Open SCSI Integrator's Manual for NonStop S-Series Servers

The SCSI SAC
Open SCSI Integrator’s Manual for NonStop Servers—422988-002
7-5
Bus-Free Phase
Bus-Free Phase
The bus-free phase is the quiescent condition that exists after a command completion,
reset, timeout, or disconnect, while the SCSI bus waits for the next command. The SCSI
bus exits the bus-free phase only in response to a selection sequence or reselection
sequence.
Selection Sequence
The selection sequence is initiated by the SCSI SAC from the bus-free phase. This
sequence consists of an arbitration phase and a selection phase.
The SCSI SAC uses the arbitration phase to gain control of the SCSI bus. If the SCSI
SAC wins bus arbitration, it enters the selection phase to select a target. Otherwise, it
waits for the bus-free phase before reentering arbitration phase. The SCSI SAC does not
time the wait for a successful arbitration. The host software should determine how long
to wait for an operation before invoking the command timeout recovery. The arbitration
phase can be terminated by a hard or soft SCSI SAC reset.
The selection sequence terminates in the bus-free phase if the SCSI SAC times out
waiting for the target to respond. The SCSI SAC waits the specified 250 milliseconds
for a BSY signal from the target during the selection phase before timing out and then
reports this condition to the host.
Except for the SCSI Bus Device Reset command, the SCSI SAC always asserts the ATN
signal before entering the selection phase. The ATN signal allows the SCSI SAC to send
an Identify message to the target.
Reselection Sequence
The reselection sequence is initiated by the target from the bus-free phase. This
sequence consists of an arbitration phase and a reselection phase. The target uses the
arbitration phase to gain control of the SCSI bus. The target then enters the reselection
phase to reconnect to the SCSI SAC.
The reselection sequence can terminate in the bus-free phase if the target elects to do so.
Otherwise, the SCSI SAC expects the target to enter the message phase to continue the
reconnection process.
Message Sequence
The message sequence is initiated when the target enters a message phase. Message-in
phase is used for sending a message from the target to the SCSI SAC. Message-out
phase is used for sending a message from the SCSI SAC to the target.
During the message sequence, the SCSI SAC transacts one message with the target. The
manner in which the SCSI SAC handles the message is described under SCSI-2
Messages Supported on page 7-7.
If the message-phase transition is invalid (see Table 7-2
on page 7-4), the SCSI SAC
initiates the termination sequence and reports an error to the host.