SCSI Request Sense Command

The SCSI Request Sense command is used to obtain sense data (error information) from a target device.

The CDB structure is:

The response is a set of SCSI sense data which is structured like this:

bit→ ↓byte	7	6	5	4	3
0	Valid	Error code
1	Segment number
2	Filemark	EOM	ILI	Reserved	Sense key
3–6	Information
7	Additional sense length (n-7)
8–11	Command-specific information
12	Additional sense code
13	Additional sense code qualifier
14	Field replaceable unit code
15	SKSV	Sense-key specific
16	Sense-key specific
17	Sense-key specific
18	Additional sense bytes (variable number of bytes)

The special control fields in the sense data have the following meaning:

Valid -
- 0 - sense data are not SCSI compliant
- 1 - sense data are SCSI compliant
Error code -
- 70h - current errors
- 71h - deferred errors
- all other values are reserved
Segment number - valid only if the current command is COPY, COMPARE, or COPY AND VERIFY
Filemark - 1 = the current command has reached a filemark or setmark (sequential-access devices only)
EOM - 1 = end-of-medium (sequential-access devices) OR out-of-paper (printers)
ILI - incorrect length indicator - 1 = requested logical block length did not match the logical block length of the data on the medium
The Sense Key, Additional Sense Code, and Additional Sense Code Qualifier fields are usually considered in aggregate to be a 20 bit field called the KCQ. A composite value of 00000h indicates no error, all other values describe specific error conditions. See the main KCQ article for a complete list.
SKSV - Sense Key Specific Valid -
- 0 - sense key specific data are not SCSI compliant
- 1 - sense key specific data are SCSI compliant
Additional sense bytes - can contain vendor-specific data
- Bytes 20-21 can be used for a Unit Error Code (UEC)