Chapter 7
G Mode Functions
© National Instruments Corp.
7-9
GPIB-SCSI-A User Manual
cmd - Build SCSI Command Descriptor Block and
Execute SCSI Command Phase
Type:
SCSI Function
Syntax:
cmd command byte[, command bytes]
<CR>
Purpose:
Use
cmd
to build the Command Descriptor Block for your
SCSI device if the high-level commands provided with the
GPIB-SCSI-A do not perform well with your device. After
being built,
cmd
transfers the Command Descriptor Block to a
SCSI Target during the Command phase.
Remarks:
Use this command in your low-level command sequence to
tell the selected Target what to do if the high-level commands
offered by the GPIB-SCSI-A do not work with your SCSI
device. This command performs the Command phase for the
SCSI bus.
command
byte(s)
indicates the bytes to go into the
Command Descriptor Block. The
command
byte(s)
go
into the Command Descriptor Block in the order that you enter
them – that is, if you enter #h3 as the first
command
byte
,
the first byte in the Command Descriptor Block will be a 3.
You must enter at least one
command
byte
. Any more are
optional. The maximum number of
command
bytes
that
can be entered are 25. This limit does not impose problems as
the SCSI specification currently indicates a maximum
Command Descriptor Block size of 12 bytes. The
GPIB-SCSI-A accepts more bytes for future expansion and for
SCSI devices that use vendor uniquely determined Command
Descriptor Block lengths greater than 12 bytes.
This command assumes that the SCSI is in a proper state to
accept command data. That is, this command presumes that
you have already issued the
getscsi
command to arbitrate
for the SCSI followed by the
selwa
or
selwo
command to
select the desired Target. This command then attempts to give
the Command Descriptor Block to the selected SCSI device.
