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'' half-height CDC Wren II 40MB drive with an integrated WD controller and was initially used in the first Compaq 386 systems in 1986.

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 connection, which includes additional pins for power.

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.

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 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 that followed CDC, such as Conner Peripherals (which later merged with Seagate Technology), made proprietary changes to what had been done by CDC.

As a result, many older ATA drives from the late 1980s are very difficult to integrate into a dual-drive setup that has newer 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 formatter you are using typically must know the specific vendor-unique commands for rewriting sector headers and remapping defects. Unfortunately, these and other specific drive commands differ from OEM to OEM, clouding the "standard" somewhat. Most ATA drive manufacturers have formatting software available on their Web sites.

Standard ATA is a 16-bit parallel interface, meaning that 16 bits are transmitted simultaneously down the interface cable. A new interface called Serial ATA was officially introduced in late 2000 and is being adopted in systems starting in 2003.

Serial ATA (SATA) sends 1 bit down the cable at a time, enabling thinner and smaller cables to be used and providing higher performance due to the higher cycling speeds allowed. SATA is a completely new and updated physical interface design, while remaining compatible on the software level with Parallel ATA.

ATA refers to the parallel version, whereas Serial ATA is explicitly referenced as SATA. Figure 1 shows how the power and data cables used by SATA compare in size to those used by Parallel ATA.

Serial ATA power and data cables

The primary advantage of ATA drives over the older, separate controller-based interfaces and newer host bus interface alternatives, such as SCSI and IEEE-1394 (iLink 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 available—but 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 on the market for a single-user, single-tasking operating system.