User's Guide

Page 24 DDS-4 SCSI Interface Manual, Rev. B
Attention Condition
The Attention Condition allows an Initiator to inform a Target that the Initiator has a
message to send. The DAT drive may read this message at its convenience by
performing a Message-Out phase. The Initiator creates the Attention Condition by
asserting the ATN signal at any time except during the Arbitration or Bus Free
phases. The DAT drive checks to see if ATN is set at every phase change. If ATN is
set, the drive goes into the Message-Out phase. The Initiator may de-assert the
ATN signal at any time. Normally, the Initiator de-asserts ATN during or before the
last REQ/ACK handshake of the Message-Out phase. The Attention signal must be
present prior to a phase change to allow the DAT drive time to respond with a
Message-Out phase at the phase change.
Reset Condition
The Reset Condition takes precedence over all phases and conditions and is used
to immediately terminate operation and clear all SCSI devices from the bus. Any
SCSI device can create a reset condition by asserting a RST signal for a minimum
reset hold time of 25 microseconds. The DAT drive never asserts reset. During the
Reset Condition, all SCSI devices release all SCSI signals (except RST) within a
Clear Reset Delay (800 nSec) of the transition of RST to low. The Bus Free phase
always follows the Reset Condition.
When a reset is issued to the DAT drive, the SCSI bus clears all uncompleted
commands, releases all SCSI device reservations, sets the DAT drive to default
modes, and returns to the Bus Free phase.
Unit Attention Condition
The Unit Attention condition in the DAT drive typically results from the following
conditions:
• A Reset was previously issued to the DAT drive.
• The DAT drive has just been powered on.
• The cartridge was removed when the tape is positioned away from BOT.
• The cartridge was removed when the tape is positioned at BOT following a
LOAD command.
• A cartridge has been inserted since the previous bus reset or power-on.
• A log exception condition occurred.
• The mode pages have been changed
The Unit Attention Condition persists for an Initiator until the Initiator issues a
command other than Inquiry for which the DAT drive returns with a Check Condition
Status. If the next command from that Initiator following the Check Condition Status
is Request Sense, then the unit attention sense key is returned.
If the Inquiry Command is received from an Initiator with a pending Unit Attention
Condition before the DAT drive reports Check Condition Status, the DAT drive
performs the Inquiry Command and does not clear the Unit Attention Condition.