Optical Drive and Library SCSI-2 Command Reference

SCSI Command Overview
Drive Control Commands
Chapter 11-4
Data phase
The data phase encompasses both the “data in” phase and the “data out”
phase to transfer data to and from the drive. Synchronous and
asynchronous data transfers are supported.
The data in phase allows the target to request that data be sent to the
initiator from the target.
The data out phase allows the target to request that data be sent from
the initiator to the target.
Command phase
The command phase sends the Command Descriptor Block (CDB) from
the initiator to the drive.
Message phase
The message phase refers to a message in or a message out phase.
Multiple messages may be sent during either phase. The first byte
transferred in either of these phases is either a single-byte message or
the first byte of a multiple-byte message. Multiple-byte messages are
wholly contained within a single message phase.
Message In phase allows the target to request that messages be sent to
the initiator from the target.
Message Out phase allows the target to request that messages be sent
from the initiator to the target. The target invokes this phase in response
to the attention condition created by the initiator.
See Table 1-1 on page 1-5 for a list of supported messages.