Small Computer System Interface (SCSI)

The SCSI standard is no longer used in consumer hardware

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 drivesoptical 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 it used in some business and enterprise server environments. More recent versions 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, with a maximum incoming speed approaching 40 Gbps.

Adaptec 2248700-R U320 PCI Express X1 1-Channel SCSI Host Bus Adapter
Adaptec SCSI Host Adapter. PMC-Sierra, Inc.

SCSI is based on an older interface developed by floppy disk drive manufacturer Shugart Associates and 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 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 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 and a maximum cable length of six meters. Faster versions came later with support for 16 devices and a 12-meter maximum cable length.

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
  • Ultra2 Wide SCSI: 80 MBps; connects 16 devices
  • Ultra3 SCSI: 160 MBps; connects 16 devices
  • Ultra-320 SCSI: 320 MBps; connects 16 devices
  • Ultra-640 SCSI: 640 MBps; connects 16 devices
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