What is SCSI? (Small computer system interface)

SCSI is a once popular type of connection for storage and other peripherals in a PC. The term refers to the cables and ports used to connect certain types of hard drives, optical drives, scanners, and other peripherals to a computer.

The SCSI standard is no longer common in consumer hardware devices, but you will still find it used in some business and enterprise server environments. Newer versions include USB connected SCSI (UAS) and serial attached SCSI (SAS).

Most computer manufacturers have completely stopped using onboard SCSI and are using much more popular standards like USB and FireWire to connect external devices to computers. USB is much faster, with maximum incoming speed approaching 40Gbps.

Adaptec SCSI host adapter.
PMC-Sierra, Inc.

SCSI is based on an older interface developed by floppy drive manufacturer Shugart Associates and called Shugart Associates System Interface (SASI), which later became System interface for small computerabbreviated as SCSI and pronounced “scuzzy”.

How does SCSI work?

SCSI interfaces used internally in computers to connect various types of hardware devices directly to a motherboard or storage controller card. When used internally, the devices are connected via a flat cable.

External connections are also common and usually connect through an external port on a storage controller card using a cable.

Within the controller is a memory chip that contains the SCSI BIOS, which is embedded software used to control attached devices.

What are the different SCSI technologies?

There are several SCSI technologies that support different cable lengths, different speeds, and a number of devices that can be attached to a cable. They are sometimes referred to by their bus bandwidth in Mbps.

Released in 1986, the first version of SCSI supported eight devices with a maximum transfer speed of 5 Mbps and a maximum cable length of six meters. Faster versions came later with support for 16 devices and a maximum cable length of 12 meters.

Here are some of the other SCSI interfaces that have existed:

  • Fast SCSI: 10Mbps; connects eight devices
  • Large and fast SCSI: 20 Mbps; connects 16 devices
  • Ultra Wide SCSI: 40Mbps; connects 16 devices
  • Ultra2 Wide SCSI: 80Mbps; connects 16 devices
  • Ultra3 SCSI: 160 MB/s; connects 16 devices
  • Ultra-320 SCSI: 320Mbps; connects 16 devices
  • Ultra-640 SCSI: 640Mbps; connects 16 devices

Gordon K. Morehouse