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SCSI Host Adapters
Most IDE/ATA hard disks are controlled today by integrated IDE controllers that are built into the chipset on the motherboard. The SCSI interface is not, for the most part, controlled by built-in motherboard SCSI controllers, although some are and this is growing in popularity. Most systems require the addition of a special card which serves as the interface between the SCSI bus and the PC.
This device is called a SCSI host adapter, or alternately a host bus adapter (sometimes abbreviated HBA). It is sometimes called a SCSI controller or even just a SCSI card, though these are technically incorrect names. They are not accurate because SCSI is a systems-level interface, and every device on the bus has its own controller. Logically, the host adapter is just a SCSI device like any other. Its job is to act as the gateway between the SCSI bus and the internal PC's I/O bus. It sends and responds to commands and transfers data to and from devices on the bus and inside the computer itself. Since it is inside the PC, of course, the host adapter really isn't the same as the other devices on the bus--it's sort of a "first among equals", if you want to think about it that way. ;^)
Since SCSI is a very "intelligent" interface--meaning it has a lot of capabilities and the devices on it are able to interact in advanced ways--many SCSI host adapters have evolved rather exceptional capabilities, and can act in many ways to improve performance. In some ways, the host adapter is the key to good SCSI implementation in the PC, since no matter how advanced the peripherals are that you attach to the bus, everything goes through that host adapter.
In this section I take a reasonably detailed look at host adapters. I focus primarily on the most important attributes and features that someone considering a SCSI setup might look for. This includes a discussion of device support, interfacing, connectors, resources and compatibility.
Tip: Motherboard support
for SCSI is actually on the rise, especially in higher-end systems, as SCSI becomes more
"mainstream". It is still not common to find it in most motherboards because it
increases cost, and most people still are not using SCSI. If you are building a new PC and
want to go with SCSI, consider a motherboard with an integrated SCSI host adapter. When
selecting such a motherboard, however, it is critical to pay specific attention to
what SCSI transfer modes and feature sets the motherboard will
support. While most built-in SCSI controllers can be disabled, having to buy a SCSI host
adapter six months after you buy a SCSI-capable motherboard--because the motherboard-based
controller doesn't do what you need it to--is just a waste of time and money.