Logical Characteristics; Scsi Bus Phases; Bus Free Phase - Seagate ST11200 Product Manual

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3.0

Logical characteristics

The operations of the SCSI bus as described in Section 3 are supported by the disc drive, as specified in
each drive's Product Manual (Vol. 1). The disc drive always functions as the target unless otherwise stated.
3.1

SCSI bus phases

The disc drive responds to 8 distinct bus phases.

Bus Free phase

Arbitration phase
Selection phase
Reselection phase
Command Phase
Data (in and out)
Status (in only)
Message (in and out)
The SCSI Bus can never be in more than one phase at a time.
3.1.1 Bus free phase
The Bus Free phase indicates that no SCSI device is actively using the SCSI bus and it is available for
subsequent users. In some cases a target reverts to the Bus Free phase to indicate an error condition that it
has no other way to handle. This is called an unexpected disconnect.
SCSI devices shall detect the Bus Free phase after SEL and BSY are both false for at least a bus settle delay.
SCSI devices shall release all SCSI bus signals within a bus clear delay after BSY and SEL are continuously
negated for a bus settle delay. If a SCSI device requires more than a bus settle delay to detect the Bus Free
phase, it shall release all SCSI bus signals within a bus clear delay minus the excess time to detect the Bus
Free phase. The total time to clear the SCSI bus shall not exceed a bus settle delay plus a bus clear delay.
Initiators normally do not expect BUS FREE phase to begin because of the target's release of the BSY signal
except after one of the following occurrences:
(1) after a reset condition is detected.
(2) after an ABORT message is sucessfully received by a target.
(3) after a BUS DEVICE RESET message is successfully received by a target.
(4) after a DISCONNECT message is successfully transmitted from a target (see 3.5.3.1).
(5) after a COMMAND COMPLETE message is successfully transmitted from a target (see 3.5.3.1).
(6) after a RELEASE RECOVERY message is successfully received by a target.
(7) after an ABORT TAG message is successfully received by a target.
(8) after a CLEAR QUEUE message is successfully received by a target.
The BUS FREE phase may also be entered after an unsuccessful selection or reselection, although in this
case it is the release of the SEL signal rather than the release of the BSY signal that first establishes the BUS
FREE phase.
If an initiator detects the release of the BSY signal by the target at any other time, the target is indicating an
error condition to the initiator. The target may perform this transition to the BUS FREE phase independent of
the state of the ATN signal. The initiator shall manage this condition as an unsuccessful I/O process termina-
tion. The target terminates the I/O process by clearing all pending data and status information for the affected
logical unit or target routine. The target may optionally prepare sense data that may be retrieved by a RE-
QUEST SENSE command. When an initiator detects an unexpected disconnect, it is recommended that a
REQUEST SENSE command be attempted to obtain any valid sense data that may be available.
Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
These phases are collectively termed the Information transfer phases