User's Manual

DLT4000 SCSI Interface
8-4 DLT4000/DLT4500/DLT4700 Cartridge Tape Subsystem
8.2.6 Data Cache and Tape Write Interaction
The DLT4000 drive contains a data cache that buffers blocks (records) until they are
written. This section defines specific times that blocks are written to the tape. A Mode
Select parameter allows the data cache to be disabled (unbuffered mode). In this mode,
every WRITE command will cause the data to be written to the media before the
STATUS byte and the COMMAND COMPLETE message is returned to the host.
Unbuffered mode is not recommended due to the poor performance that will result.
_________________________NOTE ____________________________
Unbuffered mode is not recommended due to the poor performance that
will result.
___________________________________________________________
The write data cache are written (flushed) to the media under the following
circumstances:
• When two or more WRITE FILEMARKS commands are issued without
intervening tape motion commands, the immediate bit is set.
• When a WRITE 0 FILEMARKS command is issued.
• Data in the cache longer than specified by the value of the Mode Parameter
"Write Delay Time" is automatically written to media.
• When a nonwrite type media access command is received (for example, SPACE,
READ, UNLOAD, . . .)
8.2.7 Other SCSI Functionality
1. Linked commands are supported.
2. Untagged queuing is supported.
8.2.8 Bus Phases
The DLT4000 drive conforms to the bus state transition table shown in the SCSI-2
standard, "Phase Sequences." The information in the following sections also applies.
8.2.9 ATTENTION Signal Response
The DLT4000 drive will respond to an ATN condition at least at every phase
transition, as long as the initiator sets the ATN bit before the target desserts the REQ
for the last byte of the previous phase. Generally ATN will be recognized immediately
by the DLT4000 drive, and it will change the bus phase to Message Out.