SCSI Send Diagnostic Command

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The SCSI Send Diagnostic command is used to instruct a target device to perform a self-test on a specific LUN. The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 1Dh
1	SELF-TEST CODE			PF	Reserved	SelfTest	DevOfl	UnitOfl
2	Reserved
3–4	Parameter list length
5	Control

The special parameter fields in the CDB have the following meaning:

• PF - Page Format:
  • 0 - SCSI-1 compliant (vendor-specific)
  • 1 - SCSI-2 compliant (addresses a particular SCSI diagnostic page) as defined in the parameter list. In this case the Send Diagnostic command is usually followed by a Receive Diagnostic Results command.

• SelfTest if this bit is one then the device runs its default self-test. The device will then return either good status or a check condition. This version of the command is usually followed by a Receive Diagnostic Results command. If the SelfTest bit is zero then the device performs a special diagnostic operation as specified in the parameter list.

• DevOfL - Device Off-Line - used in high-availability applications - if this is one then the target is allowed to perform diagnostic operations that could cause it to fail read/write operations to the same from other initiators.

• UnitOfL - Unit Off-Line - similar to DevofL but refers to all LUNs