Features of the SCSI Interface
The Ultrium Tape Drive’s SCSI parallel interface features the following:
vLUN identification
vBus parity errors
vDisconnect strategy
vMessages
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
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
