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SCSI Start Stop Unit Command: Wikis


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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:

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]

  1. ^ Seagate Disc Drive SCSI-2/SCSI-3 Interface, Publication Number 77738479, Rev H


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