Seagate Ultra 320 - Manual (Page 125)

Parallel SCSI Interface Product Manual, Rev. A 111
5.6 Command processing considerations and exception conditions
5.6.1 Command processing considerations and exception conditions overview
The following subclauses describe some aspects of command processing, including exception conditions and
error handling that are specific to this standard.
5.6.2 Asynchronous event notification
Notification of an asynchronous event is performed using the SEND command with the AER bit set to one. The
information identifying the condition being reported shall be returned during the data out delivery phase of the
SEND command (see SCSI Primary Commands-4 standard).
An error condition or unit attention condition shall be reported once per occurrence of the event causing it. The
target may choose to use an asynchronous event notification or to return CHECK CONDITION status on a
subsequent command, but not both. Notification of command-related error conditions shall be sent only to the
initiator that requested the task.
The asynchronous event notification protocol may be used to notify processor devices that a system resource
has become available. If a SCSI target port chooses to use this method, the sense key in the sense data sent
to the processor device shall be set to Unit Attention.
The asynchronous event notification protocol shall be used only with SCSI devices that return processor
device type with an AERC bit of one in response to an INQUIRY command. The INQUIRY command should be
sent to logical unit zero of each SCSI device responding to selection. This procedure shall be conducted prior
to the first asynchronous event notification and shall be repeated whenever the SCSI device deems it appropri-
ate or when an event occurs that may invalidate the current information. (See SYNCHRONOUS DATA
TRANSFER REQUEST message, Section 4.3.16, for examples of these events.)
Each SCSI device that returns processor device type with an AERC bit of one shall be issued a TEST UNIT
READY command to determine that the SCSI device is ready to receive an asynchronous event notification. A
SCSI device returning CHECK CONDITION status is issued a REQUEST SENSE command. This clears any
pending unit attention condition. A SCSI device that returns processor device type with an AERC bit of one and
returns Good status when issued a TEST UNIT READY command shall accept a SEND command with an
AER bit of one.
Note. A SCSI device that uses asynchronous event notification at initialization time should provide means to
defeat these notifications. This may be done with a switch or jumper wire. SCSI devices that imple-
ment saved parameters may alternatively save the asynchronous event notification permissions either
on a per SCSI device basis or as a system wide option. In any case, a SCSI device conducts a survey
with INQUIRY commands to be sure that the SCSI devices on the SCSI bus are appropriate destina-
tions for SEND commands with an AER bit of one. (The SCSI devices on the bus or the SCSI ID
assignments may have changed.)
See asynchronous event reporting in the SCSI Architecture Model-4 standard for more information on asyn-
chronous event notification.
This feature is not likely supported at this time. Check individual drive’s Product Manual, in features supported
table to see if it is supported by a particular drive model.
5.6.3 Incorrect initiator connection
An incorrect initiator connection occurs during an initial connection if a SCSI initiator port creates a nexus that
already exists and does not send an ABORT TASK SET, ABORT TASK, TARGET RESET, CLEAR TASK SET,
DISCONNECT, or LOGICAL UNIT RESET message as one of the messages of the MESSAGE OUT phase or
as one of the task management flags in the SPI command information unit.