Hard disk drive interface


s are accessed over one of a number of bus types, including parallel ATA, Serial ATA, SCSI, Serial Attached SCSI, and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses with which they cannot communicate natively, such as IEEE 1394, USB, SCSI and Thunderbolt.

Disk interface families

Disk drive interfaces have evolved from simple interfaces requiring complex controllers to attach to a computer into high level interfaces that present a consistent interface to a computer system regardless of the internal technology of the hard disk drive. The following table lists some common HDD interfaces in chronological order:
Acronym or abbreviationMeaningDescription
SMDStorage Module DeviceBit serial data interface introduced by CDC
Standard interface for many mini-computers in the 1970s and 1980s.
SASIShugart Associates System InterfaceWord serial interface introduced by Shugart Associates circa 1978;
Evolved by ANSI into SCSI.
ST-506
ST-412
ST-412RLL		Bit serial data interfaces introduced by Seagate Technology beginning 1980.
Standard interfaces for most small HDDs in the 1980s and early 1990s.
SCSISmall Computer System InterfaceWord serial interface sponsored by ANSI and introduced in mid 1980s;
Standard interfaces for most enterprise HDDs in this century; superseded by SAS
ESDIEnhanced Small Disk InterfaceBit serial data interface sponsored by ANSI and first introduced by Maxtor in late 1980s.
A higher data rate follow on to the ST-506 family into the mid-1990s, superseded by SCSI
ATA AT AttachmentWord serial interface introduced in late 1980s by Conner Peripherals, later sponsored by ANSI; successor to ST-412/506/ESDI. Standard HDD interface on all but enterprise HDDs until superseded by SATA
SATASerial ATABit serial interface successor to PATA sponsored by ANSI and introduced in 2003.
Most common interface for all but enterprise HDDs.
SASSerial Attached SCSIBit serial interface successor to SCSI sponsored by ANSI and introduced in 2004.
Most common interface for enterprise HDDs.

Early interfaces

The earliest hard disk drive interfaces were bit serial data interfaces that connected an HDD to a controller with two cables, one for control and one for data. An additional cable was used for power, initially frequently AC but later usually connected directly to a DC power supply unit. The controller provided significant functions such as serial/parallel conversion, data separation, and track formatting, and required matching to the drive in order to assure reliability. Each control cable could serve two or more drives, while a dedicated data cable served each drive.
In bit serial data interfaces the data frequency, data encoding scheme as written to the disk surface and error detection all influenced the design of the supporting controller. Encoding schemes used included Frequency modulation, Modified Frequency Modulation and RLL encoding at frequencies for example ranging from 0.156 MHz to 7.5 MHz MHz. Thus each time the internal technology advanced there was a necessary delay as controllers were designed or redesigned to accommodate the advancement; this along with the cost of controller development led to the introduction of [|Word serial interfaces].
Enhanced Small Disk Interface was an attempt to minimize controller design time by supporting multiple data rates with a standard data encoding scheme; this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives were not downward compatible. ESDI disk drives typically also had jumpers to set the number of sectors per track and sector size.

Word serial interfaces

Historical Word serial interfaces connect a hard disk drive to a bus adapter with one cable for combined data/control. The earliest versions of these interfaces typically had an 8 bit parallel data transfer to/from the drive, but 16-bit versions became much more common, and there are 32 bit versions. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
Modern bit serial interfaces connect a hard disk drive to a host bus interface adapter with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.