National Instruments GPIB-SCSI-A, Руководство пользователя

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Appendix D Operation of the SCSI
© National Instruments Corp. D-9 GPIB-SCSI-A User Manual
Control Signals
The nine control signals are RST*, ATN*, BSY*, SEL*, ACK*, REQ*,
MSG*, C/D*, and I/O*.
Handshake Lines
REQ* and ACK* form the handshake signals, which asynchronously
control the transfer of information bytes between the Initiator and Target.
The process is called a four-cycle interlock handshake and it guarantees that
the information bytes on the data lines are sent and received without a
transmission error. ACK* is asserted by the Initiator in response to the
assertion of REQ* by the Target. Similarly, ACK* is unasserted after
REQ* becomes inactive.
Phase Control Lines
MSG*, C/D*, and I/O* form the phase control signals, which the Target
uses to put the SCSI bus into specific information transfer phases. When
asserted, MSG* indicates a Message phase. When C/D* is asserted, it
indicates Control (Command or Status) information is on the SCSI data bus.
Data is specified when C/D* is unasserted. Finally, I/O* indicates the
direction of information transfer. When I/O* is asserted, the direction of
transfer is to the Initiator. When I/O* is unasserted, the direction of transfer
is from the Initiator. I/O* is also asserted by the Target during the
Reselection phase to distinguish it from the Selection phase.
Miscellaneous Control Lines
The remainder of the control signals are used for specific tasks and
indications. RST*, when active for 25
µ
sec, indicates the SCSI RESET
condition. The response to the RESET condition is device specific with
some limitations imposed by the SCSI specification.
ATN* is asserted by the Initiator to send a message to the Target. The
assertion of ATN* creates the SCSI ATTENTION condition. The Target
may respond to the ATN* signal by changing into the Message Out phase at
its convenience.