immediately followed by other messages, such
as the first of a pair of SYNCHRONOUS DATA TRANSFER REQUEST messages. If tagged queuing is used
the queue tag message immediately follows the IDENTIFY message. The IDENTIFY message establishes a
logical connection between the initiator and the specified logical unit known as an I T L nexus.
After the RESELECTION phase, the target’s first message shall be IDENTIFY. This allows the I T L nexus to
be re-established. Only one logical unit shall be identified for any connection; if a target receives a second
IDENTIFY message with a logical unit number other than zero during a connection, it shall go to BUS FREE
phase (unexpected BUS FREE).
Whenever an I T L nexus is established by an initiator that is allowing disconnection, the initiator shall ensure
that the active pointers are equal to the saved pointers for that particular logical unit. An implied restore
pointers operation shall occur as a result of a reconnection.
3.5.3
Message details
SCSI message details are defined in the following paragraphs.
3.5.3.1
One and two byte messages
Abort (06h)
The
Abort message is sent from the initiator to the target to clear the present I/O process plus any queued I/
O process for the I T L nexus. The target shall go to the BUS FREE phase following successful receipt of this
message. Pending data, status, and queued I/O processes for any other I T L nexus shall not be cleared.
If only an I T nexus has been established, the target shall go to the BUS FREE phase. No status or message
shall be sent for the I/O process and the I/O process queue shall not be affected.
It is not an error to issue this message to an I T L nexus that does not currently have an active or queued I/O
process. Transmission of this message shall terminate any extended contingent allegiance condition that
may exist between the I T L nexus.
Notes.
1.
The BUS DEVICE RESET, CLEAR QUEUE, ABORT, and ABORT TAG messages provide a means to
clear one or more I/O processes prior to normal termination. The BUS DEVICE RESET message clears
all I/O processes for all initiators on all logical units of the target. The CLEAR QUEUE message clears all
I/O processes for all initiators on the specified logical unit of the target. The ABORT message clears all I/
O processes for the selecting initiator on the specified logical unit of the target. The ABORT TAG mes-
sage clears the current I/O process only.
2.
It is permissible for an initiator to select a target that is currently disconnected for the purpose of sending
one of the above message sequences.
