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Hard disk drive interface
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Hard disk drive interface
Hard disk drives are accessed over one of a number of bus types, including parallel ATA (PATA, also called IDE or EIDE; described before the introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), 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, NVMe and Thunderbolt.
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:
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. 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:
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 (FM), Modified Frequency Modulation (MFM) and RLL encoding at frequencies for example ranging from 0.156 MHz (FM on 2311) to 7.5 MHz (RLL on ST412) 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 (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.
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.
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.
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Hard disk drive interface
Hard disk drives are accessed over one of a number of bus types, including parallel ATA (PATA, also called IDE or EIDE; described before the introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), 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, NVMe and Thunderbolt.
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:
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. 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:
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 (FM), Modified Frequency Modulation (MFM) and RLL encoding at frequencies for example ranging from 0.156 MHz (FM on 2311) to 7.5 MHz (RLL on ST412) 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 (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.
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.
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.