Understanding SCSI
Tamara M.Harahsheh

SCSI,IDE,EIDE all describes the interface between the disk and the computer. The following is a brief description of SCSI.


Almost everyone with computer experience has heard of SCSI ,this describe a standard interface and command set for transferring data between devices on a computer bus.


Let's get more specific:
History


In 1981,SCSI was developed by "Shugart Association", which introduced Shugart Association System Interface (SASI)in conjunction with NCR corporation. In 1986, a modified version of SASI was ratified by American National Standard Institute (ANSI)as SCSI.


SCSI: What? Why?


SCSI (Pronounced "skuzzy"),which is acronym for Small Computer Systems Interface ,is a standard parallel interface between computers and peripheral devices such as hard drives ,scanners ,CD-ROM/RW drives ,tap drives ,printers…etc .
The technology allows you to connect various devices to your PC or PC server ,using SCSI card that fits inside your machine. SCSI bus is completely separated from the usual bus ,such as IDE,ISA,PCI.SCSI is completely independent from computer.
SCSI hard disks are very commonly found in RAID array, workstations, servers, and high-end peripherals while ,desktop computers and notebooks more typically use the ATA/IDE interface for hard disks.


Why we use SCSI? Quite simple, it is used because it speeds up the computer. Expandability ,flexibility ,performance, ease to use, and maturity are additional great reasons why SCSI has the popularity in computer marketplace. There are no set standard SCSI, originally SCSI-1,SCSI-2 which sometimes have a hard time talking to each other. Also, some companies have developed SCSI standards which may not be able to talk with other SCSI standard. All may bring problems to the system.


SCSI basics


1. SCSI can connect multiple devices to single SCSI adaptor on the computer bus.
2. SCSI is a parallel interface.
3. SCSI can operate in either synchronous or asynchronous modes.
4. There must be at least one target and one initiator on the SCSI bus.
5. SCSI can support up to 25 meters of cable.


SCSI System Components


The basic component in the SCSI system :


1. controller" Host Adapter": It serves as the interface between all the other devices on SCSI bus and computer.
2. Device.
3. Cable.
4. BIOS: on the controller is the SCSI BIOS ,that is a small ROM or flash memory chip that contains the software needed to gain access the control devices on SCSI.


SCSI Termination


SCSI buses must be terminated in both ends. There are four ways to terminate:
1. Internal.
2. External.
3. Passive.
4. Active.


Terms In SCSI System


1. Embedded SCSI Derives: That is a device with an adapter built in. This adapter allows the device to interface and communicate with the SCSI bus.
2. Identifier(ID):The ID ranges from 0-7 in SCSI system that supports eight devices. and ranges from 0-15 for 16 devices support.
3. SCSI Configured Automatically (SCAM):The auto-selection capability in most plug and play SCSI cards.





Tamara M. Harahsheh
Mutah University/Jordan