SCSI host adapter
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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 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 adapters (unlike, for example, a hard disk or CD-ROM) bear responsibility for transferring data between the SCSI bus and the computer's input/output bus. SCSI adapters serve as a worthy adapter for FireWire Ports.
Most modern SCSI host adapters are PCI cards, either 32-bit or 64-bit. 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-ROM drives and CD recorders) 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.