Q:
What
is SCSI?
A:
The term "SCSI" is an acronym for Small
Computer System Interface. It 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:
What
are the differences between SCSI-1 and SCSI-2?
A:
I
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
for internal wiring and the equally familiar 50-pin, male, shielded
"centronics" type connector for external wiring. This "centronics"
type connector is frequently called the "SCSI-1 connector".
5 Mbyte/sec SCSI is termed "Slow" SCSI.
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 Alternative 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.
Q:
What is the difference between single-ended and differential
SCSI?
A: 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:
What is "Narrow" SCSI?
A: Narrow
SCSI is the term that is used for 8-bit SCSI. It can usually be identified
by 50-pin connectors.
Q:
What is "Wide" SCSI?
A: 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:
What is "Wide Ultra" SCSI?
A: 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:
What is "HVD" SCSI?
A: 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:
What is "LVD" SCSI?
A: 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:
What is "multimode LVD" or LVD/MSE SCSI?
A: 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:
What are the benefits of LVD SCSI?
A: 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:
Is LVD SCSI backward compatible?
A: 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:
What
is Ultra160 or U160 SCSI?
A: 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:
What
is it U160/m SCSI?
A: 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:
Is
Ultra160 SCSI backward compatible?
A: 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:
What
is Fast-20 (or Fast-40 or Fast-80) SCSI?
A: 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)
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