A SCSI host adapter is a device used to connect one or more other SCSI devices to a computer bus. It is commonly called a SCSI controller, which is not strictly correct, as all SCSI devices have a SCSI controller built into them: the difference between a host adapter and another SCSI device such as a hard disk drive or CD-ROM is that the host adapter is responsible for transferring data between the SCSI bus and the computer's input/output bus.

Most modern SCSI host adapters are 32-bit or 64-bit PCI cards. Older ones were based on the 16-bit ISA bus or the transitional 32-bit VESA and EISA 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.

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 drives and SCSI scanners. 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 burners) removed one major use, and the advent of USB 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 servers). These are almost always quite expensive, around $US200 or more. The advent of serial ATA 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.

Many companies have manufactured SCSI host adapters at one time or another, but the market structure has been remarkably stable over the years. At any given time, the host adapter market usually comprises the largest single manufacturer and undisputed market leader, Adaptec, plus a major competitor which offers broadly similar products at a lower price, and several lesser-known names. Adaptec has regularly bought out its major competitors and has maintained its market- leading stance ever since Western Digital sold off its hard drive controller division to concentrate on hard drives in 1991.

Currently, the main competition to Adaptec is Symbios Logic, which supplies SCSI controller chipsets to a number of companies to make host adapters based on them, notably Tekram. In 1998 Adaptec moved to buy out Symbios Logic as well, but abandoned the move after the US Federal Trade Commission indicated that it would oppose the acquisition on competitive grounds.