SCSI (pronounced "skuzzy")
stands for Small Computer System Interface. It is a special bus that can be
used to connect peripherals like scanners, printers,
hard drives, and CD-ROMs.
There are three basic types of SCSI:
This is the basic specification of SCSI
as developed by ANSI in 1986. It formed the basis of how SCSI should work
and created an outline of what SCSI's traits are, but there was no standard.
Different manufacturers had different ideas of how SCSI should work. This
is still a problem today.
This is a version of SCSI-1 that has
been built upon and improved upon. It was developed in 1994 and had a Common
Command Set, or CCS. The CCS was very important because it was a list of
the 18 basic commands deemed a necessity for all SCSI devices. SCSI-2 was
also much faster than SCSI-1, and it also had an increased bus width. In
addition, SCSI-2 had a new feature called command queuing. This means that
SCSI-2 adapters can send many commands to a single device. That device would
then store and process these commands and determine which ones should be
SCSI-3 was introduced in 1995, one year
after SCSI-2's debut. There is currently no standard for SCSI-3, but some
components of this bus are used.
There are three major components to an SCSI system:
The controller is also sometimes called
a host adapter. It is an interface that allows all the devices connected
to the SCSI bus to communicate with the computer system. Sometimes, the
controller is built right into your computer's motherboard,
while other times it comes in the form of a card that you can plug into
your computer system.
The devices on an SCSI bus are basically
just the peripherals that the bus connects. For example, a device could
be a scanner, a printer,
or a hard drive. Every single device on an
SCSI bus has a unique ID. This ID comes in the form of a number between
0 and 7 (for buses that can support 8 devices) or in the form of a number
between 0 and 15 (for buses that can support 16 devices). Usually, one of
the IDs is reserved for the SCSI controller. This leaves room for either
7 additional devices or 15 additional devices.
All SCSI buses must be connected somehow.
This is the task of the cables. Ribbon cables are used to connect all of
the internal devices on an SCSI bus. Ribbon cables have a connector on either
end and more connectors all along its length for adding on devices.
External cables are used to connect,
well, external devices (duh!). These cables have three layers. The data
being sent is contained by the inner layer, while commands are sent via
the media, or middle, layer. Finally, the outer layer contains information
to check to make sure the data is correct. External SCSI devices usually
have two controllers. One controller is used to connect to the device
ahead of it, while the other is used to connect to the cable itself.
SCSI buses contain a chain of devices.
At the end of this chain, the SCSI bus must have some way of being terminated,
or closed. If the bus was not closed, then the electrical signals sent down
the bus could reflect back and cause major problems. Termination is usually
accomplished by using resistor circuits. There are two terminators in an SCSI
bus- one at either end.
Types of SCSI termination can be put
into two groups- passive and active. Passive termination is for devices that
are within a few feet of the SCSI controller, and it is not as fast as active
termination. Active termination is for fast SCSI buses and for devices that
are farther away.
As you can see, SCSI is an integral part
of many computer systems today!
Continue on to the next section:
Universal Serial Bus (USB)
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