Scsi Bus Phases; Bus Free Phase - Quantum DLT 8000 Product Manual

3.5

SCSI Bus Phases

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.
3.5.1 BUS FREE Phase
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.
Quantum DLT 8000 Tape System
SCSI Description
3-11