Reset Condition; Contingent Allegiance Condition; Scsi Bus Phase Sequences; Nonarbitrating System - Seagate ST11200 Product Manual

Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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3.2.2

Reset condition

The Reset condition is used to immediately clear all SCSI devices from the bus. This condition shall take
precedence over all other phases and conditions. During the Reset condition, the state of all SCSI bus
signals other than RST is not defined.
The disc 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.
Disc drives that operate in accordance with this specification implement only the "hard" Reset option. Upon
detection of the Reset condition, the disc 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.
3.2.3

Contingent allegiance condition

The contingent allegiance condition shall exist following the return of CHECK CONDITION or COMMAND
TERMINATED STATUS and may optionally exist following an unexpected disconnect. The contingent alle-
giance condition shall be preserved for the I T x nexus until it is cleared. The contingent allegiance condition
shall be cleared upon the generation of a hard reset condition or by an ABORT message, a BUS DEVICE
RESET message, or any subsequent Untagged command for the I T x nexus. While the contingent alle-
giance condition exists the disc drive shall preserve the sense data for the initiator.
While the contingent allegiance condition exists, the disc drive shall respond to any other requests for access
to the logical unit from another initiator with a BUSY status. Execution of queued commands shall be sus-
pended until the Contingent Allegiance condition is cleared.
See individual drive Product manual to see if a particular disc drive supports the Contingent Allegiance
condition.
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
3.3-2. 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).
3.3.2

Arbitrating systems

For systems in which the Arbitration phase is implemented, the allowable sequences are shown in Figure
3.3-1. The normal progression is from the Bus Free phase to Arbitration, from Arbitration to Selection or
Reselection, and from Selection or Reselection to one or more of the information transfer phases (Com-
mand, Data, Status, or Message).
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