Advanced Technology Attachment

AT Attachment

Advanced Technology Attachment (ATA) is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. The standard is maintained by X3/INCITS committee T13. Many synonyms and near-synonyms for ATA exist, including abbreviations such as IDE and ATAPI. Also, with the market introduction of Serial ATA in 2003, the original ATA was retroactively renamed Parallel ATA (PATA).

Parallel ATA standards allow cable lengths up to only 18 inches (46 centimetres) although cables up to 36 inches (91 cm) can be readily purchased. Because of this length limit, the technology normally appears as an internal computer storage interface. It provides the most common and the least expensive interface for this application.

History

The name of the standard was originally conceived as PC/AT Attachment as its primary feature was a direct connection to the 16-bit ISA bus then known as 'AT bus'; the name was shortened to an inconclusive "AT Attachment" to avoid possible trademark issues.

An early version of the specification, conceived by Western Digital in late 1980s, was commonly known as Integrated Drive Electronics (IDE) due to the drive controller being contained on the drive itself as opposed to the then-common configuration of a separate controller connected to the computer's motherboard -- thus making the interface on the motherboard a host adapter, though many people continue, by habit, to call it a controller.

Enhanced IDE (EIDE) -- an extension to the original ATA standard again developed by Western Digital -- allowed the support of drives having a storage capacity larger than 528 megabytes (504 mebibytes), up to 8.4 gigabytes. Although these new names originated in branding convention and not as an official standard, the terms IDE and EIDE often appear as if interchangeable with ATA. This may be attributed to the two technologies being introduced with the same consumable devices -- these "new" ATA hard drives.

The interface at first worked only with hard disks, but eventually an extended standard came to work with a variety of other devices -- generally those using removable media. Principally, these devices include CD-ROM and DVD-ROM drives, tape drives, and large-capacity floppy drives such as the Zip drive and SuperDisk drive. The extension bears the name AT Attachment Packet Interface (ATAPI), which started as non-ANSI SFF-8020 standard developed by Western Digital and Oak Technologies, but then included in the full standard now known as ATA/ATAPI starting with version 4. Removable media devices other than CD and DVD drives are classified as ARMD (ATAPI Removable Media Device) and can appear as either a floppy or a hard drive to the operating system.

The move from programmed input/output (PIO) to direct memory access (DMA) provided another important transition in the history of ATA. As every computer word must be read by the CPU individually, PIO tends to be slow and use a lot of CPU resources. This is especially a problem on faster CPUs where accessing an address outside of the cacheable main memory (whether in the I/O map or the memory map) is a relatively expensive process. This meant that systems based around ATA devices generally performed disk-related activities much more slowly than computers using SCSI or other interfaces. However, DMA (and later Ultra DMA, or UDMA) greatly reduced the amount of processing time the CPU had to use in order to read and write the disks. This is possible because DMA and UDMA allow the disk controller to write data to memory directly, thus bypassing the CPU.

ATA-6 introduced 48 bit addressing, increasing the limit to 128 PiB (or 144 petabytes). Some OS environments, including Windows 2000 until Service Pack 3, did not enable 48-bit LBA by default, so the user was required to take extra steps to get full capacity on a 160 GB drive.

Parallel ATA Interface cable and Socket

Until the introduction of Serial ATA, 40-pin connectors generally attached drives to a ribbon cable. Each cable has two or three connectors, one of which plugs into an adapter that interfaces with the rest of the computer system. The remaining one or two connectors plug into drives. Parallel ATA cables transfer data 16 bits at a time.

ATA's ribbon cables had 40 wires for most of its history, but an 80-wire version appeared with the introduction of the Ultra DMA/66 (UDMA4) mode. All of the additional wires in the new cable are ground wires, interleaved with the previously defined wires. The interleaved ground wire reduces the effects of capacitive coupling between neighboring signal wires, thereby reducing crosstalk. Capacitive coupling is more of a problem at higher transfer rates, and this change was necessary to enable the 66 megabytes per second (MB/s) transfer rate of UDMA4 to work reliably. The faster UDMA5 and UDMA6 modes also require 80-conductor cables.

80-wire cables usually come with three differently colored connectors (blue, gray & black) as opposed to uniformly colored 40-wire cable's connectors (all black).

Multiple devices on a cable

If two devices attach to a single cable, one is commonly referred to as a master and the other as a slave. The master drive generally appears first when the computer's BIOS and/or operating system enumerates available drives. If there is a single device on a cable, in most cases it should be configured as master. However, some hard drives have a special setting called single for this configuration.

Serial ATA

SATA is expected to eventually replace the older technology ( Parallel ATA or PATA). Serial ATA adapters and devices communicate over a high-speed serial link. There exists a point to point link between the host and the device and no sharing of cable is used.

Architecture

At the physical layer of the Serial ATA architecture, the data-connection is formed by two pairs of (unidirectional) signal wires. Over these wires, SATA uses Low Voltage Differential Signaling (LVDS), enabling much higher (per-wire) signalling rates (1.5 Gbit/s and up) than traditional parallel ATA. Byte data is encoded and transmitted using 8B/10B encoding, which is also used in Ethernet, Fibre Channel, PCI Express, etc. The switch from a parallel to serial electrical scheme facilitates future upgrades to performance, and lowers costs (compared to a comparably fast parallel-interface.). Vendor implementations may include additional functionality (such as simulated RAID) above and beyond the SATA specification, but require device-specific