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The SCSI architecture includes eight distinct phases:
BUS FREE phase
ARBITRATION phase
SELECTION phase
RESELECTION phase
COMMAND phase
DATA phases (In/Out)
STATUS phase
MESSAGE phases (In/Out)
The last four phases are called the “information transfer phases.”
The SCSI bus can never be in more than one phase at any given time. In the
following descriptions, signals that are not mentioned are not asserted.

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The BUS FREE phase indicates that there is no current I/O process and that the
SCSI bus is available for a connection.
SCSI devices detect the BUS FREE phase after the SEL and BSY signals are both
false for at least one bus settle delay.
During normal operation, the BUS FREE phase is entered when the drive releases
the BSY signal. However, the BUS FREE phase can be entered following the
release of the SEL signal after a SELECTION or RESELECTION phase time-out.
BUS FREE might be entered unexpectedly. If, for example, an internal hardware
or firmware fault makes it unsafe for the tape drive to continue operation without a
full reset (similar to a power-up reset), or if ATN is asserted or a bus parity error is
detected during non-tape data transfers.