Read Buffer - HP C2488A Technical Reference Manual

Read Buffer
The READ BUFFER command is used in conjunction with the WRITE BUFFER command
as a diagnostic tool for testing Target memory and the SCSI bus integrity. This command
does not alter the media or the buffer. The data returned by the READ BUFFER command
contains a 4-byte header, followed by the buffer data.
Read Buffer Command Descriptor Block (COB)
Bit
Byte
7
I
6
I
5
I
4
I
3
I
2
I
1
I
0
00 Opcode = 3CH
01 Logical Unit Number
I
Reserved
1
Mode
02
Buffer ID = 0
03-05 (MSB=03) Buffer Offset = 0 (LSB=05)
06-08
(MSB=06)
Allocation Length (LSB=08)
09 Control Byte
Mode. Modes 0 (OOOb) and 3 (OUb) are supported. Mode 0 returns a 4-byte header followed
by the data bytes, and Mode 3 returns only the 4-byte header.
Buffer ID. This field is not supported and should be set to zero (0).
Buffer Offset. This field is not supported and should be set to zero (0).
Allocation Length. This field specifies the number of bytes that the initiator has allocated for
returned data. An allocation length of zero specifies that no data be transferred and is not
considered an error. The target will terminate the Data Phase when the specified number of
bytes or when all available buffer data has been transferred, whichever is less.
Read Buffer Header Format
Bit
Byte
7
I
6
I
5
I
4
I
3
1
2
I
1
I
0
00 Reserved
01-03 (MSB=Ol) A vailable Length (LSB=03)
Available Length. This field may contain up to 262,144 bytes (1 's-based 256K). An Available
Length of zero indicates that no data transfer will take place. If the Allocation Length of
the CDB is too small to allow all of the Available Length, the Available Length field is NOT
adjusted to reflect the truncation.
The data in the buffer may have been altered since the last WRITE BUFFER command. It is
recommended that the Target be placed in reserve or that the WRITE BUFFER command
and subsequent READ BUFFER command be linked to ensure that the initiator can reliably
test the Target's data buffer.
If any command has been executed by the controller between the execution of the WRITE
BUFFER command and the READ BUFFER command, a status of CHECK CONDITION
will be returned. In this case, the sense information wi11 have a Sense Key of MISCOMPARE
set. The amount of requested buffer data will be returned regardless of the MISCOMPARE
error status, but the contents should be suspect.
A-32 SCSI Command Descriptions DRAFT
1/18/93 09:58