User's Guide

Page 36 DDS-4 SCSI Interface Manual, Rev. B
Command Descriptor Block Field Descriptions
The command descriptor block fields are described in the following table.
Field Name Byte Bits Description
Group Code 0 5-7 This field indicates the SCSI command groups used
Command Code * 0 0-4 This field indicates which of 32 command codes is sent.
NOTE: The Group Code and Command Code are used
together to indicate the operation code.
Logical Unit 1 5-7 The LUN must be set to zero.
Command
Dependent
1
2-4
0-4
0-7
See specific command.
Vendor Unique 5 6-7 These bits are reserved. In the following command sections,
these bits are shown by Xs.
Flag and Link bits 5 0-1 These bits are used by all commands and described in the
following subsection, Flag and Link Bit Descriptions.
Flag and Link Bit Descriptions
The Link bit enables a chain of commands to execute sequentially without the
initiator reselecting the target. By setting the Link bit, the Initiator desires an
automatic link to the next command on successful completion of the current
command.
The Flag bit allows the initiator to track milestones in the linked command process
by requesting a linked Command Complete (with flag) message from the drive.
If the CDB has the Link bit set, the drive completes the command and takes the
following actions:
1. Goes to the Status phase and posts Intermediate Good status.
2. Goes to the Message-In phase and posts one of two possible messages
according to the setting of the Flag bit.
3. Proceeds to the Command Out phase to request the next CDB.
Command linking continues as long as the Link bit is set in successive CDBs.
Linking stops when the drive detects an error and posts a Busy or Check Condition.
The Flag and Link bits are used by all commands. These bits are defined in the
following table.
If the
Link bit
is . . .
and the
Flag bit
is . . .
the result is . . .
1 0 The Target sends an Intermediate Good Status, then a Linked
Command Complete message, and then requests the next command.
1 1 The Target returns Immediate Status and then sends a Linked
Command Complete (with flag) message, then requests the next
command.
0 0 The Target performs normal termination and goes to Bus Free.
0 1 Invalid, the Target returns Illegal Request.