SCSI Start Stop Unit Command
Encyclopedia
The SCSI Start/Stop Unit command is used to control the motor in a rotary device such as a SCSI
disk-drive. It is also used to load or eject removable media, such as a tape
or compact disc
. It is a frequently-used command and is often the first one issued after initial communication has been established between the initiator
and the target.
The CDB
structure is:
The parameter fields in the CDB have the following meaning:
In most cases, sending a "stop motor" parameter to a disk will also park the read/write heads in the landing zone.
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
disk-drive. It is also used to load or eject removable media, such as a tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
or compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
. It is a frequently-used command and is often the first one issued after initial communication has been established between the initiator
SCSI initiator
In computer data storage, a SCSI initiator is the endpoint that initiates a SCSI session, that is, sends a SCSI command. The initiator usually does not provide any Logical Unit Numbers ....
and the target.
The CDB
SCSI CDB
In SCSI computer storage, commands are sent in a Command Descriptor Block .Each CDB can be a total of 6, 10, 12, or 16 bytes, but later versions of the SCSI standard also allow for variable-length CDBs. The CDB consists of a one byte operation code followed by some command-specific parameters.A...
structure is:
| bit→ ↓byte |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| 0 | Operation code = 1Bh | |||||||
| 1 | LUN | Reserved | Immed | |||||
| 2 | Reserved | |||||||
| 3 | Reserved | |||||||
| 4 | Reserved | LoEj | Start | |||||
| 5 | Control | |||||||
The parameter fields in the CDB have the following meaning:
- Immed - immediate - if this is 1 then the target will not wait until the operation has completed before sending back the status codeSCSI Status CodeA SCSI Status Code is used to determine the success or failure of a SCSI command. At the end of any command, the target returns a Status Code byte which should be one of the following:-02h Check condition:...
.
- LoEj (load/eject) and Start - these two bits are used together:
- 00 - Stop motor
- 01 - Start motor
- 10 - Eject media
- 11 - Load media
In most cases, sending a "stop motor" parameter to a disk will also park the read/write heads in the landing zone.