Sony CDU-541 - Page 13

CDU-541 CD-ROM DRIVE SCSI INTERFACE MANUAL
4.1.5. ATTENTION Condition
The ATTENTION condition allows an initiator to inform the controller that
the initiator has a message ready. The controller may get this message by
performing a MESSAGE OUT phase.
The initiator creates the ATTENTION condition by asserting ATN at any time
except during the ARBITRATION or BUS FREE phases.
The initiator will assert the ATN signal before releasing ACK for the last
byte transferred in a bus phase for the ATTENTION condition to be honored
before transition to a new bus phase. An ATN asserted later might not be
honored
until a later bus phase and then may not result in the expected
action. A controller will respond with MESSAGE OUT phase as follows:
1. If ATN occurs during a COMMAND phase, MESSAGE OUT will occur after the
transfer of all command descriptor block bytes has been completed.
2. If ATN occurs during a DATA phase, MESSAGE OUT will occur at the
controller's convenience on a logical block boundary. The initiator must
continue REQ/ACK handshakes until it detects the phase change.
3. If ATN occurs during a STATUS phase, MESSAGE OUT will occur after the
status byte has been acknowledged by the initiator.
4. If ATN occurs during a MESS~GE IN phase, MESSAGE OUT phase will occur after
the current MESSAGE 1M byte has been acknowledged by the initiator.
5. If ATN occurs during a SELECTION phase and before the initiator releases
the BSY signal, MESSAGE OUT will occur immediately after that SELECTION phase.
6. If ATN occurs during a RESELECTION phase, MESSAGE OUT will occur after the
controller has successfully sent its IDENTIFY message for that RESELECTION
phase.
The initiator shall keep ATN asserted if more than one byte is to be
transferred. The initiator may negate the ATN signal at any time except it
shall not negate the ATN signal while the ACK signal is asserted during a
MESSAGE OUT phase. Normally, the initiator negates ATN while REQ is true and
ACK is false during the last REQ/ACK handshake of the MESSAGE OUT phase.
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