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5-134 Quantum DLT 7000 Tape System
READ BUFFER Command (3Ch) (continued)
Table 5–63 READ BUFFER Command Descriptor Block - Field Descriptions
Field Name Description
Mode The tape drive supports the following values within this field. If any non-supported
value is set, the drive terminates the command with a CHECK CONDITION status,
ILLEGAL REQUEST sense key set.
Mode Description
000b Combined Header and Data (see 5.14.1)
010b Data (see 5.14.2)
011b Descriptor (see 5.14.3)
Buffer ID Must be 0, 1, or 2.
Buffer 0:This buffer is intended to be used in conjunction with the WRITE BUFFER
command to provide a diagnostic capability for testing the SCSI bus and/or
hardware integrity.
Buffer 1: Choosing Buffer 1 results in the tape system transferring the contents of
SCSI RAM and EEPROM over the SCSI bus. For DLT 7000 tape systems, a total of
264K is transferred (256K for RAM, 8K for EEPROM).
Buffer 2:Choosing Buffer 2 results in the tape system transferring the contents of
cache RAM over the SCSI bus. For DLT7000 tape systems, a total of either 4 or 8
MB is transferred.
Buffer Offset Supported if Buffer ID is 2. The Buffer Offset field allows the host to specify where
the start of the data is within the buffer.
Allocation
Length
This field specifies the maximum number of bytes that the initiator has allocated
for returning data. The host uses this field to limit the size of data transfers to its
own internal buffer size.
The host should first send a READ BUFFER command, in Descriptor mode, to
determine the size of the buffer being returned. In response to the READ
BUFFER command, the tape system returns four bytes of data, three of which
contain the size of the buffer. The host can then use this data to establish the
Buffer Offset/Allocation Length fields of the CDB. Once the size of the buffer is
known, Mode 2 (Data Only, see Section 5.14.2) can be used to transfer the data
across the SCSI Bus.