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

The SCSI SAC

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

7-10

Message Parity Error (09h)

In the message-out phase, the SCSI SAC issues the Message Parity Error message to

inform the target that the SCSI SAC has received a message with bad parity. This

provides a means for the target to return to the message-in phase to resend the message.

The SCSI SAC expects the next phase to be a message-in phase or status phase. If the

next phase is not as expected or if the retransmitted message also fails with bad parity,

the SCSI SAC initiates the termination sequence and reports an error to the host.

Bus Device Reset (0Dh)

The Bus Device Reset message, issued by the SCSI SAC in the message-out phase,

clears all I/O processes on the indicated device. When this message is issued, the SCSI

SAC expects the target to make a transition to the bus-free phase.

This message forces a reset in the selected target. The target then clears previously

established conditions, including the data-transfer agreement and the mode-select

parameters. In the first SCSI command that follows the Bus Device Reset message, the

target reports the reset to the host by a check status and designates the unit in the sense

buffer. Host software then reestablishes operational conditions with the target.

Identify (80h)

In the message-out phase, the SCSI SAC issues an Identify message to inform the target

of its disconnection privilege and to select a logical unit number (LUN) within the

target. This is the first message sent to the target following a selection sequence.

This message is used by the target in the message-in phase immediately following a

reselection sequence by the target to identify the reconnecting LUN. Bit 5 of the Identify

message (the logical unit target, or LUNTAR, bit) is not used. A maximum of one

Identify message is issued by the SCSI SAC per selection.

With the exception of a Bus Device Reset (0Dh) condition, after the selection phase or

reselection phase, the SCSI SAC always sends the target an Identify message (80h).

SCSI SAC Data-Transfer Modes

The SCSI SAC supports both synchronous and asynchronous data-transfer modes. In

synchronous mode, the maximum REQ/ACK offset is 15.

For an asynchronous data-transfer agreement, the SCSI SAC initiates negotiation with a

REQ/ACK offset of zero (SDTR (0,0)) and responds to an SDTR message from the

target with a Message Reject message.

Asynchronous data transfer is the implied agreement in either of the following cases:

•

The SDTR message is rejected by the target.

•

A REQ/ACK offset of zero is specified.

The default data-transfer agreement is asynchronous mode. This agreement is set on the

target following a Bus Device Reset message, a SCSI RST condition, or a power on/off