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

The SCSI SAC

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

7-8

Command Complete (00)

The target issues the Command Complete message in the message-in phase to indicate

to the SCSI SAC that a SCSI operation has been completed and that valid status has

been sent to the SCSI SAC. The SCSI SAC expects the target to present this message

following a status phase. When the SCSI SAC accepts this message without a SCSI

ATN signal, the target should make a transition to the bus-free phase.

Synchronous Data Transfer Request (SDTR) (01h)

The Synchronous Data Transfer Request (SDTR) message is the only message in

SCSI-2 extended-message format supported by the SCSI SAC. Each SDTR message

transfers five bytes. The target and the SCSI SAC exchange SDTR messages to

negotiate a mutually acceptable data-transfer agreement. The objective is to establish the

permissible transfer period and REQ/ACK offset to be used for data transfer in the data

phase. This agreement is valid only in the data phase.

Once an agreement is reached, it remains in effect until either a Bus Device Reset

message is received by the target, a SCSI RST condition occurs, a power off/on cycle

takes place, or the agreement is modified.

Save Data Pointer (02h)

The target issues a Save Data Pointer message in the message-in phase to direct the

SCSI SAC to copy the current data pointer to the saved data pointer for the active SCSI

operation.

The target can use this message, together with the Disconnect message and reconnect

sequence, to effectively break a long data transfer into multiple connections.

The SCSI SAC performs the Save Data Pointer operation only as directed by the target

with this message. Under no circumstances, including a disconnect, does the SCSI SAC

do an implicit Save Data Pointer operation.

Restore Pointers (03h)

The target issues the Restore Pointers message in the message-in phase to direct the

SCSI SAC to copy the saved command, data, and status pointers to the current pointers

for the active SCSI operation.

This message is used by the target to initiate the retry of either the whole command, the

data phase sequence, or the status phase sequence.

An implicit Restore Pointers operation is performed by the SCSI SAC on a legal

reconnection with the target.

Disconnect (04h)

The target issues the Disconnect message in the message-in phase to inform the SCSI

SAC that the present connection is going to be broken. When the SCSI SAC accepts this

message without a SCSI ATN signal, the target should make a transition to the bus-free