Definition
SCSI is actually an acronym for Small Computer System Interface and it is pronounced as "skuzzy". It is the second-most popular hard disk interface used in PCs today. It's a high-speed, intelligent peripheral I/O bus with a device independent protocol for transferring data between different types of peripheral devices. The SCSI bus connects all parts of a computer system so that they can communicate with each other. The bus frees the host processor from the responsibility of I/O internal tasks. A SCSI bus can be either internal, external, or cross the boundary from internal to external. The SCSI protocol is a peer-to-peer relationship: one device does not have to be subordinated to another device in order to perform I/0 activities.

Only two of these devices can communicate on the bus at any given time. Each SCSI bus can connect up to 8 or up to 16 peripherals; one of those devices will always be the computer or the SCSI card, because they too are devices on the SCSI. SCSI devices are designated as either initiators (drivers) or targets (receivers) and the interface to the host computer is called the host adapter. Every device connected to the bus will have a different SCSI ID, ranging from 0 to 7. The host adapter takes up one ID leaving 7 ID's for other hardware. SCSI hardware typically consists of hard drives, tape drives, CD-ROMs, printers and scanners.

The reason for the slow taking of SCSI is the lack of standard. Each company seems to have its own idea of how SCSI should work. While the connections themselves have been standardized, the actual driver specs used for communication have not been. The end result is that each piece of SCSI hardware has its own host adapter. So, due to the lack of an adapter standard, a standardized software interface, and a standard BIOS for hard drives attached to the SCSI. Adapter.



History and Evolution
In the beginning, one couldn't even use a hard drive on the bus. This was mainly because the BIOS in those systems were designed to use the ST506/412 controller. With the IDE, the BIOS was easily changed because of the similarity to ST506/412 on the WD1003 controller. At the register level, though, SCSI was very different, and would have required an entirely new set of BIOS in the PC.

What we currently know of as the SCSI interface had its beginnings back in 1979. Shugart Associates, led by storage industry pioneer Alan Shugart (who was a leader in the development of the floppy disk, and later founded Seagate Technology) created the Shugart Associates Systems Interface (SASI). This very early predecessor of SCSI was very rudimentary in terms of its capabilities, supporting only a limited set of commands compared to even fairly early "true" SCSI, and rather slow signaling speeds of 1.5 Mbytes/second. For its time, SASI was a great idea, since it was the first attempt to define an intelligent storage interface for small computers. The limitations must be considered in light of the era: we are talking about a time when 8" floppy drives were still being commonly used

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INTRODUCTION
Small Computer System Interface or SCSI (pronounced scuzzy), is a set of standards for physically connecting and transferring data between computers and peripheral devices.
The SCSI standards define commands, protocols, and electrical and optical interfaces.
SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives.
SCSI is an intelligent, peripheral, buffered, peer to peer interface.
SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports.
We can connect many devices to a single SCSI port. Thus SCSI is really an I/O bus rather than simply an interface.
Up to 8 or 16 devices can be attached to a single bus.
The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device.
The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.
HISTORY
SCSI was derived from "SASI", the
"Shugart Associates System Interface", developed in 1978 and publicly disclosed in 1981.
Larry Boucher is considered to be the "father" of SASI and SCSI due to his pioneering work first at Shugart Associates and then at Adaptec.
The committee documenting the
standard would not allow it to be named after a company.
So they decided to name the standard
“Small Computer System Interface”.
ENDL's Dal Allan pronounced the new acronym as "scuzzy"
SCSI INTERFACES
There are 3 main types SCSI interfaces:
Parallel SCSI
Serial SCSI
iSCSI
PARELLEL Scsi INTERFACES
OTHER SCSI INTERFACES
SCSI CABLING
Traditional parallel SCSI
Serial attached SCSI
Internet small computer system interface
USB attached SCSI
SCSI COMMAND PROTOCOLS
In SCSI terminology, communication takes place between an initiator and a target.
There are 4 categories of SCSI commands:
N (non-data)
W (writing data from initiator to target)
R (reading data)
B (bidirectional).
There are about 60 different SCSI commands in total, with the most common being:
Test unit ready
Inquiry
Request sense
Send diagnostic and Receive diagnostic results
Start/Stop unit
Read capacity
Format unit
SCSI Read format capacities
Read (four variants)
Write (four variants)
Log sense
Mode sense
Mode select
HOW SCSI WORKS???
SCSI uses a protocol method to transfer data between devices on the bus.
It is a circular process which starts and ends up in the same layer.
The protocol layers are referred to as "SCSI bus phases".
The SCSI bus can be in only one phase at a given time.
These phases are:
Bus Free
Arbitration
Selection
Message Out
Command Out
Data Out/In
Status In
Message In
Reselection
SCSI DEVICES BEING USED by
The SCSI devices currently is being used by various companies.
Some of these companies are:
NCR Corporation
Adaptec
Optimem
Amiga
Apple Macintosh
Sun Microsystems