When you compare the performance and capabilities of ATA (AT attachment, sometimes also known as IDE for integrated drive electronics) interface and SCSI interface drives, you must consider several factors.

These two types of drives are the most popular drives used in PC systems today, and a single manufacturer might make seemingly identical drives in both interfaces. Deciding which drive type is best for your system is a difficult decision that depends on many factors.

In most cases, you will find that an ATA drive using the same head-disk assembly as a SCSI drive from the same manufacturer performs about the same or outperforms the equivalent SCSI drive at a given task or benchmark. This is particularly true when using a single disk drive with a single-user operating system, such as Windows 9x/Me.

More powerful operating systems, such as Windows NT, Windows 2000, or Windows XP, can more effectively use the command queuing and other features of SCSI, thus improving performance over ATA—especially when supporting multiple drives.

It is interesting to see that SCSI really evolved from ATA, or you could say that both evolved from the ST-506/412 and ESDI interfaces that were once used.

SCSI Hard Disk Evolution

SCSI is not a disk interface, but a bus that supports SCSI bus interface adapters connected to disk and other device controllers. The first SCSI drives for PCs were standard ST-506/412 or ESDI drives with a separate SCSI bus interface adapter (sometimes called a bridge controller) that converted the ST-506/412 or ESDI interfaces to SCSI.

This interface originally was in the form of a secondary logic board, and the entire assembly often was mounted in an external case. The next step was to build the SCSI bus interface "converter" board directly into the drive's own logic board. Today, we call these drives embedded SCSI drives because the SCSI interface is built in.

At that point, there was no need to conform to the absolute specifications of ST-506/412 or ESDI on the internal disk interface because the only other device the interface ever would have to talk to was built in as well.

Thus, the disk-interface and controller-chipset manufacturers began to develop more customized chipsets that were based on the ST-506/412 or ESDI chipsets already available but offered more features and higher performance.

Today, if you look at a typical SCSI drive, you often can identify the chip or chipset that serves as the disk controller on the drive as being exactly the same kind that would be used on an ST-506/412 or ESDI controller, or as some evolutionary customized variation thereof.

Consider some examples. An ATA drive must fully emulate the system-level disk-controller interface introduced with the Western Digital WD1003 controller series IBM used in the AT. These drives must act as though they have a built-in ST-506/412 or ESDI controller; in fact, they actually do.

Most of these built-in controllers have more capabilities than the original WD1003 series (usually in the form of additional commands), but they must at least respond to all the original commands that were used with the WD1003. If you follow the hard-drive market, you usually will see that drive manufacturers offer most of their newer drives in both ATA and SCSI versions.

In other words, if a manufacturer makes a particular 20GB ATA drive, you invariably see that the company also makes a SCSI model with the same capacity and specifications, which uses the same head disk assembly (HDA) and even looks the same as the ATA version.

The conventional wisdom has been that SCSI always is much faster than ATA; unfortunately, this wisdom is usually wrong. This incorrect conclusion was derived by looking at the raw SCSI and ISA bus performance capabilities. A 16-bit Ultra4 SCSI drive can transfer data at a claimed 320MBps, whereas an UltraATA/133 drive can transfer data at 133MBps.

Based on these raw transfer rates, SCSI seems to be faster, but the raw transfer rate of the bus is not the limiting factor. As discussed previously, the actual HDA and disk-controller circuitry place limits on this performance. The key figure to check is what is reported as the internal or media transfer rate for the drive.

Performance

ATA drives currently are used in most PC configurations on the market because the cost of an ATA drive implementation is low and the performance capabilities are high. Serial ATA drives also compare favorably in cost and performance to SCSI.

In comparing any given ATA and SCSI drive for performance, you have to look at the capabilities of the HDAs involved. To minimize the variables in this type of comparison, it is easiest to compare ATA and SCSI drives from the same manufacturer that also use the same HDA.

You will find that in most cases, a drive manufacturer makes a given drive available in both ATA and SCSI forms. For example, most hard drive companies make similar SCSI and ATA drives that use identical HDAs and that differ only in the logic board. The ATA version has a logic board with a built-in disk controller and a direct AT bus interface.

The SCSI version has the same built-in disk controller and bus interface circuits and also an SBIC chip. The SBIC chip is a SCSI adapter that places the drive on the SCSI bus. In essence, virtually all SCSI drives actually are ATA drives with the SBIC chip added.