Quantum Instruments DLT 7000 manual Read Buffer Command 3Ch

SCSI Commands

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	This field specifies the maximum number of bytes that the initiator has allocated
Length	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.

5-134 Quantum DLT 7000 Tape System