User's Guide

Page 80 DDS-4 SCSI Interface Manual, Rev. B
DC Control Page Field Descriptions
The following table describes the fields in the DC Control page.
Field Name Bytes Bits Description
DCC 2 6 The DCC (Data Compression Capable) bit indicates that the drive is
capable of data compression. This bit is always 1. Note that data
control compression may be disabled by the switches on the drive.
DCE 2 7 The DCE (Data Compression Enable) bit controls data
compression. If the value is 1, the drive compresses data received
from the host during a WRITE command before it writes the data to
tape in the DDS-DC format.
If the value is 0, the drive does not compress data sent during a
WRITE command, and the host data is written to tape in the
uncompressed DDS format.
DDE 3 7 The DDE (Data Decompression Enable) bit is for control of data
decompression; however, for either value, the drive decompresses
data that has been compressed on the tape before it sends the data
to the host during a READ command.
RED 3 5-6 The RED field specifies when Check Conditions are reported to the
host when reading and the compressed format on the tape changes.
Only RED = 0 is supported. Other mode settings will be accepted,
but will function identically as RED = 0.
Compression
Algorithm
4-7 The Compression Algorithm byte allows the host to specify the
algorithm that is to be used to compress data. If the drive does not
support the algorithm specified in the Compression Algorithm bytes, a
Check Condition is returned with the Sense Key set to Illegal Request.
Decompressi
on Algorithm
8-11 The Decompression Algorithm byte allows the host to specify
the algorithm that is to be used to decompress data. If the
drive does not support the algorithm specified in the
Decompression Algorithm bytes, a Check Condition is returned
with the Sense Key set to Illegal Request.
The following table shows the DCE bit configuration and the supported algorithms.
An algorithm value of 01, which is the default, can be used to determine the
supported algorithm for the drive, DCLZ (20). The value of 20 is returned by the
corresponding MODE SENSE command.
DCE Algorithm Description
0 XX Compression is disabled.
1 00 Compression is disabled.
1 01 Compression is enabled using the default algorithm (DCLZ).
1 02-1F Illegal Request.
1 20 Compression is enabled using the DCLZ algorithm.
1 21-FF Illegal Request.
Device Configuration Page (10h)
The Device Configuration page specifies the appropriate sequential access device
configuration. The following table illustrates this page.
Byte
Bits
7 6 5 4 3 2 1 0