Q. 1. What does the term "SCSI"
mean? Answer: The term "SCSI" is an acronym for Small
Computer System Interface. In the 1970s the name was appropriate. Today, SCSI
is used for PCs, workstations, servers, mainframes, supercomputers.
Q. 2. What is
SCSI? Answer: The Small Computer System Interface is a
high-speed, intelligent peripheral I/O bus with a device independent protocol.
It allows different peripheral devices and hosts to be interconnected on the
same bus. Depending on the type of SCSI, you may have up to 8 or 16 devices
connected to the SCSI bus. The number of devices can be dramatically expanded
by the use of LUNs (Logic Unit Numbers). There must be at least one initiator
(usually a host) and one target (a peripheral device) on a bus. There is a
large variety of peripheral devices available for SCSI, including hard disk
drives, floppy drives, CDs, optical storage devices, tape drives, printers and
scanners to name a few. There are many implementations of SCSI starting with
SCSI-1 to SCSI-2 to SCSI-3 including, Narrow, Wide, Fast, Ultra, Ultra-2 and
Ultra160 SCSI. The SCSI specifications are approved and issued by ANSI and are
developed by the X3T10 SCSI Committee.
Q. 3. What can I do with
SCSI? Answer: SCSI provides a high-speed, intelligent
interface that allows an easy connection for up to 16 devices (8 devices for
Narrow SCSI) on a single bus. These devices may be hard disks, floppy disks,
CDs, tape drives, printers and scanners to name a few. Peripherals may be
mounted in the computer or in an external enclosure. Total SCSI cable length
is dependent on the type of SCSI.
Q. 4. I seem to remember hearing the term
SASI in the past. What is it? Answer: SASI is the acronym
for Shugart Associates System Interface. It was developed in the 1970s by
Shugart, at the time a dominant manufacturer of disk drives. It was meant to
be an intelligent interface for disk drives only. Offering only 8-bit
(Narrow), single-ended, asynchronous operation, by today's standards it was
very slow (1.5 Mbytes per second). The standard connector for in-cabinet
cabling is the non-shielded, 50-pin, female, low-density, connector having two
rows of 25 pins each on 0.1 inch spacing. The standard connector for cabling
outside the cabinet is the shielded, 50-pin, male, "centronics" type
connector. In 1981 Shugart and NCR submitted SASI to the ANSI committee X3T9.2
as an open architecture I/O bus for disk drives. ANSI accepted the project,
changed the name to Small Computer System Interface and added some major
improvements to the specification. It was approved in 1986 by ANSI as document
IEEE X3.131-1986. Today it is called SCSI-1. SASI is now long obsolete and,
although many aspects of SCSI were backward compatible with SASI, it is very
problematic.
Q. 5. Does SCSI work in both
directions? Answer:
Yes. SCSI is a bi-directional bus and will not work at all if it does not work
in both directions. That also means that SCSI expanders such as a single-ended
(SE) to differential converter will work as a SE to differential or a
differential to SE converter. In other words, it does not make any difference
if the initiator is on the SE side or on the differential side of the
expander.
Q. 6. What are the differences between
SCSI-1 and SCSI-2? Answer: The initial implementation of
SCSI (now called SCSI-1) was designed primarily for Narrow (8-bit),
single-ended, synchronous or asynchronous disk drives and was very limited
relative to today's SCSI. It includes synchronous and asynchronous data
transfers at speeds up to 5 Mbytes/sec. Only passive termination was defined.
It did not include definitions of a device independent interface. The standard
connectors are the familiar 50-pin, female, low-density (0.1 inch spacing),
non-shielded connector (now termed the non-shielded Alternate 2, A-connector)
for internal wiring and the equally familiar 50-pin, male, shielded
"centronics" type connector for external wiring (now termed the shielded,
Alternate 2, A-connector). This "centronics" type connector is frequently
called the "SCSI-1 connector". 5 Mbyte/sec SCSI is termed "Slow" SCSI. SCSI
cable lengths may be up to 6 meters (20 ft) for Slow SCSI. Even before
X3.131-1986 was officially accepted by ANSI, the SCSI committee went to work
on improving it.
Released by
the ANSI Committee as specification IEEE X3.131-1994, SCSI-2 is also a
complete, stand-alone document. Arguably the most significant addition of
SCSI-2 is the expanded definition of the common command set (CCS) providing a
common software interface for all disk drives and many peripherals other than
disk drives. SCSI-2 defines the differential interface and the 16-bit and
32-bit "Wide" data bus; doubles data throughput to 10 Megatransfers per second
(called "Fast" SCSI), which translates to 10 Mbytes/sec for Narrow (8-bit)
SCSI and 20 Mbytes/sec for Wide (16-bit) SCSI; adds the smaller 50-pin, high
density, micro-D connector (termed Alternate 1, A-connector); and terms all
50-pin cables "A" cables. This 50-pin high-density connector is commonly
called the "SCSI-2 connector". SCSI-2 recommends active terminators in place
of passive terminators for the single-ended bus. Backward compatible to
SCSI-1. Note that in SCSI-2 the 16-bit bus requires two cables (one "A" cable
and one "B" cable) to make a connection. This seriously limited growth of the
Wide bus. SCSI-2 maximum recommended single-ended SCSI cable length is up to 3
m (10 ft) for Fast SCSI. Differential cable length is 25 m (82 ft) for Fast or
Slow SCSI.
Q. 7. What are the differences between
SCSI-2 and SCSI-3? Answer: SCSI-3 changes the complete
SCSI document structure and is no longer one document but a collection of
documents, each with its own revision number. Some of these documents are the
SCSI Primary Command (SPC) set layer, SCSI Block Commands (SBC) for hard disk
interface, SCSI Stream Commands (SSC) for tape drives, SCSI Controller
Commands (SCC) for RAID arrays, Multimedia Commands (MMC) , Media Changer
Commands (MCC) and the SCSI Enclosure Services (SES) commands. For a complete
overview see the SCSI Architecture
Model (SAM) on the T10 Committee
Website.
Let's take a look
at some other important SCSI-3 documents:
SPI The SCSI Parallel Interface (SPI) defines the electrical signals
and connections for parallel SCSI. A very quickly adapted new feature
defined in SCSI-3 is the 68-pin, high density, micro-D connector for 16-bit
Wide SCSI (termed the Alternate 3, P-connector). The SCSI specification
terms cables with this connector the "P" cable. This connector eliminates
the necessity of using two cables for 16-bit SCSI and gave a tremendous
boost to the growth of Wide SCSI. It is commonly referred to as the "SCSI-3"
connector.
There are several revisions of the SPI document. SPI
includes Fast SCSI data transfer speeds up to 10 Megatransfers (20
Mbytes/sec for 16-bit). The Ultra SCSI (Fast-20) modification of SPI
includes doubling the data throughput to 20 Megatransfers/sec (40 Mbytes/sec
for 16-bit). Ultra SCSI speeds reduce the maximum single-ended cable length
to 1.5 m (5 ft) with 5 or more devices and 3 m (10 ft) for systems having up
to 4 devices. The maximum recommended differential cable length remains at
25 m (82 ft).
SPI-2 SPI-2 doubles bus speed again to the Ultra 2 (Fast-40) SCSI
data throughput of 40 Megatransfers/s (80 Mbytes/s for 16-bit). To attain
this speed, a new electrical interface is defined. This interface uses 3 V
logic instead of TTL voltage levels and is known as Low Voltage Differential
(LVD) SCSI. The older TTL based differential SCSI is now called High Voltage
Differential (HVD) and it is not compatible with LVD signals. Most LVD
device interfaces are designed as LVD/SE.
Multimode operates at the
LVD voltage levels and bus speed as long as all devices connected are LVD.
Connecting a single-ended device to a multimode LVD bus causes all LVD/SE
devices to switch to the single-ended interface. It will then operate at a
maximum of 20 Megatransfers/sec (40 Mbytes/sec for 16-bit) with single-ended
cable length limitations. Connecting an HVD device to an LVD bus will cause
the bus to shut down. LVD cable length is specified as 12 m (40 ft). For a
single initiator-single target application this length may be increased to
as much as 25 m (82 ft). Note that single-ended signals cannot be used for
bus speeds greater than Ultra SCSI (Fast-20).
The low power
requirements of the LVD interface allow the differential drivers to be
included on the interface ASIC. Not having to place external driver chips on
the PCB reduces the amount of PCB real estate required and reduces the cost
of the board
design.
Another new
feature of SPI-2 is the SCSI Interlock Protocol (SIP) which defines the
parallel command set. Also, SPI-2 adds two new SCSI connectors:
The 80-pin Single Connector Attachment (SCA-2) connector (termed the
non-shielded Alternate 4, P-connector) that includes the 16-bit SCSI signals
as well as power for the peripheral. This connector is designed for hot
swapping of peripherals in SCSI backplanes.
The Very High Density Cable Interconnect (VHDCI) connector (termed the
shielded Alternate 4, P-connector) is a small connector that allows as many
as four separate 68-pin Wide SCSI connectors to be placed on one standard
width PC backplate. Some of the newer LVD host adapters include this
connector.
SPI-2 is a complete stand-alone document for all
parallel interfaces up to Ultra 2 (Fast-40) SCSI and does not refer to older
documents. To do this, it has incorporated the 50-conductor "A" cables defined
in SCSI-2 and the 68-conductor "P" cables defined in the original SPI
document.
SPI-3 SPI-3 again doubles the SCSI bus speed to Ultra 3 (also known
as Ultra160 and Fast-80) providing SCSI bus speeds up to 80
Megatransfers/sec (160 Mbytes/sec for 16-bit). For this speed, clocking on
both the rising and falling edges of the REQ and ACK clock is required. This
is called Double Transition (DT) clocking and is defined for the 16-bit bus
only.
SPI-3 also includes a 32-bit CRC (Character Redundancy Check)
for better data security and Domain Validation. Domain Validation is new for
peripheral buses. Basically, SCSI Domain Validation will not accept a
negotiated data throughput speed until a validation test is performed. To
perform this test, the initiator sends out a Write Buffer command to the
target at the full data throughput. The initiator will then read the data
back to see that it is correct. If it is not, the initiator will switch to
the next lower speed and perform the test again. When the test passes, that
speed is compatible with both the initiator and the target and is used for
data transfers between the two devices.
SPI-3 is also a complete
document defining parallel SCSI interfaces up to 80 Megatransfers/sec and
does not refer to previous SCSI documents. SPI-3 obsoletes HVD and 32-bit
data bus designs. For specifications of the HVD and 32-bit bus, refer to
SPI-2. The maximum cable length for Ultra 3 SCSI is 12 m (40 ft) or 25
meters (82 ft) for point-to-point applications.
Ultra 160
(U160/m) is a sub-set of Fast-80 that includes Double Transition clocking,
CRC and parts of Domain Validation. It is not yet a recognized form of
SCSI.
EPI For Paralan a very significant development released in the
Enhanced Parallel Interface (EPI) is the documentation of SCSI Expanders,
Bridging Expanders, Switches and some connectors not otherwise documented.
This finally incorporates into the SCSI specification the types of products
that Paralan has been designing, marketing and selling for years. EPI also
describes the design of SCSI systems, defining the electrical specifications
for cable lengths and loads. Also included is a description of how to work
with both Wide (16-bit) and Narrow (8-bit) devices on the same SCSI
bus.
Q. 8. What is the difference between
single-ended and differential SCSI? Answer:
Single-ended and differential are two methods of placing SCSI signals on the
cabling. Single-ended uses one wire driven against ground and the signal is
the voltage difference between that wire and ground. The differential
interface drives two wires. The signal is the voltage difference between the
two wires. Single-ended and differential are not directly compatible. (It
should be noted that HVD and LVD are also not directly compatible). They can
be interconnected by the use of a SCSI expander called a Single-ended to
Differential Converter. Single-ended cable lengths are 6 to 1.5 meters (20 to
5 ft), decreasing with increasing data throughput, while differential (HVD and
LVD) offers cable lengths to 25 meters (82 ft), regardless of the speed of the
bus.
Q. 9. What is meant by "Narrow"
SCSI? Answer:
Narrow SCSI is the term that is used for 8-bit SCSI. It can usually be
identified by 50-pin connectors.
Q. 10. What is meant by "Wide"
SCSI? Answer: Wide SCSI is the term that is used for
16-bit SCSI. It can usually be identified by 68-pin connectors. From SCSI-2
until the SPI-3 document in SCSI-3, this term also applied to 32-bit SCSI.
SPI-3 obsoleted the 32-bit SCSI bus.
Q. 11. What is HVD
SCSI? Answer: This is the "old" differential SCSI using
TTL voltage levels that was originally defined in SCSI-2, offering 25 meter
(82 ft) cable length. It was functionally replaced by LVD (Low Voltage
Differential) SCSI in the SPI-2 document of SCSI-3 and obsoleted in the SPI-3
document of SCSI-3. HVD and LVD SCSI are not directly compatible but can be
interconnected by the use of a SCSI expander called an LVD to HVD
Converter.
Q. 12. What is Wide Ultra
SCSI? Answer:
Ultra SCSI, defined in the SPI-2 document of SCSI-3 offers a maximum data
throughput of 20 Mbytes/sec for Narrow (8-bit) SCSI. Ultra Wide SCSI is the
16-bit version that offers 40 Mbytes/sec data transfers. Ultra Wide
single-ended SCSI has a maximum cable length of 1.5 m (5 ft) with more than 4
active IDs and 3 m (10 ft) with 4 or fewer active IDs. Ultra Wide differential
SCSI has a maximum cable length of 25 m (82 ft).
Q. 13. What is LVD
SCSI? Answer: LVD, which stands for Low Voltage
Differential, was introduced in the SPI-2 document of SCSI-3. It is also
called Ultra 2 or Fast-40 SCSI. It uses 3 volt instead of 5 volt logic level
and is not directly compatible with the "old" differential (HVD) SCSI. LVD
again doubles SCSI data throughput to 40 Megatransfers/sec. Cable lengths are
12 m (40 ft). Single initiator-single target applications may use up to 25 m
(82 ft) of cable. The "multimode" implementation of LVD is backward compatible
with single-ended SCSI. However, connecting one single-ended peripheral to a
multimode LVD bus will cause the entire bus to switch to the single-ended mode
with the single-ended limitations on data throughput and cable length. LVD can
be interconnected with HVD by the use of a SCSI expander called an LVD to HVD
Converter.
Q. 14. What is "multimode LVD" or LVD/MSE
SCSI? Answer:
Multimode LVD and LVD/MSE (Multimode Single-Ended) are terms for the same
interface. It is an implementation of SCSI that automatically switches between
the LVD and the single-ended mode. When a single-ended device is connected to
a multimode LVD/MSE bus, the entire bus switches to the single-ended mode.
Otherwise LVD/MSE devices operate in the LVD mode.
Q. 15. What are the benefits of LVD
SCSI? Answer: In addition to the obvious benefits of
longer maximum cable length than single-ended and a doubling of data
throughput, there are a number of other benefits. LVD/MSE and single-ended
offer some compatibility. The lower operating voltage of the LVD bus means
lower power dissipation, so the differential drivers can be included on the
LVD ASIC rather than having to mount them external to the chip. This results
in smaller boards, less heat dissipation, higher reliability and lower cost.
Also, manufacturers will no longer have to design and build devices with both
single-ended and differential interfaces. This results in lower costs.
Q. 16. Is LVD SCSI backward
compatible? Answer: LVD is backward compatible through
the single-ended interface if it is multimode LVD. It is doubtful that
anyone will build LVD devices that are not multimode. Remember that connecting
a single-ended device to a LVD/MSE bus will cause the entire bus to switch to
the single-ended mode with its data throughput and cable length limitations.
To add a single-ended peripheral to an LVD bus and preserve the data
throughput and cable length of LVD, you can use a SCSI expander called an LVD
to SE or LVD/MSE to LVD/MSE converter. This converter divides the SCSI domain
into two bus segments - one segment will operate at the LVD data throughput
and cable length and the other bus segment will operate at the single-ended
data throughput and cable length.
Q. 17. What is Ultra160 or U160
SCSI? Answer: Ultra 160 is defined in SPI-3. It offers
data throughput of 80 Megatransfers/sec or 160 Mbytes/sec for Wide (16-bit)
SCSI which is the only defined bus width. For this speed, clocking on both the
rising and falling edges of the REQ and ACK clock is required. This is called
Double Transition (DT) clocking. Also called Fast-80 or Ultra 3 SCSI.
Q. 18. I have heard of U160/m SCSI. What is
it? Answer: The SPI-3 document defines 5 new features for
SCSI: Double Transition Clocking, CRC, Domain Validation, Quick Arbitration
and Select (QAS), and Information Units (Packetization). In order to be
compliant with the SPI-3 U160 specification, at least one of these features
must be implemented. A group of industry leaders agreed to incorporate three
of these features in order to speed up introduction of U160 products. These
three features are Double Transition Clocking, CRC and Domain Validation. U160
devices with these three features are called U160/m.
Q. 19. Is Ultra160 SCSI backward
compatible? Answer: Ultra 160, also called Ultra 3 is
backward compatible through the single-ended interface, if it is multimode
Ultra 160. It is doubtful that anyone will build Ultra 160 devices that are
not multimode. Remember that if a single-ended device is placed directly on a
multimode Ultra 160 bus the entire bus will switch to the single-ended mode
with its limitations on data throughput and cable length.
Q. 20. Is Ultra 160 SCSI better than fibre
channel? Answer: This is a discussion that will go on for
some time and there is no simple answer. It depends on the application. At
least until now, fibre channel (FC) implementations of SCSI have been FC-AL or
FC-Arbitrated Loop, so I will limit my comments to FC-AL. At the time of
writing this FAQ, SCSI is beginning to ship devices that are capable of 160
Mbytes/sec data transfer rate while fibre channel (FC) is stalled at a maximum
of 100 Mbytes/sec and the only peripherals with true FC interface are disk
drives -- and only one manufacturer makes them. Mark up the data throughput
advantage to SCSI.
FC proponents say that connectivity is more important than data
throughput and that FC can have up to 126 nodes. If that were true why would
every increase in SCSI data throughput be immediately adopted? Data
throughput rules in nearly every serious application we have encountered.
Anyway, on a practical basis, FC is limited to only a couple dozen nodes
which is very similar to what SCSI can handle. Call the connectivity issue a
draw.
Well then, FC can have up to 10 kilometers of fiber. Well, there are a
number of manufacturers of SCSI Extenders that offer the same or longer
fiber cable lengths. Call this one a draw.
Again, at the time of this writing, an issue that FC proponents do not
raise is interconnectivity. There are great problems in getting FC devices
from different manufacturers to work together. Sometimes this is true with
products from the same manufacturer. SCSI went through these problems many
years ago. There are now very few problems with SCSI interconnectivity. Of
course, these FC interconnectivity problems will eventually be solved, but
for now, score this one for SCSI.
Cost is always a factor. FC is more expensive. As FC gets wider
application its cost will go down, but for now there is no contest. Score
this one for SCSI.
Overall, at this time, FC has a place in large scale storage and backup
systems but there is almost no reason for the home user and very little
reason for the small business to consider FC.
Q. 21. Is Ultra 160 SCSI better than
EIDE? Answer: Again, the answer depends on the
application. SCSI is an intelligent interface that can perform data transfers
with no intervention from the host CPU. SCSI is multi-tasking. An initiator
can issue a command to a target. The target can then disconnect from the bus
to perform the task and free the bus up for another task. Ultra 160 SCSI can
have up to 16 devices connected to the bus and they can be any of a large
variety of peripherals, including hard drives, floppy drives, tapes, CDs,
scanners, printers, etc. The number of devices can be substantially increased
through the use of LUNs. EIDE can have two internal drives connected. Your PC
probably has two EIDE buses, so it may have up to four peripherals. Ultra 160
SCSI allows up to 12 m (40 ft) of cabling, which may be internal or external
to the computer. For point to point applications you may have up to 25 m (82
ft) of cable. EIDE is for internal cabling only and the maximum cable length
is only 18 inches. And, don't forget that 160 Mbytes/sec is much faster than
any EIDE bus.
For a home user with a single hard drive, EIDE is
probably better as it is less expensive and almost as fast as Ultra SCSI. In a
compute-intensive or storage-intensive application, however, SCSI is the clear
choice.
Q. 22. What is Fast-20 [or Fast-40 or
Fast-80] SCSI? Answer: The term "Fast-xx" refers to the
maximum data throughput that a particular version of SCSI is capable of,
expressed in Megatransfers/sec. For example, Fast-20 is 20 Megatransfers/sec
which is 20 Mbytes/sec for 8-bit (Narrow) SCSI and 40 Mbytes/sec for 16-bit
(Wide) SCSI.
Fast-10 is the same as Fast SCSI
Fast-20 is the same as Ultra SCSI
Fast-40 is the same as Ultra 2 (uses LVD transmissions)
Fast-80 is the same as Ultra 3 or Ultra 160 SCSI (uses LVD
transmissions)
Q. 23. Can I connect an Ultra 2 Wide (LVD)
disk to an Ultra Wide adapter? Answer:
The answer is definitely yes. However, exactly how you do it depends on the
type of Ultra Wide host adapter you have. If it is single-ended SCSI and the
LVD disk is multimode LVD, you can connect the disk directly to the host
adapter. The multimode LVD interface on the disk will switch to the
single-ended mode.
If the Ultra Wide adapter is differential (HVD),
they can still be connected, however, you will have to use a SCSI expander
called an HVD to SE or an HVD to LVD Converter.
Q. 24. What is Double Transition
clocking? Answer:
Double Transition (DT) clocking is used to double the data transfer rate from
Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having to
increase the clock speed. That means that both edges of the REQ and ACK
signals are used to clock data. The REQ and ACK signals run at 40 MHz on Ultra
SCSI, so double clocking increases the rate at which data is clocked to 80
MHz. This provides data throughput of 160 Mbytes/sec for Wide SCSI.
Q. 25. What is "Domain
Validation"? Answer: Domain Validation is a method used
in Ultra 160 (Fast-80 or Ultra 3) to test for the optimum rate for data
exchange. Once the host adapter (initiator) has located a peripheral and
negotiated a data transfer rate, the initiator sends a Write Buffer command to
the target at that negotiated data transfer rate and then reads it back to
determine if what it reads is what it wrote. If not, it will resend the Write
Buffer command at the next lower data transfer rate. This will continue until
a speed is reached where the test is successful. This is all accomplished
automatically.
Q. 26. What is CRC? Answer:Character
Redundancy Check is a means of detecting errors that is much more effective
than the simple parity check that SCSI has used for years. CRC detects all
single bit errors, all two bit errors, all errors with an odd number of bit
errors, and all burst errors up to 32-bits long. CRC uses a 32-bit polynomial
checksum to test data integrity. A similar process is used in Fibre Channel,
Ethernet and other buses.
