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

The SCSI SAC
Open SCSI Integrator’s Manual for NonStop Servers—422988-002
7-6
Command Sequence
Command Sequence
The command sequence is initiated when the target enters the command phase. During
the command phase, the command descriptor block (CDB) is sent to the target by the
SCSI SAC.
The target is permitted to retry the command phase as described under Target-Initiated
Retry on page 7-11. Retrying the command phase by any other means is invalid. There
are other command-phase transitions that are not permitted (see Table 7-2
on page 7-4).
If the CDB transfer is unsuccessful or if the command-phase transition is invalid, the
SCSI SAC initiates the termination sequence and reports an error to the host.
Data Sequence
The data sequence is initiated when the target enters a data phase. Data-in phase is used
for sending data from the target to the SCSI SAC. Data-out phase is used for sending
data from the SCSI SAC to the target.
Data transfer is done in asynchronous or synchronous mode, depending on the
established agreement between the SCSI SAC and the target.
The SCSI SAC permits retry of the data phase and supports targets that break data
transfer into multiple connections. Interrupting and resuming the data phase by any other
means is invalid. There are other data-phase transitions that are considered invalid (see
Table 7-2
on page 7-4).
If the data-phase transition is invalid, the SCSI SAC initiates the termination sequence
and reports an error to the host. If a bus parity error occurs during the data phase
transfer, the SCSI SAC initiates the termination sequence and reports an error to the
host.
Status Sequence
The status sequence is initiated when the target enters the status phase. The status phase
normally occurs at the end of a command but in some cases might occur before the
command phase.
The SCSI SAC accepts one byte of status from the target and passes it to the host in the
SCSI-status field of the response packet. The SCSI SAC checks for and handles only
this status byte. No other status contents are decoded or interpreted by the SCSI SAC.
The target is permitted to retry the status phase as described in Target-Initiated Retry
on
page 7-11. Retrying the status phase by any other means is invalid. There are other
status-phase transitions that are not permitted (see Table 7-2 on page 7-4).
If the status-phase transition is invalid, the SCSI SAC initiates the termination sequence
and reports an error to the host.