The interface used to connect a hard disk drive to a modern PC is typically called IDE (Integrated Drive Electronics). An interesting fact is that the true name of the interface is ATA (AT Attachment), which refers to the fact that this interface originally was designed to connect a combined drive and controller directly to the bus of the 1984 vintage IBM AT (Advanced Technology) computer, otherwise known as the ISA (Industry Standard Architecture) or AT bus.

IDE is a term originated by the marketing departments of some drive manufacturers to describe the drive/controller combination used in drives with the ATA interface. Integrated Drive Electronics refers to the fact that the interface electronics or controller is built in to the drive and is not a separate board, as with earlier drive interfaces.

Although technically the correct name for the type of IDE interface we most commonly use is ATA, many persist in using the IDE designation today. If you are being picky, you could say that IDE refers generically to any drive interface in which the controller is built in to the drive, whereas ATA refers to the specific implementation of IDE that is used in most PCs.

Today, ATA is used to connect not only hard disks, but also CD and DVD drives, high-capacity SuperDisk floppy drives, and tape drives. Even so, ATA is still thought of primarily as a hard disk interface.

And it evolved directly from the separate controller and hard drive interfaces that were used prior to ATA. Because the ATA interface is directly integrated into virtually all motherboard chipsets, ATA is the primary storage interface used by most PCs.

Precursors to IDE

A variety of hard disk interfaces have been available for PC hard disks over the years. As time has passed, the number of choices has increased, and many older designs are no longer viable in newer systems. The primary job of the hard disk controller or interface is to transmit and receive data to and from the drive.

The various interface types limit how fast data can be moved from the drive to the system and offer different features as well as levels of performance. If you are putting together a system in which performance is a primary concern, you need to know how these various interfaces affect performance and what you can expect from them.

Many statistics that appear in technical literature are not indicative of the real performance figures you will see in practice. I will separate the myths presented by some of these overly optimistic figures from the reality of what you will actually see. Several types of hard disk interfaces have been used in PC systems over the years:

• ST-506/412 - 1978–1989 (obsolete)
• ESDI - 1983–1991 (obsolete)
• Non-ATA IDE - 1987–1993 (obsolete)
• SCSI - 1986–present
• ATA (IDE) - 1986–present
• Serial ATA - 2003–present

Of these interfaces, only ST-506/412 and ESDI are what you could call true disk-controller–to–drive interfaces, and they are obsolete. Non-ATA versions of IDE were used primarly in the IBM PS/2 systems and are obsolete.

Current SCSI, ATA, and Serial ATA are system-level interfaces that usually incorporate a chipset-based controller interface internally. For example, most SCSI, ATA, and Serial ATA drives incorporate the same basic controller circuitry inside the actual drive.

The SCSI interface then adds another layer that connects between the drive controller and the PCI (or ISA) bus, whereas ATA and Serial ATA have a more direct controller to the AT bus attachment interface.

Despite their differences, we call a SCSI, an ATA, or a Serial ATA card a host interface adapter instead of a controller card because the actual controllers are inside the drives. Virtually all modern disk drives use ATA, Serial ATA, or SCSI interfaces to connect to a system.

IDE Origins

Any drive with an integrated controller could be called an IDE drive, although normally when we say IDE, we really mean the specific version of IDE called ATA. No matter what you call it, combining the drive and controller greatly simplifies installation because there are no separate power or signal cables that run from the controller to the drive.

Also, when the controller and drive are assembled as a unit, the number of total components is reduced, signal paths are shorter, and the electrical connections are more noise-resistant. This results in a more reliable and less expensive design than is possible when a separate controller, connected to the drive by cables, is used.

Placing the controller, including the digital-to-analog encoder/decoder (endec), on the drive offers an inherent reliability advantage over interfaces with separate controllers such as ST506 and ESDI. Reliability is increased because the data encoding, from digital to analog, is performed directly on the drive in a tight noise-free environment.

The timing-sensitive analog information does not have to travel along crude ribbon cables that are likely to pick up noise and insert propagation delays into the signals. The integrated configuration enables increases in the clock rate of the encoder and the storage density of the drive.

Integrating the controller and drive also frees the controller and drive engineers from having to adhere to the strict guidelines imposed by the earlier interface standards. Engineers can design what essentially are custom drive and controller implementations because no other controller will ever have to be connected to the drive.

The resulting drive and controller combinations can offer higher performance than earlier standalone controller and drive setups. IDE drives sometimes are called drives with embedded controllers.

The earliest IDE drives were called hardcards and were nothing more than hard disks and controllers bolted directly together and plugged into a slot as a single unit. Companies such as the Plus Development Division of Quantum took small 3 1/2'' drives (either ST-506/412 or ESDI) and attached them directly to a standard controller.

The assembly then was plugged into an ISA bus slot as though it were a normal disk controller. Unfortunately, the mounting of a heavy, vibrating hard disk in an expansion slot with nothing but a single screw to hold it in place left a lot to be desired—not to mention the possible interference with adjacent cards because many of these units were much thicker than a controller card alone.

Several companies got the idea to redesign the controller to replace the logic board assembly on a standard hard disk and then mount it in a standard drive bay just like any other drive. Because the built-in controller in these drives still needed to plug directly into the expansion bus just like any other controller, a cable was run between the drive and one of the slots. This is the origin of IDE.

IDE Interfaces Variations

There have been four main types of IDE interfaces based on three bus standards:

• Serial AT Attachment (SATA)

• Parallel AT Attachment (ATA) IDE (based on 16-bit AT-bus, also called ISA)

• XT IDE (based on 8-bit ISA, obsolete)

• MCA IDE (based on 16-bit Micro Channel, obsolete)

Of these, only the ATA versions are used today. ATA and Serial ATA have evolved with newer, faster, and more powerful versions. The improved versions of Parallel ATA are referred to as ATA-2 and higher. They are also sometimes called EIDE (Enhanced IDE), Fast-ATA, Ultra-ATA, or Ultra-DMA.

Even though ATA appears to have hit the end of the road with ATA-7, Serial ATA picks up where parallel ATA leaves off and offers greater performance plus an established roadmap for future upgrades.

In 1987, IBM developed its own Micro Channel Architecture (MCA) IDE drives for systems such as the PS/2 Model 70, and connected them to the bus through a bus adapter device called an interposer card.

These bus adapters (sometimes called paddle boards or angle boards) needed only a few buffer chips and did not require any real circuitry because the drive-based controller was designed to plug directly into the bus.

The paddle board nickname came from the fact that they resemble game paddle or joystick adapters, which do not have much circuitry on them. MCA IDE uses a non-standard 72-pin connector, and was designed for MCA bus systems only.

An 8-bit variation of IDE appeared in 8-bit ISA systems such as the PS/2 Model 30. The XT IDE interface used a 40-pin connector similar to, but not compatible with, the 16-bit version. Neither MCA nor XT versions of IDE became very popular, and they have been off the market for several years.

In most modern systems you will find at least two ATA connectors on the motherboard, and systems that offer ATA RAID have two additional ATA connectors which can be used for an ATA RAID array or for additional ATA drives running as independent devices.

If your motherboard does not have one of these connectors and you want to attach an ATA drive to your system, you can purchase an adapter card that adds an ATA interface (or two) to a system via the ISA or PCI bus slots. Some of the cards offer additional features, such as an onboard ROM BIOS or cache memory. Only the parallel and serial ATA versions of IDE are in use today.