| SCSI Revision Levels
SCSI - Small Computer System Interface
An intelligent peripheral
interface characterized by its use of high level communication between devices.
Communications are defined as being between an "Initiator" and a
"Target". The Initiator is normally a computer, and the Target is normally a
peripheral. Data may be transferred in asynchronous (not clocked) or synchronous (clocked)
mode. All messages and commands are always transferred in asynchronous mode.
This term is often used to
describe the published ANSI standard now called SCSI-1 (X3.131-1986).
SCSI-2
This is a term describing
the published ANSI standard (X3.131-1994). SCSI-2 was an upgrade from the original SCSI
interface. Changes included faster data rates and mandated message and command structure
to improve compatibility. Synchronous data transfer rate for SCSI-2 is 2.5 to 10
Mbytes/sec for an 8-bit data bus (N/ND models), and 5.0 to 20 Mbytes/sec for a 16-bit data
bus (W/WC/WD/DC models).
Click here for a PDF version of the Interface
Manual.
SCSI-3
This term describes a set
of related ANSI standards that are currently being developed for the SCSI bus. The SCSI-2
document is very large (400+ pages) and covers the full range of topics. SCSI-3 split this
large document into a series of smaller documents, each covering a "layer" of
the interface definition.
The basic layers are:
physical
(connectors, pin assignments, electrical specifications)
protocol
(physical layer activity is organized into bus phases, packets, etc.)
architecture
(a description of how command requests are organized, queued, and responded to by any
protocol)
primary
commands (description of commands that must be supported by all SCSI devices)
device
specific commands (commands that are specific to a particular class of devices; CD-ROMs or
-WORM drives, for example)
The set of standards
needed to do a SCSI-3 parallel interface disc drive implementation are:
- SPI (SCSI Parallel
Interface) for the physical layer
- SIP (SCSI Interlocked
Protocol) for the protocol layer
- SAM (SCSI Architecture
Model) for the architecture
- SPC (SCSI Primary Commands)
for the primary command set
- SBC (SCSI Block Commands)
for the disc drive specific command set
The SCSI-3 standards are
layered in this manner to allow substitution of parts of the structure as new technology
emerges. For example, a comparable set of standards for a SCSI Fiber Channel interface
disc drive replaces the physical and protocol layers with new documents but uses the same
documents for the other 3 layers. The main point to remember here is that the terms SCSI-2
or SCSI-3 do not imply any particular performance per se, rather they refer to the
generation of documents to which a product conforms. Since the newest features are only in
SCSI-3 and tend to be higher performing, SCSI-3 devices should demonstrate better
performance than SCSI-2 in most cases.
Click here for a PDF version of the Interface
Manual.
SCSI
FAST
This refers to timings
defined in SCSI-2 for a 10 MegaTransfer/sec transfer rate. A "MegaTransfer" (MT)
is a unit of measure referring to the rate of signals on the interface regardless of the
width of the bus. For example, a 10 MT/sec rate on a 1 byte wide (narrow) bus results in a
10 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in a 20 Mbytes/sec
transfer rate.
SCSI
FAST-20
This refers to timings
defined in SCSI-3 SPI for a 20 MT/sec transfer rate, which achieves data rates twice as
fast as SCSI FAST rates. For example, a 20 MT/sec rate on a 1 byte wide (narrow) bus
results in a 20 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in a 40
Mbytes/sec transfer rate.
SCSI
FAST-40
This refers to timings
being defined for a future revision of the SCSI-3 SPI that achieve 40 MT/sec, which is
twice as fast as SCSI FAST-20 rates. For example, a 40 MT/sec rate on a 1 byte wide
(narrow) bus results in a 40 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it
results in an 80 Mbytes/sec transfer rate. For more information, see Ultra2
SCSI.
SCSI
FAST-80
This refers to timings
being defined for a future revision of the SCSI-3 SPI that achieve 80 MT/sec, which is
twice as fast as SCSI FAST-40 rates. For example, an 80 MT/sec rate on a 1 byte wide
(narrow) bus results in an 80 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it
results in a 160 Mbytes/sec transfer rate.
Ultra
SCSI
This is a term describing
the latest published ANSI standard (X3T10/1071D rev. 6), commonly known as Fast-20. Ultra
SCSI, like all synchronous transfers, is a negotiated clock rate. For more information,
see The UltraSCSI Buzz.
Ultra2 SCSI
This is a term describing
the latest published ANSI standard (X3T10/1071D rev. 6), commonly known as Fast-40. Ultra2
SCSI, like all synchronous transfers, is a negotiated clock rate.
SCSI
Narrow
This term refers to the 1
byte wide data bus on a 50-pin parallel interface that is defined in the ANSI standard
SCSI-1 (X3.131-1986). The narrow bus consists of 8 data lines with parity, a series
of control lines and the matching ground lines. Seagate designates a narrow SCSI
interface with an "n" in the model number.
SCSI
WIDE
This term usually refers
to the 2 byte wide data bus on a 68 pin parallel interface that is defined in the SCSI-3
SPI document. The term can be generically applied to any implementation wider than 1 byte,
but at the time of this writing, there are no implementations wider than 2 bytes. Future
implementations may include more data bytes because FAST transfer rates are giving plenty
of life to 2 byte transfers until serial interfaces (like Fibre Channel or FireWire)
become more popular. Seagate designates a wide SCSI interface with a "w" in the
model number.
SCSI
FAST-WIDE
This refers to a
combination of a FAST transfer rate with a 2 byte wide connector, resulting in an
increased data transfer rate. Wide FAST-20 (40 Mbytes/sec) products will be available in
the year of this writing. Wide FAST-40 (80 Mbytes/sec) and FAST-80 (160 Mbytes/sec)
products will be available in the near future.
Differential or High Voltage
Differential (HVD)
Differential (D, ND, WD,
WDC) is a logic signal system used in some SCSI drives. It uses a paired plus and minus
signal level to reduce the effects of noise on the SCSI bus. Any noise injected into the
signal would be present in both a plus and minus state, thereby being canceled. Seagate
designates a differential (high voltage) SCSI interface with a "d" in the model
number.
Due to changing
definitions, Differential is now often referred to as High Voltage Differential (HVD).
Low Voltage Differential (LVD)
Low Voltage Differential
is a differential logic scheme using lower voltage levels than HVD. For more information,
see Ultra2 SCSI. Seagate designates low voltage differential
SCSI interface with an "l" in the model number.
Fibre Channel Arbitrated Loop (FCAL)
This is the formal name
for the Fibre Channel system used by SCSI. It is more commonly known as Fibre Channel
SCSI. The loop part of the name refers to the way the system is connected as one large
ring. Because of the loop characteristics, this interface has more in common with local
area networks than with parallel SCSI.
Click here for a PDF version of the Interface
Manual.
Fibre Channel SCSI
This refers to products
with fibre channel physical and protocol layers using the SCSI command set. The Fibre
Channel interface is completely different from parallel SCSI in that it is a serial
interface, meaning command and data information is transmitted on one signal stream
organized into packets. The fibre may be either a copper coaxial cable or a fiber optic
cable. The signal on the first implementation of fibre channel uses a 1 GHz rate, thereby
achieving 100 Mbytes/sec over the cable. Fibre channel also implements increased software
control of configuration and pushes the total device count on the bus to 126 IDs, as
opposed to only 8 or 16 on a parallel bus. For more information, see our Technology Paper
on Fibre Channel. Seagate designates a fiber channel SCSI interface
with an "fc" in the model number.
ASA-2
SCSI
This is a Seagate specific
term describing the basic structure of the SCSI firmware included with a Seagate disc
drive and standing for Advanced SCSI Architecture, generation 2. It provides better
performance than ASA-1 code in certain user environments (such as sequential 1 block data
transfers). The labels ASA-1 and ASA-2 can apply to code shipped on a wide range of
products, meaning different products have firmware originating from the same base
firmware, but such firmware is individually adapted to the particular servo and read/write
channel hardware on that product. The performance of a disc drive is still primarily
determined by the seek times and data transfer rates, but the code base label provides a
level of commonality and lack of bugs assurance in SCSI features for all drives with that
label.
The most attractive
feature of a common code architecture is that most enhancements made on one drive (such as
a Barracuda 4) can be quickly migrated to other drives (such as the Barracuda 2 or Hawk 4)
developed from the same code base. All new products since January 1995 use the ASA-2 code
base, and many older products developed with ASA-1 code have been updated to the new
structure.
For more information on
ASA-2, please read the Seagate ASA-2 Technology Paper.
SCA-2
This is a miniature D
style, 80 pin connector used on SCSI drives to plug to backplane connections. The SCA-2
connector provides grounds, voltage, and control lines needed to allow hot-plugging of
parallel interface SCSI drives. Seagate designates an SCA-2 SCSI interface with a
"c" in the model number.
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