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SCSI

SCSI stands for small computer system interface. Pronounced "scuzzy," SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers.

SCSI interfaces provide for faster data transmission rates (up to 320 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

Typical SCSI Devices

Hard Drives
CD/DVD ROM
Scanners
Tape Drives
Zip Drives
Removable Drives
Printers

SCSI chain – a series of SCSI devices working together through a host adapter

SCSI card or SCSI Host Adapter – comes as a PCI or ISA and has an internal and external connector that would connect to a device

scsi host adapter

Whatever devices are at the both ends of the SCSI chain must be terminated

Ways to terminate – PnP, software, jumper settings, switch and using a terminator

Below are examples of internal SCSI interfaces:

Below are examples of external SCSI interfaces:

DB25 (found on SCSI 1 and common in Macintosh computers)

SCSI 50

SCSI 68

SCSI ID – Each device on the SCSI chain must have a unique ID with 0 being the highest priority and 15 being the lowest priority and the SCSI card is usually ID 7

LUN – logical unit numbers allows a device to share an ID up to seven sub units per ID

Command Queuing – Ability of SCSI to accept multiple commands

Although SCSI is an ANSI standard, there are many variations of it; so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors

While SCSI has been the standard interface for Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

SCSI Cables

SE - Single Ended system allows eight wires on the data cable to carry data
HVD - High Voltage Differential employs two wires per bit of data
LVD - Low Voltage Differential employs requires less power than HVD

SCSI Flavor CHART

Other Types of SCSI

iSCSI
iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged. iSCSI advocates project the iSCSI standard, an embedding of SCSI-3 over TCP/IP, as displacing Fibre Channel in the long run, arguing that Ethernet data rates are currently increasing faster than data rates for Fibre Channel and similar disk-attachment technologies. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology.

Serial SCSI
Four recent versions of SCSI, SSA, FC-AL, IEEE1394, and Serial Attached SCSI (SAS) break from the traditional parallel SCSI standards and perform data transfer via serial communications. Although much of the documentation of SCSI talks about the parallel interface, most contemporary development effort is on serial SCSI. Serial SCSI has number of advantages over parallel SCSI—faster data rates, hot swapping, and improved fault isolation. Serial SCSI devices are more expensive than the equivalent parallel SCSI devices, but this is likely to change soon.

SCSI – is used primarily in RAID drives and if the client wants the ability to daisy chain and have fast devices.