A SCSI host adapter is a device used to connect one or more other SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

 devices to a computer bus

Computer bus

In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

. It is commonly called a SCSI controller

Controller (computing)

In computing and especially in computer hardware, controller is a chip, an expansion card, or a stand-alone device that interfaces with a peripheral device...

, which is not strictly correct as any component understanding the SCSI protocol can be called a controller. In this sense all SCSI devices have a SCSI controller built into them, while host adapter

Host adapter

In computer hardware, a host controller, host adapter, or host bus adapter connects a host system to other network and storage devices...

s (unlike, for example, a hard disk

Hard disk

A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

 or CD-ROM

CD-ROM

A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....

) bear responsibility for transferring data between the SCSI bus and the computer's input/output bus.

Most modern SCSI host adapters are PCI

Peripheral Component Interconnect

Conventional PCI is a computer bus for attaching hardware devices in a computer...

 cards, either 32-bit or 64-bit. Older ones were based on the 16-bit ISA

Industry Standard Architecture

Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...

 bus or the transitional 32-bit VESA

VESA Local Bus

The VESA Local Bus was mostly used in personal computers. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.-Historical overview:In the early 1990s, the I/O bandwidth of...

 and EISA

Extended Industry Standard Architecture

The Extended Industry Standard Architecture is a bus standard for IBM PC compatible computers...

 buses. It is not uncommon for a SCSI host adapter to be built into a PC motherboard as an integral part, however this typically makes the board quite expensive and it is more usual to retain the SCSI host adapter as a separate component which can be easily replaced or transferred to another machine.

Types

SCSI host adapters traditionally fall into two broad classes:

• Simple, relatively low-performance cards that provide an inexpensive way to connect SCSI devices that do not demand a great deal of bandwidth: notably CD-ROM
  CD-ROM
  A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
  
   drives and SCSI scanner
  Image scanner
  In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...
  
  s. The cost varies but is typically in the order of $US40 or so. These were very common up until about 2000, but are now fairly rare: the demise of the ISA slot added a little to the cost; the great improvement in IDE technology (particularly in the case of CD-ROM
  CD-ROM
  A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
  
   drives and CD recorders) removed one major use, and the advent of USB
  Universal Serial Bus
  USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
  
   made the expense of a SCSI interface unnecessary for most scanners.

• High-end cards designed for maximum performance, usually for very high-speed hard drives, and especially for situations that require several high-speed drives (notably server
  Server (computing)
  In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
  
  s). These are almost always quite expensive, around $US200 or more. The advent of serial ATA
  Serial ATA
  Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
  
   is expected to reduce the usage of high-end SCSI host adapters, but there is no current expectation that they will disappear in the near future.

Manufacturers

Many companies have manufactured SCSI host adapters at one time or another, but the market structure has been remarkably stable over the years. Two leading manufacturers are Adaptec

Adaptec

Adaptec is a computer hardware brand owned by PMC-Sierra that is used on some of its host adapters for connecting storage devices to computers. The production line of Adaptec is in Indonesia. Products are made to interface with SCSI, Serial ATA, and Serial attached SCSI. Some of its host adapters...

 and LSI Logic.