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Upgrading and Repairing PCs: The ATA/IDE Interface

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This chapter, from the 21st edition of Upgrading and Repairing PCs, provides a wealth of technical information about the IDE/ATA drive interface specification.

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This chapter is from the book

An Overview of the IDE Interface

The interface used to connect disk drives to a PC is typically called IDE (Integrated Drive Electronics); however, the official name of this interface is ATA (AT Attachment). The ATA designation refers to the fact that this interface was originally designed to connect a combined drive and controller directly to the 16-bit bus found in the 1984 vintage IBM PC-AT (Advanced Technology) and compatible computers. The AT bus is otherwise known as the ISA (Industry Standard Architecture) bus. Although ATA is the official name of the interface, IDE is a marketing term originated by some of the 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 into the drive and is not a separate board, as it was with earlier drive interfaces. Although the correct name for the particular IDE interface we most commonly use is technically 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 into the drive, whereas ATA refers to the specific implementation of IDE that is used in most PCs.

ATA was originally a 16-bit parallel interface, meaning that 16 bits are transmitted simultaneously down the interface cable. A newer interface called Serial ATA (SATA) was officially introduced in late 2000 and was adopted in desktop systems starting in 2003 and in laptops starting in late 2005. SATA sends one bit down the cable at a time, enabling thinner and smaller cables to be used, as well as providing higher performance due to the higher cycling speeds it enables. Although SATA is a completely different physical interface design, it is backward compatible on the software level with Parallel ATA (PATA). Throughout this book, ATA refers to both the parallel and serial versions. PATA refers specifically to the parallel version, and SATA refers specifically to the serial version.

Precursors to IDE

Several types of hard disk interfaces have been used for PC hard disks over the years, as shown in Table 7.1. As time has passed, the number of choices has increased; however, many of the older interface standards are obsolete and no longer viable in newer systems.

Table 7.1. PC Drive Interfaces


When Used


1978–1989 (obsolete)


1983–1991 (obsolete)


1987–1993 (obsolete)







ST = Seagate Technology

ESDI = Enhanced Small Device Interface

IDE = Integrated Drive Electronics

SCSI = Small Computer Systems Interface

ATA = AT (Advanced Technology) Attachment

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 primarily in the IBM PS/2 systems and are also obsolete. Current SCSI, ATA, and SATA are system-level interfaces that usually internally incorporate a chipset-based controller interface. For example, many SCSI, PATA, and SATA 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 PATA and SATA have a more direct connection from the controller to the AT bus attachment interface. Despite their differences, we call a SCSI, PATA, or SATA card a host interface adapter instead of a controller card because the actual controllers are inside the drives. Virtually all modern disk drives use SATA or PATA 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 no separate power or signal cables 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.

The earliest IDE drives were called hardcards and were nothing more than hard disks and controller cards 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-inch drives (either ST-506/412 or ESDI) and attached them directly to a standard controller card. The drive/controller assembly then was plugged into an ISA bus slot as though it were a normal disk controller card. 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 physical 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 was the origin of IDE.

Origins of ATA

Control Data Corporation (CDC; its disk drive division was later called Imprimis), Western Digital, and Compaq actually created what could be called the first ATA IDE interface drive and were the first to establish the 40-pin ATA connector pinout. The first ATA IDE drive was a 5 1/4-inch half-height CDC Wren II 40MB drive with an integrated WD controller and was initially used in the first Compaq 386 systems in 1986. I remember seeing this drive for the first time in 1986 at the fall COMDEX show. Besides the (at the time) unique 40-pin ribbon cable, I remember being surprised by the green activity LED on the front bezel. (Most drives up until then used red LEDs.)

Compaq was the first to incorporate a special bus adapter in its system to adapt the 98-pin AT-bus (also known as ISA) edge connector on the motherboard to a smaller 40-pin, header-style connector into which the drive would plug. The 40-pin connector was all that was necessary because it was known that a disk controller never would need more than 40 of the ISA bus lines. Smaller 2 1/2-inch ATA drives found in laptop computers use a superset 44-pin or 50-pin connection, which includes additional pins for power and configuration. The pins from the original ISA bus used in ATA are the only signal pins required by a standard-type AT hard disk controller. For example, because a primary AT-style disk controller uses only interrupt request (IRQ) line 14, the primary motherboard ATA connector supplies only that IRQ line; no other IRQ lines are necessary. Even if your ATA interface is integrated within the motherboard chipset South Bridge or I/O Controller Hub chip (as it would be in newer systems) and runs at higher bus speeds, the pinout and functions of the pins are still the same as the original design taken right off the ISA bus.

left-arrow.jpg See the Chapter 4 section, “Motherboard Connectors,” p. 228.

left-arrow.jpg See the Chapter 4 section, “The ISA Bus,” p. 245.

Eventually, the 40-pin ATA connector and drive interface design was placed before one of the ANSI standards committees that, in conjunction with drive manufacturers, ironed out some deficiencies, tied up some loose ends, and then published what was known as the CAM ATA (Common Access Method AT Attachment) interface. The CAM ATA Committee was formed in October 1988, and the first working document of the ATA interface was introduced in March 1989. Before the CAM ATA standard, many companies, such as Conner Peripherals (which later merged with Seagate Technology), made proprietary changes to the original interface as designed by CDC. As a result, many older ATA drives from the late 1980s are difficult to integrate into a dual-drive setup because minor differences in the interfaces can cause compatibility problems among the drives. By the early 1990s, most drive manufacturers brought their drives into full compliance with the official standard, which eliminated many of these compatibility problems.

Some areas of the ATA standard have been left open for vendor-specific commands and functions. These vendor-specific commands and functions are the reason it is important to use the OEM-specific programs for testing ATA drives. To work to full capability, the diagnostic program you are using typically must know the specific vendor-unique commands for remapping defects. Unfortunately, these and other specific drive commands differ from OEM to OEM, thus clouding the standard somewhat. Most ATA drive manufacturers publish their drive-formatting/initialization software on their websites.

As I noted at the start of this chapter, PATA is a 16-bit parallel interface that has been largely phased out in favor of the serial interface of SATA. SATA’s thinner and smaller cables provide higher performance due to the higher cycling speeds allowed and are considerably easier to work with than the wide PATA ribbon cables. Figure 7.1 shows how the power and data cables SATA uses compare in size to those PATA uses.

Figure 7.1

Figure 7.1. SATA data cables are much smaller than those used by PATA, whereas the power cables are similar in size.

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