User's Guide

SCSI Interface Description Page 17
Group Code* This field indicates which of eight possible SCSI command groups is specified.
Seagate drives support Group 0.
Command
Code*
This field indicates which of 32 possible command codes for a particular group
code is specified.
Logical Unit The LUN must be set to zero.
Vendor Unique When set, these bits select vendor unique functions in specified commands.
Flag bit The Flag bit is used only in conjunction with the Link bit and must be set to zero if
the Link bit is zero. When the Link bit is set, the value of the Flag bit determines the
appropriate message to send to the Initiator when a linked command completes
successfully. A 0 value indicates that the Linked Command Complete message is
required. A 1 value indicates that the Linked Command Complete with Flag
message is required. Typically, the Flag bit is used to cause an interrupt in the
Initiator at the end of, or at logical intervals in, linked command processing.
Link bit** The Link bit is used to indicate that the Initiator desires automatic linking to the next
command on successful completion of the current command. When the Link bit is
one, on successful termination of the command, the drive returns an Intermediate
Status followed by one of the two Command Complete messages as determined by
the Flag bit. (Refer to description of Flag bit above.)
* Together, group code and command code make up the op code.
** If the Link bit is used, all applicable commands must have the Immediate bit set to zero or a Check
Condition status is returned and Extended Sense Key is set to Illegal Request.
Logical Unit Support
The Seagate DDS-4 and DAT 72 drive only supports Logical Unit (LUN) 0, except
for the Autoloader, which supports both LUN 0 and LUN 1. However, it responds to
a host command that tries to select or identify any other LUN. The response of the
drive to illegal LUNs varies depending on the command and the manner in which the
host specifies the LUN.
The host can specify an LUN I only by sending an Identify message after the
Selection Phase
Because of the redundancy of specifying the LUN, the Seagate DDS-4 and DAT 72
drive responds appropriately if the LUN is specified in both the Identify message and
in the CDB. If the drive is selected and a valid Identify message is received with
LUN = 0 (or LUN = 1 if the drive is an Autoloader), the LUN field of the CDB is
ignored. Thus, the Identify message overrides the CDB LUN specification.
Because the SCSI INQUIRY command determines what LUNs a particular SCSI
device supports, the response from the drive to this command is unique. With the
INQUIRY command when the drive receives an unsupported LUN, the command
completes normally. The drive indicates that it does not support the specified LUN
by returning a 7F hexadecimal value in byte 0 of the INQUIRY data. Regardless of
what method is used to specify the LUN, the 7F hex value is returned.
If an unsupported LUN is specified for a REQUEST SENSE command, the
command completes normally with Good Status, and the sense data is set to Illegal
Request.
For all other commands, the illegal LUN is detected in the Identify message. If an
illegal LUN is specified in the Identify message, the drive enters the Command
Phase and accepts the CDB. It immediately skips to the Status Phase and posts a
Check Condition. The Sense data is set to Illegal Request. If the Identify message is
correct, the CDB LUN is ignored.