Sony DDS-4 Product Description Manual page 41

5. SCSI INTERFACE
2. At this point, the target responds to the Host ATTENTION condition by initiating a MESSAGE OUT phase and
receiving a message from the Host which tells it whether or not the Host allows Disconnects for the desired logical
unit on the target.
3. If the Host does not send an Identify message, the target assumes that Disconnects are not permitted.
4. The I/O activity from this point is controlled entirely by the target. The target initiates the COMMAND phase and
reads in the Command Descriptor Block (CDB) from the Host. If the Host has sent an Identify message with the
Disconnect permission bit set, then the target will immediately disconnect from the bus after the entire CDB has
been received.
5. The Disconnect process occurs when the target initiates a MESSAGE IN phase and sends a DISCONNECT mes-
sage back to the Host. This message may be preceded by a SAVE DATA POINTERS message byte if the Discon-
nect is performed in the middle of the DATA phase. Following the MESSAGE IN phase, the target frees the bus by
releasing the BSY.
6. The host now waits for the target to perform Reselection. When ready, the target reselects the Host and sends an
Identify message via a MESSAGE IN phase, indicating the specific logical unit with which the subsequent commu-
nications are to take place. Usually, the target will proceed from the MESSAGE IN phase to either a DATA or a
STATUS phase. If the Host had been previously told to SAVE DATA POINTERS, then the target's Identify mes-
sage implies that the Host restores its data pointers upon reselection; in other words, it is not necessary for the target
to send a specific RESTORE DATA POINTERS to the Host in such context.
7. When doing a large data transfer, the target may choose to divide the entire transfer into several smaller transfers.
This allows the target to free the SCSI bus whenever it needs to perform its own system tasks not immediately
related to the actual data transfer over the SCSI bus. The target will not lock the bus into the DATA phase if it is not
presently ready to efficiently execute the transfer.
After the data transfer is completed, the target enters a STATUS phase and sends a single status byte to the Host. It
then proceeds to a MESSAGE IN phase, during which the target sends a COMMAND COMPLETE message.
Finally, the target frees the bus and the current command is considered completed.
8. The Host may change Disconnect permission at any time after the SELECTION phase by asserting ATN and
sending an Identify message via a MESSAGE OUT phase. If the Host, however, performs this action during a
DATA phase, it is possible that the target will not notice the change in Disconnect permission status until past the
end of the current bus phase.
5 - 4 SONY SDT-10000/SDT-11000 DDS Tape Drive