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3.0
Logical characteristics
The operations of the SCSI bus as described in this section are supported by the drive as specified in each
individual drive's Product Manual. The drive always functions as the target unless otherwise stated.
3.1
SCSI bus phases overview
The drive responds to the following phases:
BUS FREE phase
ARBITRATION phase
SELECTION phase
RESELECTION phase
COMMAND phase
Data (IN and OUT)
STATUS (IN only)
MESSAGE (IN and
OUT)
The COMMAND, DATA, STATUS, and MESSAGE phases are collectively called the information transfer
phases.
The SCSI bus can never be in more than one phase at a time. Signals that are not mentioned in a particular
context shall not be asserted.
3.1.1
BUS FREE phase
The BUS FREE phase indicates that there is no current task and that the SCSI bus is available for a physical
connection or physical reconnection. SCSI devices shall detect the BUS FREE phase after the SEL and BSY
signals are both false for at least one bus settle delay.
SCSI devices shall release all SCSI bus signals within one bus clear delay after BSY and SEL are continuously
negated (false) for one bus settle delay. If a SCSI device requires more than one bus settle delay to detect the
BUS FREE phase, it shall release all SCSI bus signals within one bus clear delay minus the excess time to
detect the BUS FREE phase. The total time to clear the SCSI bus shall not exceed one bus settle delay plus
one bus clear delay.
During normal operation a SCSI target port enters the BUS FREE phase when it releases the BSY signal.
3.1.1.1
Unexpected and expected bus free phases
In some cases a SCSI target port (connected to a SCSI initiator port) unexpectedly reverts to the BUS FREE
phase to indicate an error condition that it has no other way to handle. This is called an unexpected disconnect.
SCSI target ports shall create a BUS FREE phases after any of the following:
Parallel SCSI Interface Product Manual, Rev. A
These phases are collectively termed the Information transfer phases
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