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SCSI Interface Overview
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2.7.4
Buffered Mode
Buffered Mode allows the most efficient operation of a tape drive. The drive defaults
to Buffered mode. In this mode, the drive signals Command Complete when all
requested data for a WRITE command has been transferred from the host to the
tape drive buffer. This mode provides data to maintain operation while the host
readies a new WRITE Command.
If an error occurs in writing data to the tape after the tape drive signals Command
Complete, an error status is sent on the next Command issued.
2.7.5
Immediate Commands
For Initiators that do not support the disconnect feature, the immediate bit in certain
commands provides a means of returning status while the drive is busy completing a
function such as repositioning the tape. For the Parallel SCSI interface, this allows
the bus to be released. If the Initiator sends a command after a previous immediate
command was accepted, the drive continues the immediate command it is currently
performing and may disconnect, execute, or reject the new command depending on
the new command.
An immediate bit of zero means that the status is returned to the Initiator when the
operation is completed. (For example, the status is returned when the tape has been
repositioned.) An immediate bit of one means that good status is returned to the
Initiator when the command is validated.
2.7.6
Residual Length Function
When performing a WRITE command, the drive returns a Good Status and
Command Completion Message when the last byte requested by the command is
placed in the Data Buffer, rather than when it is written onto tape. If an error occurs
while data is being written onto tape, the drive calculates the Residual Length and
places this value in the information bytes of the Sense Data Block. Also, the
Residual Length functions for other commands, such as Read.
Residual Length is calculated by:
RL
=
TL
-
AL
Where:
AL
(Actual Length) =
Blocks transferred from the host to the tape drive.
TL
(Transfer Length) =
The Transfer Length from bytes 2-4 of the WRITE
command (Request Transfer Length).
RL
(Residual Length) =
The amount of blocks or bytes not written to tape.
2.7.7
Early Warning Function
Early Warning on the LTO drive is a logical warning given when 64 megabytes of
storage space remain on the tape. The position is calculated by the drive. When this
physical position is reached on a tape, the following occurs.
Summary of Contents for LTO 4
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