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Interfaces:
The SCSI Command Sets
Command
Sets are the basic commands that SCSI devices use to communicate
with each other. They are similar to a programming or operating
language, and includes all the protocol information such as codes,
fields, and data types, that the device and adapter may use between
each other to communicate.
Each
new version of SCSI incorporates the command sets of previous versions,
therefore retaining backwards compatibility with older SCSI devices.
The SCSI Command Sets are split into the basic major groups of SCSI-1,
SCSI-2, and SCSI-3. These are the protocols of the SCSI standard
and do not include the actual physical and electrical interface
or connections. The physical interface may use 8-bit or 16-bit connections,
different cabling and connectors, and therefore offer varying levels
of performance.
Alongside
the actual electrical interface standard is another set of standards
regarding the command sets used. As the command set is simply a
set of instructions that the device understands - a language or
protocol.. The first SCSI Command Set standard was introduced with
the electrical connection and interface specification and was entitled
SCSI-1.
The
next Command Set improved the protocol and provided for a faster
and more sophisticated electrical interface. It was entitled SCSI-2.
Therefore any device meeting this criteria by supporting and implementing
the SCSI-2 Command Set has the right to call itself a SCSI-2 device.
Because
in the real world most people are more concerned with the physical
data throughput and transfer rates, most manufacturers and users
simplify the process by quoting the physical specification of the
device as the SCSI standard it relates to. This is why 2 hard drives
can exist side-by-side both claiming a DTR of 40MB/sec whilst stating
they are Ultra-SCSI Wide, SCSI-2 Ultra Wide, SCSI-3 Wide, SCSI-3
Ultra-Wide etc, etc.
SCSI
3 Command Set
The SCSI-3 command set was specifically introduced to allow integration
of Fibre Channel technology and SCSI technology. Fibre Channel integrates
the SCSI-3 command set into its own specifications and therefore
has full support for all SCSI-3 commands. This makes all SCSI-3
devices compatible with a Fibre Channel bus and controller. As Fibre
Channel may use either fibre or copper cabling, building a large
disk farm from SCSI hard drives and then combining these into a
single RAID array attached to the host or network via a Fibre Channel
controller is an ideal use of legacy and cheaper SCSI drives utilising
the faster and wider bandwidth of Fibre Channel. The host side of
the Fibre Channel controller may be a fibre link up to 10KM in length.
Single-ended (SE) SCSI is being slowly replaced by Ultra2 or LVD
SCSI, and on fixed mass storage products such as hard drives it
is fast becoming the only SCSI interface readily available. Most
SCSI manufacturers have already switched their standard SCSI hard
drive interface to the faster newer specification leaving only removable
storage devices such as CD-ROM, DVD, and digital tape drives available
with single-ended interfaces. In an LVD physical interface, each
signal has a dedicated return conductor to ensure signal integrity
and reduce interference. This is in direct contrast to a single-ended
interface devices. A differential signaling is generated by a balanced
LVD driver whereby each SCSI signal has a plus signal (+) on one
conductor and its exact complement minus signal (-) on the other
conductor. The LVD driver works to either assert a signal (when
it pushes current to the positive signal or pulls current from the
negative signal) or negates a signal when it pulls current to the
positive signal and pushes current from the negative signal.
LVD signaling is inherently less noisy than single-ended signaling
and this improved stability offers many I/O system benefits including
greater cable distances, more device connectivity, and faster data
transfer rates. Introduced initially at a marginal increase cost
to the user, Ultra2's rapid acceptance and availability has stabilised
prices quickly to the level of previous generation single-ended
devices.
Related
topics:
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