User's Guide

SCSI Interface Description Page 29
When reading in variable mode, the host must know the size of the block to be read
from the tape in advance in order to avoid causing the DAT drive to return a Check
Condition with Incorrect Length indicated in the Sense data (ILI). Also, the data
transfer may be truncated (cut off) when the recorded block does not match the
transfer length in variable mode or the current block size in fixed mode.
The SCSI-2 READ command includes a SILI bit to Suppress ILI Check conditions.
When the SILI bit is set, the host usually specifies the maximum block size before
reading so that the data blocks are not truncated, and no Check Conditions are
generated.
The SCSI READ BLOCK LIMITS command returns the minimum and maximum
block sizes that the DAT drive can support. The Block Limits data is not modified to
reflect the current mode of writing—fixed or variable. The Block Limits returned data
is not modified to reflect the current block size for the next fixed-mode WRITE. The
MODE SENSE command is used for that purpose.
Autoloader Operation
General SCSI Information
The Scorpion 240 is, in SCSI-3 terminology, a tape drive with an independent
medium changer: SCSI Stream Device, i.e., tape, commands are addressed to
Logical Unit Number 0 and SCSI Medium Changer (SMC) commands to LUN 1.
SCSI Primary commands (SPC) may be addressed to either LUN.
The Standard Inquiry Data of LUN 0 reports a Peripheral Device Type of 01h
(sequential access device) and a MCHNGR bit of 0. LUN 1 reports a Peripheral
Device Type of 08h (medium changer device).
(By contrast, Scorpion 240 is not a SCSI-3 attached medium changer device. These
devices have only logical unit 0, and all commands are addressed to that LUN. Their
Inquiry data has a Peripheral Device type of 01h and MCHNGR bit of 1.)
Sense data is maintained for each Initiator-Target nexus; in other words, while the
Request Sense command may be addressed to either LUN, the same data is
reported for each LUN.
SCSI Medium Changer Commands
Element Addresses
SMC commands control the process of moving cassettes (volumes) between the
magazine and the drive. They refer to the components of the Autoloader by element
addresses. (Element addresses are not to be confused with SCSI ID or LUN.)
• The medium transport element (MTE) is the elevator, which actually moves the
cassettes; its element address is 0. The MTE may not be either the source or
destination when a cassette is moved.
• The data transfer element (DTE) is the DAT drive itself. Its address is 1.
• The storage elements (SE) are the slots in the magazine. Slot 1 has element
address 2, slot 2 has address 3, etc. through address 7 for slot 6.