SCSI Start Stop Unit Command
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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:
| 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 code.
- 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.[1]
- ^ Seagate Disc Drive SCSI-2/SCSI-3 Interface, Publication Number 77738479, Rev H
