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5.SCSI Interface
5-4 SONY AIT-5 drive SDX-1100V series Ver.1.0
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
message back to the Host. This message may be preceded by a SAVE DATA POINTERS message byte if
the Disconnect 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 Re-selection. When ready, the target re-selects the Host and
sends an Identify message via a MESSAGE IN phases, indicating the specific logical unit with which the
subsequent communications 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 message implies that the Host restores its data pointers upon re-selection; 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.
5.3. Message Specification
This clause includes all SCSI messages. Both supported and non-supported messages are listed. Elements of this
clause come from clause 5, Logical Characteristics, of the SCSI standard. The message system allows
communication between an initiator and a target for the purpose of physical path management. The table below
lists the Messages supported by the SDX-1100V.
The SDX-1100V supports the Synchronous Data Transfer Request Extended Message, and the Wide Data
Transfer Request Message.
Table 5-2: Message Descriptions
Code Description Direction
In Out
00 COMMAND COMPLETE *
01 EXTENDED MESSAGE * *
02 SAVE DATA POINTER *
03 RESTORE POINTERS *
04 DISCONNECT *
05 INITIATOR DETECTED ERROR
06 ABORT *
07 MESSAGE REJECT *
08 NO OP * *
09 MESSAGE PARITY ERROR *
0C BUS DEVICE RESET *
23 IGNORE WIDE RESIDUE *
80-CO IDENTIFY * *