ARCHIVED: For the PC, what are the various interface standards, and what are their specifications?

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Current standards

ATA/ATAPI-4

Fourth ANSI-accepted standard, published in 1998 under NCITS 317-1998; also known as UDMA/33, ATA-33, and UltraATA/33
Allowed a transfer rate of up to 33MBps (MB per second) by introducing UDMA (Ultra Direct Memory Access) 2 (also called UDMA/33).
Officially integrated ATAPI into the standard
Introduced the 80-pin drive cable, which was the standard 40-pin IDE cable with 40 ground wires added individually between each pair of standard wires. This was thought necessary to achieve the 33MBps speed, since the ground wires helped eliminate the crosstalk interference induced by the proximity of the individual wires, but this was not the case. This cable standard became necessary in the following standards.
The cables were no longer reversible but had "motherboard" and "drive" ends. Previously, IDE cables would work when reversed and would still meet the standard. (In practice, the placement of the plugs and the existence of the "cable select" mode [which designated a drive as master or slave not by drive settings but by its placement along the ribbon cable] usually made reversing the cable impractical.)
All other characteristics match ATA-3

ATA/ATAPI-5

Fifth ANSI-accepted standard, published in 2000 under NCITS 340-2000
Also referred to as Ultra ATA/66 in some marketing literature
Allowed a transfer rate of up to 66MBps
Majority of drives manufactured to this standard fell between 20 and 120GB; smaller drives were not produced due to lack of demand. ATA-5 supports up to 128GB disk capacity; as with previous standards, the limitation is due to the standard's addressing scheme. Unlike previous standards, ATA-5 drives were produced that actually reached that capacity. This is the last standard to have the 128GB limit.
Used the 80-pin cable introduced in ATA-4; the 80-pin cable was required for achieving the full transfer rate. ATA/ATAPI-5 drives will work with a 40-pin cable, but will transfer data at only 33.3MBps.

Ultra 100

Marketing term, not an ANSI standard; used by Seagate in anticipation of the ATA-6 standard
Basically an ATA/ATAPI-5 drive that included the UDMA mode 5, which allowed for 100MBps transfer rates

ATA/ATAPI-6

Sixth ANSI-accepted standard, published in 2002 under NCITS 361-2002
Allowed a transfer rate of up to 100MBps
Allows 48-bit addressing, which increases the theoretical disk size limit to 144 petabytes (PB; a Petabyte is one million gigabytes). Most drives currently adhering to this standard are 100GB or larger; drives smaller than that tend to be laptop/notebook/portable 2.5" drives. Those 2.5" ATA-6 drives can be as small as 20GB, although the 60 to 80GB capacity drives are the most common. In the full-sized hard drives, even 80GB is considered small, with 100 to 250GB hard drives becoming increasingly common.
Introduced acoustic Management technologies, which allow users to select between higher performance or less noise from the hard drive

ATA/ATAPI-7

Seventh ANSI-accepted standard, published in 2005 under NCITS 397-2005
Allows transfer rates of up to 133MBps
Also uses 48-bit addressing, which sets a theoretical maximum capacity of 144PB. Practical capacities exceed the 300GB range, with even 500GB single drives being available.

Serial ATA (SATA)

Not a separate standard so much as a specific implementation of the previous one. Volume 3 of the ATA/ATAPI-7 standard specified how to implement a serial transport version of itself, now known as Serial ATA (SATA). The technical name for this implementation is Serial Transport Protocols and Physical Interconnect for AT Attachment devices; it's also known as SATA 1.0 (or SATA I). Other informal names for the standard are U150 and SATA150.
Currently allows transfer rates of up to 150MBps
Still uses 48-bit Logical Block Addressing (LBA), like ATA/ATAPI-7, so the theoretical maximum capacity is still the same
While some drives can be as small as 30GB+, (Western Digital's 36.7GB Raptor [model wd360gd], for example), most drives being manufactured are bigger than 100GB. As of July 2006, the largest SATA hard drive being produced is Seagate's 750GB Barracuda (model ST3750640AS).
Uses a 7-pin serial cable rather than a 40-pin ribbon cable, so is not physically backward-compatible with previous ATA standards. For side-by-side images, visit:
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The standard is backward compatible in terms of software drivers, since it is simply a specific implementation of the ATA/ATAPI-7 standard. So the plugs are different, but the drivers and the way they integrate into the operating system are still the same.
While the cables and connectors have changed for this standard, for desktop computers the drives themselves still adhere to the 3.5" form factor.
The move to the serial standard developed for many reasons, among them the problems with crosstalk interference arising from the parallel wire configuration of the previous standards.

ATA/ATAPI-8

As of July 2006, still in development

eSATA

External SATA; attempt to extend the SATA standard to external drives and compete with USB 2.0 and FireWire

SATA II/2.0

Both the term for the organization authoring the next SATA specification, and the informal name attached to the 3.0GBps implementation of SATA

Older standards still in use

EIDE

Enhanced IDE (EIDE) was developed by Western Digital in anticipation of the ATA-2 standard. Like Fast ATA, this standard is not recognized by ANSI, but is a manufacturer-specific standard.
The standard contains all the specifications of ATA-2, plus ATAPI, so transfer rates and maximum capacities are the same as ATA-2 (in the beginning days of the standard) and ATA-3 (after other specifications were added later on).
Due to its addition of LBA, EIDE could support hard drives up to 128GB. However, hard drives approaching this size were not available until well after the introduction of the standard.
Western Digital continually adds ANSI standards to the definition of EIDE, so that eventually EIDE ended up adhering to the ATA-3 (aka "Ultra ATA") specifications as well.

ATAPI

AT Attachment Packet Interface; mainly for CD-ROM and tape backup drives. This added additional CD-ROM and tape backup commands to the standard ATA command set, which allowed the use of those drives on the same ATA hard drive interfaces. This standard is an addition to ATA/IDE, and never existed as a stand-alone interface. Since it was a standard for removable media drives, storage capacities are not associated with the standard, and neither are data transfer rates.

ATA-2

The second ANSI-accepted standard, published in 1996 as ANSI standard X3.279-1996, "AT Attachment Interface with Extensions". Developed to deal with the growing demand for more size and speed from hard drives, and as a response to manufacturer-specific extensions to the original ATA standard to provide a universal, official, manufacturer-independent standard
Can achieve a transfer rate of up to 16.6MBps
Supported disk capacities up to 128GB. However, given BIOS as well as operating system limitations, disks were not produced in sizes approaching this limit. For that reason, during the beginning of the standard, many mistakenly thought that the ATA-2/EIDE/Fast ATA disk size limits were as low as 8GB (an early BIOS barrier, the next one encountered after the 504MB and 2.1GB limits affecting ATA-1 and IDE) and as high as 32GB (the maximum partition size FAT32 is capable of). Those limits were practical ones imposed by other components, not a limitation of the ATA-2 standard itself.
Added many new features, such as:
- Commands allowing block transfers, a method that allows the system to group read/write commands into a single mass request to the CPU
- PIO modes 3 and 4 and multiword DMA modes 1 and 2
- LBA; this feature originally appeared in IDE, but its appearance here made it an official part of the ANSI standard
- Improved "identify drive" command, which makes the hard drive provide info about its geometry, number of heads and cylinders, and other physical properties
Continued to use the standard 40-pin connector cable
Officially declared an obsolete standard; withdrawn by ANSI in 2001

ATA-3

Third ANSI-accepted standard, published in 1997 as ANSI standard X3.298-1997
Can achieve a transfer rate of up to 16.6MBps
Supported up to 128GB of disk capacity, a carry-over of the addressing scheme employed in ATA-1. As a practical matter, drives of that capacity were never made to the ATA-3 standard; most drives produced were in the 1 to 20GB range, with most 2.1 to 8GB.
Improved reliability of PIO mode 4 operations, but otherwise did not improve transfer modes
Added Self-Monitoring Analysis and Reporting Technology (S.M.A.R.T.), based on IBM's mainframe drive Predictive Failure Analysis technology and a Compaq pre-standard called IntelliSafeTM. Both diagnostic technologies were designed to warn of impending drive failures.
Included a more sophisticated power management standard
Continued to use the standard 40-pin cable
Declared obsolete by ANSI in 2002

Obsolete standards

MFM

Original hard drive standard found on PCs
Allowed a transfer rate of up to 5 megabits (Mb) per second
Drives of this standard were manufactured in capacities from 5 to 100MB.
Two cables were required to connect: A 20-pin that handled data, and a 34-pin that controlled the physical components
Physically large, conformed to the 5.25" form factor, which meant these hard drives were the size of today's CD/DVD drives, and much heavier.
Adhered to the ST-506 hard drive controller standard, one based on early Seagate drive controllers.

RL

Successor to MFM
Allowed a transfer rate up to 7.5Mb per second (Mbps)
Manufactured in sizes from 30 to 200MB
RLL recording involved squeezing more bits into a track. This was accomplished by using a larger number of sectors and required a better quality magnetic medium than required for MFM. RLL recording resulted in a roughly 50% increase in capacity over MFM recording, given the same physical disks.
Same cabling as MFM
Still physically large drives
The controller card determined whether the computer used MFM or RLL encoding on the attached hard drive. It was possible, though not recommended, to low-level format MFM drives using an RLL controller and vice versa; data loss was possible when using an MFM drive with an RLL controller, as the disks' magnetic surfaces in MFM drives were not designed to handle the density of RLL data. Using an RLL drive with an MFM controller reduced the RLL drive to MFM capacities and speeds. Drive manufacturers usually made two versions of drives to accommodate both standards; the RLL drives would have better magnetic media but were otherwise identical.

ESDI

Enhanced Small Device Interface; created to improve on the reliability found in the MFM/RLL ST-506 standard
Allowed a maximum transfer rate of 24Mbps (which is equal to 3MB per second), but in actual use achieved roughly only half of that
Also used a pair of ribbon cables, exactly like MFM's and RLL's 34- and 20-pin cables.
Created in the mid 1980s by a consortium of manufacturers led by Maxtor. It moved some drive controller functions to the hard drive from the controller card, which somehow improved the reliability of drives manufactured to this standard. It never became popular due to competition by the upcoming ATA standard in the mainstream market, and SCSI in the high end one. It all but disappeared in the early 1990s.

Note: At this point, varying competing standards began to become more common. Manufacturers began implementing features they anticipated would appear in upcoming official standards, in hopes of beating their competitors to the market with the updated products. The result was separate, manufacturer-specific, competing standards that, despite much overlap and similarities, were not the same. Since all were extended from actual ANSI standards, they were mostly compatible with each other (with the exception of the most cutting edge features or specifications). However, since some differences were very technical and minute, understanding the differences between specifications can be difficult.

The following information distinguishes between official ANSI-approved standards and manufacturer-specific standards. Some unofficial standards will be very close to those from ANSI; most of these came out before the official ones, for the reasons described above.

IDE

Integrated Drive Electronics. Essentially, these are drives with RLL controllers built directly on rather than being elsewhere in the computer.
Allowed a transfer rate up to 8.3MB per second
Were found in sizes up to 500MB and greater, doubling in capacity roughly every three or four months
Used a single, 40-pin ribbon cable to connect to the controller
Smaller form factors (i.e., 3.5") became available with this standard
Saw the introduction of LBA
Because the controller was aboard the drive, manufacturers started using techniques to increase capacity, speed, and reliability, for example:
- Active error checking
- Moving contents of failing blocks to blocks set aside for this purpose (the controller re-mapped the sectors, so the computer doesn't even know the block has changed location)
- Zone recording (fewer sectors on the inside tracks and more on the outside)
- Higher disk rotation speeds
- Re-mapping of drive geometry to suit the user
- Connected to the computer's bus as follows:
  - In older computers, through interface cards most commonly known as paddle cards, and also known as I/O or Super I/O cards
  - Starting with 386 computers, through chipsets and electronics integrated into the motherboard. This practice became de facto for Pentium computers and beyond.
The standard was capable of up handling disks up to 8.4GB, but due to motherboard BIOS limitations, drives above that size were not manufactured until succeeding standards and advances to BIOS technology came about.
Note: Many people consider IDE to be a standard. It is not; ANSI has not accepted it as a standard. It is more accurate to consider IDE to be a marketing term for early implementations of ATA, used by Conner Peripherals, Compaq, and Western Digital. See the ATA-1 section below for more information.

ATA (AT Attachment), ATA-1

The ATA interface standard was submitted in 1990, published in 1994 (ANSI standard X3.221-1994, AT Attachment Interface for Disk Drives), and had these characteristics:

Allowed transfers at up to 8.3MBps
At first mostly found in sizes around 504MB or less due to BIOS limitations. The block addressing protocol used by ATA-1 allowed for disk sizes up to 128GB; however, the 504MB limit was imposed by other components. After a period of time and some advances in technology, a barrier of 2.1GB was reached, but again, that limit was imposed by the BIOS as well.
Also used a 40-pin ribbon cable to connect the drives to the motherboard or controller card
Since this standard was common at a time when motherboard interfaces were changing, it could be found with either an 8- or a 16-bit interface. It has a single channel populated by up to two hard drives in a master/slave configuration. It supports PIO (Programmed Input/Output) modes 0,1, and 2. It also supports DMA single word modes 0, 1, and 2, and multiword DMA mode 0.
Officially declared an obsolete ANSI standard as of August 6, 1999
Hard drives were nearly universally found in the 3.5" form factor, although exceptions like the Quantum Bigfoot existed. Those, however, were still not as big as CD or DVD drives: The "Full Height 5.25" standard allowed for 3.25" tall drives, which is the size of the average CD or DVD drive. The "Half Height" drives were 1.6" tall. Quantum Bigfoots were "Low Profile" and "Ultra Low Profile" drives, going from 1" tall down to 3/4".
Although the ATA standard itself was not published until 1994, manufacturers were producing drives adhering to the specifications in the initial submission of the standard since 1990.
Note: when you see the marketing term "IDE", it generally refers to the ANSI standard ATA-1. Current drives, including Serial ATA (SATA), SCSI, USB, and FireWire, could be considered IDE in the sense that they have logic boards embedded into the drives themselves. Technically, however, drives use only the ATA or SCSI interface (Serial ATA [SATA] is a new application of the ATA standard); external drives use one of the external interfaces like FireWire or USB. There are no other standards. The term IDE can be applied to any of these current standards, but vendors who used it really meant "ATA-1".

Fast ATA

This manufacturer's standard was created by Seagate and received support from Quantum as well. It's related to ATA-2, but not quite the same thing; it supports only PIO mode 3 and multiword DMA mode 1. Other aspects of the standard, such as transfer rate and disk capacity, are the same as ATA-2 (see below).
This manufacturer-specific, unofficial term is not recognized by ANSI. Fast ATA came out before ATA-2, and was developed as part of a race against Western Digital's unofficial extensions to ATA called "Enhanced IDE (EIDE)". It also was developed in anticipation of the new, official standard coming out that eventually became ATA-2.