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.

A PCI-based Adapter Wide Ultra2 SCSI host adapter.
Note the numerous connectors, which allow several chains
of internal and external devices to be attached to the host adapter. You
can see a 50-pin connector for internal devices on the right side near
the top, pointing straight out towards you. There are two high-density
Wide Ultra2 connectors on the top of the card. There is also one
high-density external connector on the expansion slot insert on the
left-hand side of the card. The PCI interface connector is on the bottom.

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.