page_title: Hard disk drive interface
section_title: Word serial interfaces
hierarchical_section_title: Hard disk drive interface / Word serial interfaces
caption_attribution_description: English: Several PATA hard disk drives.
context_page_description: Hard disk drives 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.
context_section_description: Historical Word serial interfaces connect a hard disk drive to a bus adapter with one cable for combined data/control. (As for all early interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.) 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.
  CTL-I (Controller Interface) was an 8-bit word serial interface introduced by IBM for its mainframe hard disk drives beginning with the 3333 in 1972.  The 3333 was the first unit in a string of up to eight 3330 type hard disk drives; it contained a CTL-I controller and two 3330 type disk drives.  Subsequently, the first drive (containing a CTL-I controller) in a string of drives was designated by IBM as an A-unit.  The drives within an A-unit and all other drives in a string had interfaces similar to the early interfaces, above.  A-units connected to IBM Directors or integrated attachments.
  Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, is an early (circa 1978) industry standard interface explicitly deployed to minimize system integration efforts. SCSI disks became standard on servers and workstations. Commodore Amiga, and Apple Macintosh deployed SCSI drive through the mid-1990s, by which time most models had been transitioned to ATA (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind ATA disk technology, though the highest-performance disks are still available in SCSI, SAS and Fibre Channel only. The range limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended (common mode) data transmission, but server class SCSI could use differential transmission, either low voltage differential (LVD) or high voltage differential (HVD). ("Low" and "High" voltages for differential SCSI are relative to SCSI standards and do not meet the meaning of low voltage and high voltage as used in general electrical engineering contexts, as apply e.g. to statutory electrical codes; both LVD and HVD use low voltage signals (3.3 V and 5 V respectively) in general terminology.)
  Parallel ATA, originally IDE and then standardized under the name AT Attachment (ATA), with the alias P-ATA or PATA retroactively added upon introduction of the new variant Serial ATA. The original name (circa 1986) reflected the integration of the controller with the hard drive itself. (That integration was not new with IDE, having been done a few years earlier with SCSI drives.) Moving the HDD controller from the interface card to the disk drive helped to standardize the host/contoller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively swifter versions of this standard ultimately added the requirement for an 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80-conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
caption: Several Parallel ATA hard disk drives
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