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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
Tape Drives
Zip Drives
Removable Drives

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 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

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 Types Bus Speed
Bus Width
Maximum Device Supported ID
SCSI-1 5 8 6   25 8 0-7
Fast SCSI 2 10 8 3   25 8 0-7
Fast Wide SCSI 2 20 16 3   25 16 0-15
Ultra SCSI 3 20 8 3   25 8 0-7
Ultra 2 SCSI 3 80 16   12 25 16 0-15
Ultra SCSI 160 (SCSI 3) 160 16   12 5 16 0-15
Ultra 4 SCSI 320 (SCSI 4) 320 16   12 5 16 0-15
Ultra 4 SCSI 640 (SCSI 5) 640 16   12   16 0-15

Other Types of SCSI

iSCSI use mostly for high end networking

SAS(Serial Attach SCSI) use for storage 300 use SFF 8482, 8484, 8470 connector and is use for high end Serial Transmission

(Narrow SCSI 8bit and Wide SCSI 16bit)
Ultra SCSI 3, 320 and 640 use 68 and 80 pins and uses Parallel Transmission

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

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