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Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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3.2
SCSI bus conditions
The SCSI bus has three asynchronous conditions; the Attention condition, the Reset condition and the Con-
tingent Allegiance condition. These conditions cause the SCSI device to perform certain actions and can alter
the phase sequence.
3.2.1
Attention condition
The Attention condition allows an initiator to inform a target that the initiator has a message ready. The target
gets this message at its convenience 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.
The initiator shall assert the ATN signal two deskew delays before negating the ACK signal for the last byte
transferred in a bus phase for the attention condition to be honored before transition to a new bus phase.
Asserting the ATN signal later might not be honored until a later bus phase and then may not result in the
expected action. The initiator shall negate the ATN signal two deskew delays before asserting the ACK signal
while transferring the last byte of the message. If the target detects that the initiator failed to meet this
requirement, then the target shall go to BUS FREE phase (see unexpected BUS FREE, 3.1.1).
The disc drive responds with MESSAGE OUT phase as follows:
1.
If ATN occurs during a Data phase, Message Out occurs at a convenient time. It may not occur until
several logical blocks after ATN is first asserted.
2.
If ATN occurs during a Command phase, Message Out occurs after transfer of all Command Descriptor
Block bytes has been completed.
3.
If ATN occurs during a Status phase, Message Out occurs after the status byte has been acknowledged
by the initiator.
4.
If ATN occurs during a Message In Phase, Message Out occurs after the last byte of the current mes-
sage has been acknowledged by the initiator.
5.
If ATN occurs during a Selection or Reselection phase, Message Out occurs immediately after that
Selection or Reselection phase. In the Reselection case, the disc drive enters the Message Out phase
after it has 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 while the ACK signal is asserted during a Message Out phase. Recommended
practice is that the initiator negates ATN while REQ is true and ACK is false during the last REQ/ACK hand-
shake of the Message Out phase.
