2
Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
_____________________________________________________________________________________
The Seagate SCSI interface described herein consists of a 9 or 18 bit bidirectional bus (8 data + 1 parity or 16
data + 2 parity) plus 9 control signals supporting multiple initiators, disconnect/ reconnect, self configuring
host software, automatic features that relieve the host from the necessity of knowing the physical architec-
ture of the target (logical block addressing is used), and some other miscellaneous features.
The SCSI physical interface uses either single ended drivers and receivers or differential drivers and receiv-
ers and uses asynchronous or synchronous communication protocols. The bus interface transfer rate for
asynchronous or synchronous is given in individual disc drive Volume 1 Product Manuals. The bus protocol
supports multiple initiators, disconnect/reconnect, additional messages plus 6 byte and 10 byte Command
Descriptor Blocks.
Unless specified otherwise in the individual drive Product Manuals (Vol. 1), the disc drive is always a target,
and never an initiator. For certain commands, which may or may not be supported by a particular drive
model, the drive must act as an initiator, but does not otherwise do so. For purposes of this specification,
“disc drive” may be substituted for the word “target” wherever “target” appears.
GLOSSARY
Arbitration - SCSI bus phase wherein SCSI devices try to gain control of the SCSI bus to operate as an
initiator or target (see Section 3.1.2).
Byte - This term indicates an 8 bit hexadecimal construction.
Command Descriptor Block (CDB) - The structure used to communicate requests from an initiator to a
target.
Connect - The function that occurs when an initiator selects a target to start an operation.
Disconnect - The function that occurs when a target releases control of the SCSI bus, allowing it to go to the
Bus Free phase.
FRU (Field Replaceable Unit) - An assembly that is believed faulty based on test results. A value of 00h
indicates an unknown cause or the end of a list of known possible causes. Nonzero values have product
unique meanings.
Initiator - A SCSI device (usually a host system) that requests an operation to be performed by another SCSI
device.
Intermediate Status - A status code sent from a target to an initiator upon completion of each command,
except the last command, in a set of linked commands.
I/O Process - An I/O process consists of one initial connection and zero or more reconnections, all pertaining
to a single command or group of linked commands. More specifically, the connection(s) pertain to a nexus as
defined below in which one or more command descriptor blocks are usually transferred. An I/O process
begins with the establishment of a nexus. An I/O process normally ends with the BUS Free phase following
successful transfer of a COMMAND COMPLETE, ABORT, ABORT TAG, or CLEAR QUEUE message. An I/
O process also ends when a hard RESET condition occurs, an unexpected BUS FREE phase occurs, or
when the BUS FREE phase occurs following a BUS DEVICE RESET message.
I T nexus - A nexus prior to the successful receipt of an IDENTIFY message, at which time the nexus is
changed to an I T L nexus. (See glossary word “Nexus”).
I T L nexus - A nexus that exists between an initiator and a Logical Unit. This relationship replaces the prior
I T nexus. (See glossary word “Nexus”).
I T L Q nexus - A nexus between an initiator, a Logical Unit, and a queue tag following the successful receipt
of one of the QUEUE messages. This relationship replaces the prior I T L nexus. (See glossary word “Nexus”).
Logical Unit - A physical device or virtual device addressable through a target. The disc drive is a target but
also a Logical Unit.
Logical Unit Number - An encoded three bit identifier for the logical unit. The disc drive is considered Logical
Unit number zero.
LSB
-
Least significant byte
MSB - Most significant byte
ms - millisecond
LUN -
Logical unit number
mm - Millimetre
