Sony SDX-700C, Product Specifications Manual

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5.SCSI Interface
SONY AIT-3 drive SDX-700 series Ver.2.1
5-3
1. After waiting for the bus clear time the Host arbitrates for the SCSI bus. It does this by asserting BSY and the
data line corresponding to its bus ID. If any other devices wish to compete for the bus, they also assert BSY
and the appropriate data line. Each arbitrating device then inspects the data bus. The device with the highest
ID wins the bus. All the other devices must release BSY and their data lines.
2. The Host attempts to select the target by asserting SEL and releasing BSY. The Host maintains its ID and
asserts the target’s ID on the data bus. Each target then checks the data lines. If the target’s ID matches that
on the data bus, it accepts selection by asserting BSY. Once the Host has detected BSY being asserted, it
asserts ATN to indicate that it wants the target to go to the MESSAGE OUT phase. The Host releases SEL.
3. The target now has control of the SCSI bus and it is the target which switches between phases. The target
responds to the ATTENTION condition and initiates the MESSAGE OUT phase. The Host sends an IDENTIFY
message which tells the target which logical unit the Host wishes to talk to. The fact that the target responds to
the ATN indicates to the Host that the target can accommodate more than just a COMMAND COMPLETE
message.
4. The target initiates the COMMAND phase and transfers the Command Descriptor Block from the Host. In the
COMMAND phase, the target decodes the command and either executes the command (TEST UNIT READY)
or sets itself up for a data transfer to the Host (READ, WRITE, INQUIRY etc.). The target then either switches
to the STATUS phase, if the command is complete, or if it is ready to transfer data, the DATA phase.
5. The data transfer length is set by the Host in the Command Descriptor Block. The target will remain in the
DATA phase until all the data is transferred.
6. The target then initiates a STATUS phase and transfers one byte to the Host to indicate whether the target has
successfully completed the command. If the target has detected an error, the next command that the Host is
expected to send is REQUEST SENSE. This allows the target to return further status information to the Host.
7. The target completes the SCSI sequence by going to the MESSAGE IN phase and transferring a COMMAND
COMPLETE message to the Host. The target then releases BSY allowing the bus to go to the BUS FREE
state.
5.2.2.
Disconnect
In order to improve bus usage and performance, the unit is capable of temporarily disconnecting from the bus,
consequently allowing other initiator-target communications to take place. To do so, however, the Host needs to
support Disconnect/ Re-select. The mechanism for performing the Disconnect/ Re-select procedure is implemented
as follows:
1.
The Host arbitrates for the SCSI bus and upon winning the bus it proceeds to select target device. Prior to
releasing SEL and completing the selection phase, the Host asserts the ATN line. The Host then releases SEL
and BSY to allow the target to assume control of the SCSI bus. By asserting ATN, the host indicates that the
target should go to a MESSAGE OUT phase.
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
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 phase, 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.