Hard disk drive interface

The earliest hard disk drive (HDD) interfaces were bit serial data interfaces that connected an HDD to a controller with two cables, one for control and one for data.[a] 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 (after formatting) in order to assure reliability. Each control cable could serve two or more drives, while a dedicated (and smaller) data cable served each drive.

Examples of such early interfaces include:

• Many early IBM drives, e.g., IBM 2311,^[1] had such an interface.
• The SMD interface was popular on minicomputers in the 1970s.
• MFM
  (Modified Frequency Modulation) for the data encoding method.
• ST412, an
  RLL
  (Run Length Limited) encoding variants.
• Enhanced Small Disk Interface (ESDI) was an industry standard interface similar to ST412 supporting higher data rates between the processor and the disk 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

Enhanced Small Disk Interface (ESDI) 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 (i.e. a 15 or 20 megabit disk drive would not run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

Word serial interfaces

Historical Word serial interfaces connect a hard disk drive to a bus adapter^[b] 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)^[3] was an 8-bit word serial interface introduced by IBM for its mainframe hard disk drives beginning with the 3333 in 1972.^[4] 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.^[5] 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.
• 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.)

• 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 crosstalk
  . 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.

Bit serial interfaces

Modern bit serial interfaces connect a hard disk drive to a host bus interface adapter (today in a PC typically integrated into the "south bridge") with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.

• DECs Standard Disk Interconnect (SDI) was an early example of a modern bit serial interface.
• 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 networks (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 fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers
  .

• FireWire, and differential SCSI
  .
• 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 identical 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 hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

Notes

1. IBM 2305
2. ^ Today typically integrated but separate boards or boxes in early embodiments

References

1. ^ IBM 2311 Field Engineering Theory of Operation, October 1967, Chapter 3 and Fig. 3-1
2. ^ "Reed Solomon Codes – Introduction"
3. ^ IBM 3880 Storage Control,Models 1, 2, 3, and 4 Description Manual, GA26-1661-9. September 1987
4. ^ Via 'New Attachment Strategy' IBM Meant to Frustrate PCMs
5. ^ IBM 3990 Storage Control Reference: GLOSSARY, GA32-0099-06, © Copyright IBM Corp. 1988, 1994
6. ^ “Intelligent systems interface eases peripheral integration,” H. Meyer & J. Korpi, Electronic Design, August 20, 1981, pp. 97-103

External links

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• Computer History Museum's HDD Working Group Website
• HDD Tracks and Zones
• HDD from inside
• Hard Disk Drives Encyclopedia
• Video showing an opened HD working Archived 2012-09-14 at the Wayback Machine
• Average seek time of a computer disk
• What to consider before buying a hard disk drive