Viper 60s Theory Of Operation And Maintenance Manual page 52

INTERFACE
4.16 Attention Condition
The Attention Condition allows an Initiator to inform a Target the Initiator has a message
to be sent. The Viper 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 Viper drive
checks to see if ATTN is set at every phase change. If ATTN is set, the drive goes into the
message phase. The Initiator may deassert the ATN signal at any time. Normally, the
Initiator deasserts A TN 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 Viper
drive time to respond with a Message-Out phase at the phase change.
4.17 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!l5ec. The Viper 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 Viper drive, the SCSI bus clears all uncompleted commands,
releases all SCSI device reservations, sets the Viper drive to default modes, and returns to
the Bus Free phase.
4.18 Unit Attention Condition
The Unit Attention condition in the Viper drive results from the following conditions:
• A Reset was previously issued to the Viper drive.
• The Viper drive has just been powered on.
• The tape cartridge was removed when the tape is positioned away from BOT.
• The tape cartridge was removed when the tape is positioned at BOT following a
Prevent or a Load command.
The Unit Attention Condition perSists for each Initiator until that Initiator issues a
command other than Inquiry for which the Viper 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 Viper drive reports Check Condition Status, the Viper drive per-
forms the Inquiry Command and does not clear the Unit Attention Condition.
If the Request Sense Command is received from an Initiator with a pending Unit Attention
Condition before the Viper drive reports Check Condition Status, the Viper dtive reports
unit attention sense key and clears the Unit Attention Condition for that Initiator.
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