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

The SCSI SAC
Open SCSI Integrator’s Manual for NonStop Servers—422988-002
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SCSI ATN (Attention) Signal
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A valid Command Complete message received from the target
When the target makes a transition to the bus-free phase at any other time, the SCSI
SAC treats this as a target error condition. Unexpected disconnection causes the SCSI
SAC to terminate the SCSI operation and report an error to the host.
SCSI ATN (Attention) Signal
The SCSI ATN (attention) signal is used by the SCSI SAC to request the target to enter
the message-out phase. The SCSI SAC asserts the ATN signal if it has a message to send
to the target. The SCSI SAC can assert the ATN signal at any time except during the
arbitration phase or bus-free phase.
During the selection sequence, with the exception of the SCSI Bus Device Reset
command, the SCSI SAC always selects the target with the ATN signal asserted. This
enables the SCSI SAC to send an Identify message to the target.
The SCSI SAC initiates the termination sequence by asserting the ATN signal. The SCSI
SAC does this when it detects an error affecting the SCSI operation or when it must
perform a soft reset.
SCSI RST (Reset) Signal
The SCSI RST (reset) signal is used by the SCSI SAC to implement the SCSI Bus
Device Reset command. Except for implementing this command, the SCSI SAC does
not arbitrarily use this signal, even on a hard reset.
An RST signal asserted on the SCSI bus causes a SCSI reset on the SCSI SAC.
An RST signal on the SCSI bus forces a hard reset on all targets. Previously established
conditions, including the data-transfer agreement and mode-select parameters, are
cleared by the target. The target reports this to the host by a check status, with unit
attention in the sense buffer, on the first SCSI command following the SCSI RST signal.
Host software must reestablish operational conditions with the target.