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

The SCSI SAC

Open SCSI Integrator’s Manual for NonStop Servers—422988-002

7-2

SCSI SAC Bus-Phase Sequences

The SCSI SAC supports all SCSI-2 bus phases. Table 7-1 shows how SCSI SAC bus-

phase sequences correspond to the bus phases described in the SCSI-2 specification.

A typical control flow for a SCSI access command is shown in Figure 7-1

. After the

bus-free phase, the SCSI SAC initiates the selection phase to start the SCSI operation.

Normal completion of the SCSI operation occurs when the target returns from the

message-in phase to the bus-free phase after sending status and a Command Complete

message.

Transitions between bus-phase sequences are normally controlled by the target. The

SCSI SAC, however, can alter certain phase sequences by asserting the ATN or RST

signal on the bus. The ATN signal is used by the SCSI SAC to request a transition to the

message-out phase. The RST signal forces a transition to the bus-free phase.

Table 7-1. SCSI-2 Bus Phases and Corresponding SCSI SAC Bus-Phase

Sequences

SCSI-2 Bus Phase Corresponding SCSI SAC Bus-Phase Sequence

Bus-free Bus-free

Arbitration Selection

Selection

Arbitration Reselection

Reselection

Message (in/out) Message (in/out)

Command Command

Data (in/out) Data (in/out)

Status Status

Error condition occurring in any bus

phase except bus-free and arbitration

Termination; SCSI SAC reports errors to host