An Overview of the IDE Interface
| The interface used to connect hard disk and optical drives to a modern PC is typically called IDE (Integrated Drive Electronics); however, I always like to point out that the true 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 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 in to the drive and is not a separate board, as 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 in to the drive, whereas ATA refers to the specific implementation of IDE that is used in most PCs. Over the years, ATA has been used to connect not only hard disks, but also optical (CD and DVD) drives, high-capacity floppy drives such as SuperDisk and Zip, and some 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. This chapter covers the standard parallel version of ATA as well as the newer Serial ATA interfaces in detail; it also briefly mentions the original interfaces from which ATA evolved. Because the ATA interface is directly integrated into virtually all motherboard chipsets, ATA is the primary storage device interface used by 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, was officially introduced in late 2000 and was adopted in desktop systems starting in 2003 and in laptops starting in late 2005. Serial ATA (SATA) sends 1 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. SATA is a completely new and updated physical interface design, while remaining compatible on the software level with Parallel ATA. Throughout this book, ATA refers to either just the parallel or both the parallel and serial versions, whereas Parallel ATA (PATA) refers specifically to the parallel version and Serial ATA (SATA) refers specifically to the serial version. Note Even Apple recognized the value of ATA and began incorporating it into virtually all modern Macintosh systems released from 1995 to the present (before that, Apple used a semiproprietary version of SCSI). In fact, Apple has quietly adopted many other industry standards taken directly from the PC, including x86 processors, PCI Express, PCI, AGP, USB, and both SDR and DDR SDRAM, to name a few. Precursors to IDEA variety of hard disk interfaces has been used for PC hard disks over the years. 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. 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 PCs over the years, as shown in Table 7.1.
Of these interfaces, only ST-506/412 and ESDI are what you could call true disk-controllertodrive 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 Serial ATA are system-level interfaces that usually internally incorporate a chipset-based controller interface. For example, many SCSI, parallel 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 PATA and Serial ATA have a more direct connection from the controller to the AT bus attachment interface. Despite their differences, we call a SCSI, ATA, or 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 parallel ATA, Serial ATA, or SCSI interfaces to connect to a system. IDE OriginsAny 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 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 desirednot 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 is the origin of IDE. IDE VariationsThere have been four main types of IDE interfaces based on three bus standards:
Of these, only the parallel and Serial ATA versions are used today. ATA and Serial ATA have evolved with newer, faster, and more powerful versions. The newer 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 parallel ATA has hit the end of the evolutionary road with ATA-7, Serial ATA picks up where parallel ATA leaves off and offers greater performance, higher reliability, easier installation, lower cost, and 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. Note It is important to note that only the ATA IDE interface has been standardized by the industry. The XT and MCA IDE interfaces never were adopted as industry-wide standards and never became very popular. These interfaces were only used from 1987 to 1993 and only in IBM PS/2 (and some early ThinkPad) systems. 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. Because only the parallel and serial ATA versions of IDE are in use today, they are what this chapter focuses on. Origins of ATAControl 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" 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, and 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 connectors were 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" ATA drives found in notebook 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.
Note Many people who use systems with ATA connectors on the motherboard believe that a hard disk controller is built in to their motherboards, but in a technical sense the controller is actually in the drive. Although the integrated ATA ports on a motherboard often are referred to as controllers, they are more accurately called host adapters (although you'll rarely hear this term). A host adapter can be thought of as a device that connects a controller to a bus. 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 AT Attachment 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 very 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 main reason it is so difficult to low-level format ATA drives. To work to full capability, the format 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, clouding the "standard" somewhat. Most ATA drive manufacturers publish their drive formatting/initialization software on their websites. Note Many people are confused about 16- versus 32-bit bus connections and 16-versus 32-bit hard drive connections. A PCI bus connection allows for a 32-bit (and possibly 64-bit in some versions) connection between the bus and the ATA host interface, which is typically in the motherboard chipset South Bridge or I/O Controller Hub (ICH) chip. However, the actual parallel ATA interface between the host connector on the motherboard and the drive (or drives) itself is only a 16-bit interface. Thus, in a parallel ATA drive configuration, you are still getting only 16-bit transfers between the drive and the motherboard-based host interface. Even so, the clock speeds of the ATA interface are high enough that one or two hard drives normally can't supply the controller enough data to saturate even a 16-bit channel. The same is true with Serial ATA, whichalthough it transmits only 1 bit at a timedoes so at extremely high speeds. As I noted at the start of this chapter, Parallel ATA is a 16-bit parallel interface that is slowly being phased out in favor of the serial interface of SATA. Serial ATA'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 used by SATA compare in size to those used by parallel ATA. Figure 7.1. Serial ATA data cables (lower right) are much smaller than those used by parallel ATA (upper right), whereas the power cables (left) are similar in size. The primary advantage of ATA drives over the older, separate controller-based interfaces and newer host bus interface alternatives, such as USB, SCSI, and IEEE 1394 (i.LINK or FireWire), is cost. Because the separate controller or host adapter is eliminated and the cable connections are simplified, ATA drives cost much less than a standard controller and drive combination.
In terms of performance, ATA drives are often some of the highest performance drives availablebut they can also be among the lowest performance drives. This apparent contradiction is a result of the fact that all ATA drives are different. You can't make a blanket statement about the performance of ATA drives because each drive is unique. The high-end models, however, offer performance equal or superior to that of any other type of drive available in a similar form factor. |
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