Origins of IDE

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, as was the case with the early ST412, ST506, and ESDI interfaces used by PCs and early servers in the 1980s.

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 the ST506 and ESDI. Reliability is increased because the digital-to-analog data encoding 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 are sometimes 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 in to a slot as a single unit. Companies such as the Plus Development Division of Quantum attached a small 3.5-inch drive (either ST-506/412 or ESDI) directly to a standard controller. The drive/controller assembly was then plugged in to an ISA bus slot as though it were a normal disk controller card. To improve reliability and free up the expansion slot blocked by the relatively thick card-and-drive combination, 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 in to 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.