Viper 60s Theory Of Operation And Maintenance Manual page 53

VIPER SCSI 60. 125 & 150 THEORY OF OPERATION AND MAINTENANCE
4.19 Buffered Mode
The Buffered Mode allows maximum streaming operation. In this mode, the drive signals
Command Complete when all requested data for a Write of Verify Command has been
transferred from the host to the Viper buffer. This provides data to maintain streaming
while the host readies a new Write Command.
If an error occurs in writing data to the tape after the Viper drive signals Command
Complete, an error status is sent on the next Command issued.
4.20 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 reposi-
tioning 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, accepts the new command CDB, and immediately completes the new com-
mand with Busy Status.
An immediate bit of zero means the status is returned to the Initiator when the tape has
been repositioned. An Immediate bit of one means the status is returned to the Initiator as
soon as the function is started.
4.21 Residual Length Function
When performing a Write Command in the Buffered Mode, 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 unrecoverable
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. It is pOSSible for this
amount to be greater than the number of blocks specified by the command in progress at
the time of the error. For example, the following sequence of commands is issued:
In the Non-buffered Mode, the first command in a sequence of commands terminates at
the time of the error and returns a Check Condition. The Initiator then issues a Request
Sense Command and receives a Residual Length in the Request Sense Extended Sense
bytes 3-6.
4-22