Features of the SCSI Interface
The Ultrium Tape Drive’s SCSI parallel interface features the following:
v LUN identification
v Bus parity errors
v Disconnect strategy
v Messages
LUN Identification
Identify messages are used to identify the LUN to which an initiator is connecting
and to identify which LUN is reconnecting to an initiator. These are required. The
LUN field in SCSI-2 commands is not used.
Bus Parity Errors
On detecting a bus parity error during a Command or Data Out phase or receiving
an Initiator Detected Error message during a Data In or Status phase, the drive
attempts to retry the Bus phase. A Restore Pointers message is sent to the initiator
and the transfer is repeated. Only one retry is attempted for any given burst. If the
retry fails or the Restore Pointers message is rejected by an Initiator Detected Error,
Message Reject, or Message Parity message, then the drive goes to the Status
phase and attempts to report Check Condition status. If this fails with an Initiator
Detected Error message, the drive goes to Bus Free. The Sense Key is set to
Aborted Command and the ASC/ASCQ is set to Command Phase Error (4A00) (if
the error was in the Command phase) or to Data Phase Error (4B00) (if the error
was in the Data phase). If the error was in the Status phase, the sense data
remains as that from the command.
If an Initiator Detected Error or Message Parity Error message is received during
the Message In phase, the initiator has detected an error in the message. The drive
goes to Message In and resends the message that was in error. If the subsequent
message is rejected with an Initiator Detected Error, then the drive goes to the
Status phase and sends Check Condition status. The Sense Key is set to Aborted
Command and the ASC/ASCQ is set to Initiator Detected Error (4800). If the
subsequent message is rejected with a Message Parity Error, then the drive goes to
the Status phase and sends Check Condition status. The Sense Key is set to
Aborted Command and the ASC/ASCQ is set to Message Error (4300).
On detecting a bus parity error during a Message Out phase, the drive handshakes
in all message bytes until ATN is deasserted. It then stays in the Message Out
phase to indicate to the initiator that the whole Message Out phase should be
resent.
Disconnect Strategy
The disconnect strategy used by the drive is based on the assumption of
maximizing bus utilization for large sequential data transfers from a large data
buffer. The drive disconnects whenever it believes that it can provide better bus
utilization. This may be between Command and Data phases, between bursts of
data, or before sending status. However, the drive guarantees that it sends the
configured maximum burst size or the remaining data in the transfer in any single
Data phase burst if the maximum burst size has been set to a value other than 0.
Chapter 5. Attachment Features 97
