User's Guide

SCSI Interface Description Page 25
If the Request Sense Command is received from an Initiator with a pending Unit
Attention Condition before the DAT drive reports Check Condition Status, the DAT
drive reports unit attention sense key and clears the Unit Attention Condition for that
Initiator.
Buffered Mode
Buffered Mode allows the most efficient operation of the DAT 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
DAT 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 DAT drive signals Command
Complete, an error status is sent on the next Command issued.
Immediate Function
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 returns a Busy Status for 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.
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 and SPACE.
Residual Length is calculated by: RL = TL - AL
Where :
AL (Actual Length) = Blocks transferred from the host to the DAT drive across
the SCSI bus.
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.