Quantum Instruments DLT 7000 - Manual (Page 233)

SCSI Commands
Quantum DLT 7000 Tape System 5-129
READ Command (08h)
Table 5–61 READ Command Descriptor Block - Field Descriptions (continued)
Field Name Description
Fixed This bit specifies whether fixed-length or variable-length blocks are to be transferred,
and gives meaning to the Transfer Length field of the READ command.
When set to 0, variable-block mode is requested. A single block is transferred with
the Transfer Length specifying the maximum number of bytes the initiator has
allocated for the returned data.
When the Fixed bit is set to 1, the Transfer Length specifies the number of blocks to
be transferred to the initiator. This is valid only if the logical unit is currently
operating in Fixed Block mode.
When the Transfer Length is 0, no data is transferred and the current position on the
logical unit does not change. This is not an error condition.
A successful READ with Fixed bit set to 1 transfers (current block length) x (# of
blocks x block size) bytes of data to the host. Upon termination of READ, the
medium is logically positioned after the last block of data transferred (EOM).
Note that a READ command in fixed mode with an odd number of blocks returns a
CHECK CONDITION: the DLT 7000 tape drive does not support odd block number
transfers.
Filemark, End-of-Data, and End-of-Medium/Partition Handling
If the tape drive reads a Filemark, it returns a CHECK CONDITION status.
Within the sense data, the Filemark and Valid bits are set and the Sense Key
field is set to NO SENSE. The information fields contain the residue count. The
Additional Sense Code and Additional Sense Code Qualifier fields are set to
FILEMARK DETECTED. Upon termination, the medium is logically positioned
after the Filemark.
If the drive detects End-of-Data (EOD) during a READ, the drive returns a
CHECK CONDITION status. Within the sense data, the Valid bit is set and the
Sense Key field is set to BLANK CHECK. The End-of-Medium (EOM) bit may be
set if the drive determines that the tape is positioned past the PSEN marker. The
information fields contain the residue count. The Additional Sense Code
Qualifier fields are set. Upon termination, the medium is physically positioned
before EOD and after the last block on tape.