Editing Hard disk drive (section)

== Access and interfaces ==
{{Main|Hard disk drive interface}}
[[File:Seagate ST33232A hard disk inner view.jpg|thumb|right|Inner view of a 1998 [[Seagate Technology|Seagate]] HDD that used the [[Parallel ATA]] interface]]
[[File:2.5-inch SATA drive on top of a 3.5-inch SATA drive, close-up of data and power connectors.jpg|thumb|right|2.5-inch SATA drive on top of 3.5-inch SATA drive, showing close-up of (7-pin) data and (15-pin) power connectors]]

Current hard drives connect to a computer over one of several [[bus (computing)|bus]] types, including parallel [[Advanced Technology Attachment|ATA]], [[Serial ATA]], [[SCSI]], [[Serial Attached SCSI]] (SAS), and [[Fibre Channel]]. Some drives, especially external portable drives, use [[IEEE 1394 interface|IEEE&nbsp;1394]], or [[USB]]. All of these interfaces are digital; electronics on the drive process the analog signals from the read/write heads. Current drives present a consistent interface to the rest of the computer, independent of the data encoding scheme used internally, and independent of the physical number of disks and heads within the drive.

Typically, a [[Digital signal processor|DSP]] in the electronics inside the drive takes the raw analog voltages from the read head and uses [[PRML]] and [[Reed–Solomon error correction]]<ref name="AutoMK-79" /> to decode the data, then sends that data out the standard interface. That DSP also watches the error rate detected by [[error detection and correction]], and performs [[bad sector]] remapping, data collection for [[Self-Monitoring, Analysis, and Reporting Technology]], and other internal tasks.

Modern interfaces connect the drive to the host interface with a single data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit. Older interfaces had separate cables for data signals and for drive control signals.
* [[Small Computer System Interface]] (SCSI), originally named SASI for Shugart Associates System Interface, was standard on servers, workstations, [[Amiga peripherals|Commodore Amiga]], [[Atari ST]] and [[Apple Macintosh]] computers through the mid-1990s, by which time most models had been transitioned to newer interfaces. The length limit of the data cable allows for external SCSI devices. The SCSI command set is still used in the more modern [[Serial Attached SCSI|SAS interface]].
* [[Integrated Drive Electronics]] (IDE), later standardized under the name [[AT Attachment]] (ATA, with the alias PATA ([[Parallel ATA]]) retroactively added upon introduction of SATA) moved the HDD controller from the interface card to the disk drive. This helped to standardize the host/controller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection transfers 16&nbsp;bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements led to an [[AT Attachment#Parallel ATA interface|"ultra DMA" (UDMA)]] mode using an 80-conductor cable with additional wires to reduce [[crosstalk]] at high speed.
* EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of [[direct memory access]] (DMA) to transfer data between the disk and the computer without the involvement of the [[CPU]], an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
* [[Fibre Channel]] (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the [[Fibre Channel Arbitrated Loop]] (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of [[storage area network]]s (SANs). Recently other protocols for this field, like [[iSCSI]] and [[ATA over Ethernet]] have been developed as well. Confusingly, drives usually use ''copper'' twisted-pair cables for Fibre Channel, not fiber optics. The latter are traditionally reserved for larger devices, such as servers or [[disk array controller]]s.
* [[Serial Attached SCSI]] (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically compatible data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA HDDs. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
* [[Serial ATA]] (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like [[EIA-422]]. That requires that data be transmitted serially. A similar [[differential signaling]] system is used in [[RS485]], [[LocalTalk]], [[USB]], [[FireWire]], and differential [[SCSI]]. SATA I to III are designed to be compatible with, and use, a subset of SAS commands, and compatible interfaces. Therefore, a SATA hard drive can be connected to and controlled by a SAS hard drive controller (with some minor exceptions such as drives/controllers with limited compatibility). However, they cannot be connected the other way round&mdash;a SATA controller cannot be connected to a SAS drive.