User's Guide

Page 112 DDS-4 SCSI Interface Manual, Rev. B
READ DATA BUFFER (3Ch)
The READ DATA BUFFER command returns the 4-byte Read Buffer header plus
the number of bytes specified by the allocation Length field. This command is used
in conjunction with the WRITE DATA BUFFER command as a diagnostic function
for testing the drive buffer memory and confirming the SCSI bus integrity. The tape
is not accessed during execution of this command.
READ DATA BUFFER Command Descriptor Block
The following table shows the layout of the CDB.
Byte
Bits
7 6 5 4 3 2 1 0
0 0 0 1 1 1 1 0 0
1 0 0 0 0 0 | Mode
2 Buffer ID
3 MSB—Offset
4 Offset
5 Offset—LSB
6 MSB—Allocation Length
7 Allocation Length
8 Allocation Length—LSB
9 vendor unique* 0 0 0 0 Flag Link
* This vendor-unique field is reserved and must be set to zero.
Command Descriptor Block Field Description
The following table defines the fields in the CDB.
Field
Name Bytes Bits Description
MODE 1 0-2 If MODE is 0, data is read starting at offset 0. If MODE is 3, a
maximum of four bytes of READ DATA BUFFER Header information is
returned.
Buffer ID 2 Only Buffer ID=0 is valid.
Offset 3-5 If MODE is 2, Offset is the offset where data reading begins. If MODE
is 0 or 3, Offset must equal 0.
Allocation
Length
6-8 The Allocation Length specifies the maximum number of bytes the
Initiator allocated for returned data. For Mode 0, it includes a four-byte
header followed by the drives buffer data bytes returned to the Initiator
during the Data-In Phase. For Mode 2, it is the total number of drive
buffer data returned.