9.3 Attention Condition
The attention condition allows an initiator to inform the file that a MESSAGE OUT phase is desired. The
initiator may create the attention condition by asserting the ATN signal at any time except during the
ARBITRATION or BUS FREE phases.
The initiator must create the attention condition by asserting the ATN signal least two deskew delays before
releasing ACK for the last byte transferred in a bus phase to guarantee that the attention condition will be
honored before transition to a new bus phase. This will guarantee a predictable file response to message
received during the MESSAGE OUT phase for this attention condition. If the ATN signal is asserted later,
it might be honored in the current bus phase or the next bus phase and then may not result in the expected
action.
After the initiator asserts the ATN signal, the file will respond with the MESSAGE OUT phase as follows:
Current Phase Response
COMMAND Message Out phase will occur after part or all of the Command Descriptor Block has
been transferred to the file. The initiator must continue REQ/ACK handshakes
during the Command phase until the file enters the MESSAGE OUT phase.
DATA The MESSAGE OUT phase will occur after part or all of the data bytes have been
transferred and not necessarily on a logical block boundary. The initiator must con-
tinue REQ/ACK handshakes (asynchronous transfer) until it detects the phase
change.
Note: In synchronous transfer, the initiator must continue sending ACK pulses to
reach an offset of zero.
STATUS The MESSAGE OUT phase will occur after the REQ/ACK handshake of the status
byte has been completed.
MESSAGE IN The MESSAGE OUT phase will occur before the file sends another message.
SELECTION If ATN occurs during a SELECTION phase and before the initiator releases the BSY
signal, the MESSAGE OUT phase will occur immediately after that SELECTION
phase.
RESELECTION The MESSAGE OUT phase will occur after the file has sent its IDENTIFY message
for that RESELECTION phase. (First the file tries to complete the reselection.)
The initiator must keep the ATN signal asserted if more than one message byte is to be transferred during
the MESSAGE Out phase. The file will process each message byte (multiple-bytes for an extended message)
prior to receive the next message from the initiator. The file will continue to handshake and process byte(s)
in the MESSAGE OUT phase until ATN goes false unless one of the following condition occurs:
1. The file receives an illegal or inappropriate message and goes to the MESSAGE IN phase to send a
MESSAGE REJECT message.
2. The file detects a catastrophic error condition and goes to the BUS FREE phase.
9.4 SCSI Bus Related Error Handling Protocol
This protocol is used to handle error that threaten the integrity of a connection between the Target and an
Initiator.
SCSI MESSAGE SYSTEM 175
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