Technical data

B.3 SCSI Bus Phases
A description of the SCSI bus phases are provided in Table B–4.
Table B–4: SCSI Bus Phases
Bus Phase Description
Bus control phases:
Bus Free Indicates that no SCSI device is actively using the bus.
Arbitration Allows one SCSI device to gain control of the bus.
Selection Allows an initiator to select a target.
Reselection Allows the target to reconnect to an initiator.
Information
transfer phases:
Command Allows the target to request command information from the initiator.
Data Provides data transfer between the initiator and the target.
Status Provides status information from the target to the initiator.
Message Allows the transfer of messages between the initiator and the target.
B.4 SCSI Bus Conditions
The SCSI bus has two asynchronous conditions, the Attention (AN) condition, and the Reset
(RST) condition.
B.4.1 Attention Condition
The Attention (AN) condition allows an initiator to inform a target that the initiator has a
message ready. The target may get this message by performing a Message Out phase.
B.4.2 Reset Condition
The Reset (RST) condition is used to immediately clear all SCSI devices from the bus. This
condition takes precedence over all other phases and conditions. During the Reset condition,
the state of all SCSI bus signals other than RST is not defined.
Small Computer System Interface (SCSI) B–5