Small Computer System Interface (SCSI)

The SCSI standard is no longer used in consumer hardware

Photo of an Adaptec 2248700-R U320 PCI Express X1 1-Channel SCSI Host Bus Adapter
Adaptec SCSI Host Adapater. © PMC-Sierra, Inc.
by Tim Fisher
Tim Fisher has 30+ years' professional technology support experience. He writes troubleshooting content and is the General Manager of Lifewire.
Updated May 01, 2019

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

The SCSI standard is no longer common in consumer hardware devices, but you'll still find SCSI in some business and enterprise server environments. More recent versions of SCSI include USB Attached SCSI (UAS) and Serial Attached SCSI (SAS).

Most computer manufacturers have stopped using onboard SCSI completely and use much more popular standards like USB and FireWire for connecting external devices to computers. USB is much faster than SCSI with a sustained speed of 5 GBps and maximum incoming speed approaching 10 Gbps.

SCSI is based on an older interface called Shugart Associates System Interface (SASI), which later evolved into Small Computer System Interface, abbreviated as SCSI and pronounced "scuzzy."

How Does SCSI Work?

SCSI interfaces used internally in computers to connect different types of hardware devices directly to a motherboard or storage controller card. When used internally, devices are attached through a ribbon cable.

External connections are also common for SCSI and typically connect via an external port on a storage controller card using a cable.

Within the controller is a memory chip that holds the SCSI BIOS, which is a piece of integrated software that's used to control the connected devices.

What Are the Different SCSI Technologies?

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

Debuting in 1986, the first version of SCSI supported eight devices with a maximum transfer speed of 5 MBps. Faster versions came later with speeds of 320 MBps and support for 16 devices.

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

  • Fast SCSI: 10 MBps; connects eight devices
  • Fast Wide SCSI: 20 MBps; connects 16 devices
  • Ultra Wide SCSI: 40 MBps; connects 16 devices
  • Ultra3 SCSI: 160 MBps; connects 16 devices
  • Ultra-640 SCSI: 640 MBps; connects 16 devices