Bus Phases
96122 • Revision: F Chapter 3 SCSI Bus Operations 17
Arbitration
The Arbitration phase allows an initiator (or target during reselection) to gain
control of the SCSI bus. All devices requiring use of the bus assert their SCSI IDs to
gain control. If multiple devices attempt to gain control of the bus at the same time,
the device with the highest-priority SCSI ID obtains control over the bus.
Selection
The Selection phase allows an initiator to select a target to perform some operation.
In the Selection phase, the initiator asserts both its SCSI ID and the SCSI ID of the
target being selected on the bus. This selection process informs the device that it is
being selected and identifies the initiator that is performing the selection.
Reselection
The Reselection phase allows the target to reconnect to an initiator after
disconnecting.
Information Transfer
Four Information Transfer phases transfer data or provide status over the SCSI bus:
TABLE 3-1 Information Transfer Data
Command The Command phase allows the device to request command
information from the initiator.
Data Two types of Data phases transfer data in asynchronous modes:
Data In The target sends data to the initiator.
Data Out The initiator sends data to the target.
Message There are two types of Message phases:
Message In The target sends messages to the initiator.
Message Out The target receives messages from the initiator. The target invokes
this phase when the initiator asserts the attention (ATN) signal.
Status The Status phase allows the target to send status information to
the initiator.
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