Logical Characteristics; Scsi Bus Phases; Bus Free Phase; Arbitration Phase - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
3.0

Logical characteristics

The operations of the SCSI bus as described in Section 3.0 are supported by the drive, as specified in each
individual drive's Product Manual, Volume 1. The drive always functions as the target unless otherwise stated.
3.1

SCSI bus phases

The 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 subse-
quent 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 successfully 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 Section 3.5.3.1).
5. after a Command Complete message is successfully transmitted from a target (see Section 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 termination.
The target terminates the I/O process by clearing all pending data and status information for the affected logi-
cal unit or target routine. The target may optionally prepare sense data that may be retrieved by a Request
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.
3.1.2 Arbitration phase
The Arbitration phase allows one SCSI device to gain control of the SCSI bus so that it can assume the role of
an initiator or target. The drive arbitrates for the bus as a target implementing reselection or when performing
AEN (if AEN is implemented). The drive supports arbitration by multiple SCSI devices.
These phases are collectively termed the Information transfer phases
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