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5.SCSI Interface
The I/O activity from this point is controlled entirely by the target. The target initiates the COMMAND phase
4.
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.
The Disconnect process occurs when the target initiates a MESSAGE IN phase and sends a DISCONNECT
5.
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.
When doing a large data transfer, the target may choose to divide the entire transfer into several smaller
7.
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.
The Host may change Disconnect permission at any time after the SELECTION phase by asserting ATN and
8.
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-800V.
The SDX-800V supports the Synchronous Data Transfer Request Extended Message, and the Wide Data Transfer
Request Message.
Code
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
5-4
Table 5-2: Message Descriptions
Description
Direction
In
*
*
*
*
*
*
*
*
SONY AIT-3Ex drive SDX-800V series Ver.1.0
Out
*
*
*
*
*
*
*
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