Reset Condition; Scsi Bus Phase Sequences; Nonarbitrating System - Seagate SCSI Interface Product Manual

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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 require-
ment, then the target shall go to Bus Free phase (see unexpected Bus Free in Section 3.1.1).
The 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 message
has been acknowledged by the initiator.
5. If ATN occurs during a Selection or Reselection phase, Message Out occurs immediately after that Selec-
tion or Reselection phase. In the Reselection case, the 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.
3.2.2

Reset condition

The Reset condition is used to immediately clear all SCSI devices from the bus. This condition shall take pre-
cedence over all other phases and conditions. During the Reset condition, the state of all SCSI bus signals
other than RST is not defined.
The drive never asserts the Reset signal.
All SCSI devices shall release all SCSI bus signals (except RST) within a bus clear delay of the transition of
RST to true. The Bus Free phase always follows the Reset condition.
Drives that operate in accordance with this specification implement only the "hard" Reset option. Upon detec-
tion of the Reset condition, the drive shall:
1. Clear all uncompleted commands,
2. Release all SCSI device reservations,
3. Return any SCSI device operating modes (Mode Select, etc.) to their default or last saved conditions.
4. Activate Unit Attention Condition for all Initiators.
3.3

SCSI bus phase sequences

The order in which phases are used on the SCSI bus follows a prescribed sequence.
In all systems, the Reset condition can abort any phase and is always followed by the Bus Free phase. Also,
any other phase can be followed by the Bus Free phase.
3.3.1

Nonarbitrating system

For systems in which the Arbitration phase is not implemented, the allowable sequences are shown in Figure 5.
The normal progression is from the Bus Free phase to Selection, and from Selection to one or more of the
information transfer phases (Command, Data, Status, or Message).
SCSI Interface Product Manual, Rev. J