National Instruments GPIB-SCSI-A, Руководство пользователя

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Appendix D Operation of the SCSI
© National Instruments Corp. D-3 GPIB-SCSI-A User Manual
suspended by the Target. The I/O* signal
is active during this phase to differentiate
it from the Selection phase.
The inclusion of the Reselection phase allows a Target to
disconnect from the SCSI bus to perform a time-
consuming task relative to the last received command.
For example, a SCSI disk drive may disconnect from the
SCSI after determining that a physical head seek will be
required to process the command. The Target would then
disconnect from the SCSI, at which point an Initiator
could gain control of the SCSI to process its command or
a Target could gain control of the SCSI to finish a
previous command if it has disconnected. When the
Target disk drive finishes its head seek, it reconnects to
the SCSI to finish the command.
The remaining phases are the information transfer phases. During the
information transfer phases, the C/D*, I/O*, and MSG* SCSI signals are
used to distinguish the type of data being sent along the SCSI. The Target
is responsible for driving these signals and, therefore, controls all changes
from one phase to another.
Additionally, the Target controls the direction of the information transfer by
means of the I/O* signal. If the signal is active, the direction of the
information transfer is from the Target to the Initiator. If the signal is not
asserted, the direction of the information transfer is from the Initiator to the
Target.
The SCSI signals REQ* and ACK* form an interlocked handshake between
the Initiator and Target during the information transfer phases. ACK* is
asserted by the Initiator in response to the assertion of REQ* by the Target.
Similarly, ACK* is unasserted after REQ* becomes inactive.
Command In this phase, the Target requests command information
from the Initiator. The Target asserts the C/D* signal and
negates the I/O* and MSG* signals during the
REQ*/ACK* handshakes for this phase.