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

The SCSI SAC

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

7-3

Supported Bus-Phase Transitions

Table 7-2 lists all allowable bus-phase transitions supported by the SCSI SAC. The

leftmost column of this table lists the SCSI SAC bus phases that wait for a target bus-

phase transition. The column headings list all possible SCSI SAC bus-phase transitions.

The individual cells of Table 7-2

indicate whether a given transition is allowed (OK),

whether it would represent an error condition (Error), or, in certain cases, the conditions

that would be necessary for the transition to take place. For example, “~ABT & ~BDR”

in a cell means that the transition is allowed only if the message sent is neither Abort

(~ABT) nor Bus Device Reset (~BDR). Similarly, WR OP or RD OP in a cell means

that the indicated transition can take place only during a write or read operation,

respectively.

Note that the SCSI SAC can enter the selection and reselection phases only from the

bus-free phase, and that the termination sequence is initiated by the SCSI SAC.

If an unexpected phase transition occurs or if any error is detected by the SCSI SAC

while the SCSI operation is active, the SCSI SAC enters the termination sequence

before reporting the error to the host. The SCSI SAC does not wait for the completion of

the termination sequence before reporting an error to the host. The SCSI operation might

not be properly terminated at this point. A SCSI bus-hung condition could occur if the

target does not respond. It is the responsibility of the host to make sure the SCSI

operation is terminated and, if needed, to recover from the bus-hung condition.

Figure 7-1. Typical SCSI SAC Bus-Phase Sequence

Bus-Free

Phase

Message-Out

Phase

Command

Phase

Selection

Phase

Data

Phase

Status

Phase

Message-In

Phase

Bus-Free

Phase

011CDT .CDD