National Instruments GPIB-SCSI-A, Руководство пользователя

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Chapter 7 G Mode Functions
© National Instruments Corp. 7-83 GPIB-SCSI-A User Manual
selwa - Select a SCSI Target With SCSI ATN*
Asserted
Type: SCSI Function
Syntax:
selwa
<CR>
Purpose: Use
selwa
to select a Target on the SCSI bus if the
high-level commands performed by the GPIB-SCSI-A do not
meet your needs and you must perform low-level commands
with your device. This command also alerts the Target that
the Initiator has a message byte ready for the Target by
selecting the Target with the SCSI ATN* signal active.
Remarks: This is the second command that you must use in your
low-level command pattern if the high-level commands
offered by the GPIB-SCSI-A do not work with your SCSI
device. This command performs the Selection phase for the
SCSI bus.
Because the SCSI ATN* line is asserted during the Selection
phase, this signals to the Target that the GPIB-SCSI-A as an
Initiator has some message to send to the Target. The Target
may then go into the Message Out phase and any information
transferred across the SCSI is treated as a message.
If the Target cannot respond to the ATN* signal-that is, the
Target does not support the Message Out phase, the Target
ignores the ATN* signal. If the Target does not go into the
Message Out phase and retrieve the byte from the Initiator, the
GPIB-SCSI-A continues on and does not hang.
The SCSI specification states that it is at the discretion of the
Target as to when a Message Out phase occurs. That is, the
Target does not necessarily have to go into the Message Out
phase immediately, but can wait until a later time. In order for
the high level SCSI commands offered by the GPIB-SCSI-A
to work, the Target must go into the Message Out phase
immediately after the Selection phase and before the
Command phase.