The AT Attachment interface is the original standard for connecting magnetic hard drives and optical drives that contain integrated drive electronics (IDE drive controllers) to a computer system bus. The original Advanced Technology Attachment (ATA) as developed by IBM is an industry-standard 98-pin, 16-bit ISA bus interface used to connect intelligent hard drives, CD/DVD drives to the computer motherboard bus. That standard has been revised and simplified over the years.
Today's standard, governed by Technical Committee 13 ( http://www.t13.org/ ) specifies a 40-pin, 80 conductor physical interface that supports the ATA in both Parallel and Serial configurations at speeds up to 3GB per second.
The first microcomputers manufactured by IBM were called "Advanced Technology" personal computers, or ATs. The AT Attachment interface connects a disk drive with an integrated drive controller to the computer's system bus. The ATA interface has been through several revisions.
Parallel ATA
The original ATA interface, introduced around 1986, was actually a 16-bit parallel interface transmitting 16 bits simultaneously in parallel down the bus to the drive. The name Parallel-ATA (PATA) is now used to differentiate drives using the 40-pin (36 pins for data, 4 pins for power), 80-conductor ATA connector supporting speeds up to 133MBps from drives using the newer and faster SATA connector.
Serial ATA
SATA was introduced in desktop computers around 2003 and I first started seeing SATA in laptop computers around 2005. Being serial, the SATA interface passes 1 bit at a time across the ATA bus. The SATA cables require fewer wires and are smaller and thinner than the parallel interface cables.
Integrated Drive Electronics (IDE)
Integrated Drive Electronics (IDE) is a marketing term created for disk drives with the ATA controller logic built into the drive and it's interface to the system bus. Placing the control logic for the disk on the disks themselves simplifies interfacing storage devices to the computer main bus. By building the controller logic into the drive, the computer only needs to know how to send simple commands and the drive, whatever type it is, executes whatever hardware actions are necessary to perform the requested action. Previous to the advent of ATA/IDE, computer systems used a separate controller card to manage access the drives and the controller cards had to be managed by the computer. Today, if a separate card is used, it is called a Host Bus Adapter card.
AT Attachment Packet Interface (ATAPI)
ATAPI is software that utlizes its own form of commands from the SCSI/ASPI bus interface that allowed drive manufacturers adapt their SCSI designs to the ATA interface.