Quantum 10K II - Manual (Page 344)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–23
B.6 SCSI Bus Conditions
The SCSI bus has two asynchronous conditions: Attention and Reset.
B.6.1 Attention Condition
The attention condition informs a drive that an initiator has a message ready. The
drive gets the message by performing a MESSAGE OUT phase. The attention
condition requires the following timing:
• The initiator creates the attention condition by asserting ATN at any time
except during the ARBITRATION or BUS FREE phases.
• The initiator negates the ATN signal at least two deskew delays before asserting
the ACK signal while transferring the last byte of the message.
If the drive detects that the initiator failed to meet this requirement, then the
drive goes to BUS FREE.
• Before transition to a new bus phase, the initiator asserts the ATN signal, then
waits at least two deskew delays before negating the ACK signal for the last byte
transferred in the current bus phase.
• Asserting the ATN signal later cannot be honored until a later bus phase and
then cannot result in the expected action.
A drive responds with a MESSAGE OUT phase as described in Table B-9.
Table B–9 Drive MESSAGE OUT Phase Response
ATN Signal True in Phase... The Drive Enters MESSAGE OUT…
COMMAND After transferring part or all of the command descriptor
block bytes.
DATA At the drive’s earliest convenience (often on a logical block
boundary). The initiator continues REQ/ACK handshakes
until it detects the phase change.
STATUS After the status byte has been acknowledged by the
initiator.
MESSAGE IN Before it sends another message. This permits a MESSAGE
PARITY ERROR message from the initiator to be associated
with the appropriate message.
SELECTION Immediately after that SELECTION phase and before the
initiator releases BSY
RESELECTION After the drive has sent its IDENTIFY message for that
RESELECTION phase. The initiator should only assert the
ATN signal during a RESELECTION phase to transmit a
TARGET RESET or DISCONNECT message.