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SCSI Interface Overview
Buffered Mode
Immediate Function
Residual Length Function
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.
For Initiators that do not support the disconnect feature, the Immediate bit provides a
means of releasing the bus while the drive is busy completing a function such as
repositioning the tape. If a command is sent by the Initiator after a previous
Immediate Command was accepted, the drive continues the Immediate Function 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 the status is returned to the
Initiator as soon as the function is started.
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:
Where:
AL (Actual Length) =
TL (Transfer Length) =
RL (Residual Length) =
RL = TL - AL
Blocks transferred from the host to the tape drive across
the SCSI bus.
The Transfer Length from bytes 2-4 of the Write com-
mand (Request Transfer Length).
The amount of blocks or bytes not written to tape.
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