Seagate Ultra 160 User Manual download pdf (Page 65)

Parallel SCSI Interface Product Manual, Rev. A 51

3.0 Logical characteristics

The operations of the SCSI bus as described in this section are supported by the drive as specified in each

individual drive’s Product Manual. The drive always functions as the target unless otherwise stated.

3.1 SCSI bus phases overview

The drive responds to the following phases:

The COMMAND, DATA, STATUS, and MESSAGE phases are collectively called the information transfer

phases.

The SCSI bus can never be in more than one phase at a time. Signals that are not mentioned in a particular

context shall not be asserted.

3.1.1 BUS FREE phase

The BUS FREE phase indicates that there is no current task and that the SCSI bus is available for a physical

connection or physical reconnection. SCSI devices shall detect the BUS FREE phase after the SEL and BSY

signals are both false for at least one bus settle delay.

SCSI devices shall release all SCSI bus signals within one bus clear delay after BSY and SEL are continuously

negated (false) for one bus settle delay. If a SCSI device requires more than one bus settle delay to detect the

BUS FREE phase, it shall release all SCSI bus signals within one bus clear delay minus the excess time to

detect the BUS FREE phase. The total time to clear the SCSI bus shall not exceed one bus settle delay plus

one bus clear delay.

During normal operation a SCSI target port enters the BUS FREE phase when it releases the BSY signal.

3.1.1.1 Unexpected and expected bus free phases

In some cases a SCSI target port (connected to a SCSI initiator port) unexpectedly 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 target ports shall create a BUS FREE phases after any of the following:

BUS FREE phase

ARBITRATION phase

SELECTION phase

RESELECTION phase

COMMAND phase

Data (IN and OUT)

These phases are collectively termed the Information transfer phases

STATUS (IN only)

MESSAGE (IN and

OUT)

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Seagate Ultra 160 User Manual Page 65