Small Computer System Interface (SCSI)

Small Computer System Interface (SCSI) Definition

Photo of an Adaptec 2248700-R U320 PCI Express X1 1-Channel SCSI Host Bus Adapter
Adaptec SCSI Host Adapater. © PMC-Sierra, Inc.

What Does SCSI Mean?

SCSI is a type of connection for storage and other devices in a PC.

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

Generally, it refers to the types of cables and ports used to connect certain types of hard drives, optical drives, scanners, and other peripheral devices to a computer.

How Does SCSI Work?

SCSI interfaces can be used internally 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 basically a piece of integrated software that's used to control the connected devices.

What Are the Different SCSI Technologies?

There are a number of different SCSI technologies that support different cable lengths, speeds, and 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 very first version of SCSI supported 8 devices with a maximum transfer speed of 5 MBps. Faster versions came later, with speeds like 320 MBps with support for 16 devices.

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

  • Fast SCSI: 10 MBps; connects 8 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

Check out the chart on this HowStuffWorks page.

It lists some other SCSI interfaces, some of which are much slower than the ones I have listed and are certainly no longer in use.

What Newer Standards Are Replacing SCSI?

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

The IDE standard (PATA and SATA) is by far the leading storage interface standard in computers today.

Some computer manufacturers have stopped using on-board SCSI completely, and use standards that are now much more popular, like USB and FireWire, for connecting external devices to computers. USB is also much faster, with maximum speeds of 1250 MBps (10 Gbps).