User's Guide

Page 16 DDS-4 SCSI Interface Manual, Rev. B
Command Descriptor Block
A request to a peripheral device is performed by sending a Command Descriptor
Block (CDB) to the Target. For several commands, the request is accompanied by a
list of parameters sent during a Data Out Phase. If an invalid parameter is contained
in the CDB, the drive terminates the command without altering the medium.
The Seagate DDS-4 and DAT 72 drives implementation supports Group 0 and
selected Group 1 and Group 2 commands. Group 0 CDBs are six-bytes. Group 1
and 2 CDBs are ten-bytes.
The CDB contains both reserved bit fields and defined bit fields. Defined bit fields
are: Group Code, Command Code, Logical Unit Number (LUN), Vendor Unique
(VU), Flag, and Link, whereas reserved bit fields are defined by zeros such as those
appearing on the last line of the following table. This table shows a typical Group 0,
six-byte, Command Descriptor Block.
Note: The LUN field must be 0 in the CDB. Target LUN numbers are sent in the
Identify Message.
Byte
Bits
7 6 5 4 3 2 1 0
0 Group Code Command Code
1 LUN Command Dependent
2 Command Dependent
3 Command Dependent
4 Command Dependent
5 VU VU 0 0 0 0 Flag Link
The following table shows a typical Command Descriptor Block for Group 1 and 2
commands.
Byte
Bits
7 6 5 4 3 2 1 0
0 Group Code Command Code
1 LUN Command Dependent
2 Command Dependent
3 Command Dependent
4 Command Dependent
5 Command Dependent
6 Command Dependent
7 Command Dependent
8 Command Dependent
9 VU VU 0 0 0 0 Flag Link
The following table describes the CDB fields common to all Group 0, 1, and 2
commands.
CDB Field Description