Serial ATA

History

SATA was announced in 2000. It provided several advantages over the earlier PATA interface such as reduced cable size and cost (seven conductors instead of 40 or 80), native hot swapping, faster data transfer through higher signaling rates, and more efficient transfer through an (optional) I/O queuing protocol. Revision 1.0 of the specification was released in January 2003.

Serial ATA industry compatibility specifications originate from the Serial ATA International Organization (SATA-IO). The SATA-IO group collaboratively creates, reviews, ratifies, and publishes the interoperability specifications, the test cases and plugfests. As with many other industry compatibility standards, the SATA content ownership is transferred to other industry bodies: primarily INCITS T13 and an INCITS T10 subcommittee (SCSI), a subgroup of T10 responsible for Serial Attached SCSI (SAS). The remainder of this article strives to use the SATA-IO terminology and specifications.

Before SATA's introduction in 2000, PATA was simply known as ATA. The "AT Attachment" (ATA) name originated after the 1984 release of the IBM Personal Computer AT, more commonly known as the IBM AT. The IBM AT's controller interface became a de facto industry interface for the inclusion of hard disks. "AT" was IBM's abbreviation for "Advanced Technology"; thus, many companies and organizations indicate SATA is an abbreviation of "Serial Advanced Technology Attachment". However, the ATA specifications simply use the name "AT Attachment", to avoid possible trademark issues with IBM.

SATA host adapters and devices communicate via a high-speed serial cable over two pairs of conductors. In contrast, parallel ATA (the redesignation for the legacy ATA specifications) uses a 16-bit wide data bus with many additional support and control signals, all operating at a much lower frequency. To ensure backward compatibility with legacy ATA software and applications, SATA uses the same basic ATA and ATAPI command sets as legacy ATA devices.

The world's first SATA hard disk drive was the Seagate Barracuda SATA V, released in January 2003.

SATA replaced parallel ATA in consumer desktop and laptop computers; SATA's market share in the desktop PC market was 99% in 2008. PATA has mostly been replaced by SATA for any use; with PATA in declining use in industrial and embedded applications that use CompactFlash (CF) storage, which was designed around the legacy PATA standard. A 2008 standard, CFast, to replace CompactFlash is based on SATA.

Features

Hot plug

The Serial ATA spec requires SATA devices be capable of hot plugging; that is, devices that meet the specification are capable of insertion or removal of a device into or from a backplane connector (combined signal and power) that has power on. After insertion, the device initializes and then operates normally. Depending upon the operating system, the host may also initialize, resulting in a hot swap. The powered host and device do not need to be in an idle state for safe insertion and removal, although unwritten data may be lost when power is removed.

Unlike PATA, both SATA and eSATA support hot plugging by design. However, this feature requires proper support at the host, device (drive), and operating-system levels. In general, SATA devices fulfill the device-side hot-plugging requirements, and most SATA host adapters support this function.

For eSATA, hot plugging is supported in AHCI mode only. IDE mode does not support hot plugging.

Advanced Host Controller Interface

Advanced Host Controller Interface (AHCI) is an open host controller interface published and used by Intel, which has become a de facto standard. It allows the use of advanced features of SATA such as hotplug and native command queuing (NCQ). If AHCI is not enabled by the motherboard and chipset, SATA controllers typically operate in "IDE emulation" mode, which does not allow access to device features not supported by the ATA (also called IDE) standard.

Windows device drivers that are labeled as SATA are often running in IDE emulation mode unless they explicitly state that they are AHCI mode, in RAID mode, or a mode provided by a proprietary driver and command set that allowed access to SATA's advanced features before AHCI became popular. Modern versions of Microsoft Windows, Mac OS X, FreeBSD, Linux with version 2.6.19 onward, as well as Solaris and OpenSolaris, include support for AHCI, but earlier operating systems such as Windows XP do not. Even in those instances, a proprietary driver may have been created for a specific chipset, such as Intel's.

Revisions

SATA revisions are typically designated with a dash followed by Roman numerals, e.g. "SATA-III", to avoid confusion with the speed, which is always displayed in Arabic numerals, e.g. "SATA 6 Gbit/s". The speeds given are the raw interface rate in Gbit/s including line code overhead, and the usable data rate in MB/s without overhead.

SATA revision 1.0 (1.5 Gbit/s, 150 MB/s, Serial ATA-150)

Revision 1.0a was released on January 7, 2003. First-generation SATA interfaces, now known as SATA 1.5 Gbit/s, communicate at a rate of 1.5 Gbit/s, and do not support Native Command Queuing (NCQ). Taking 8b/10b encoding overhead into account, they have an actual uncoded transfer rate of 1.2 Gbit/s (150 MB/s). The theoretical burst throughput of SATA 1.5 Gbit/s is similar to that of PATA/133, but newer SATA devices offer enhancements such as NCQ, which improve performance in a multitasking environment.

During the initial period after SATA 1.5 Gbit/s finalization, adapter and drive manufacturers used a "bridge chip" to convert existing PATA designs for use with the SATA interface. Bridged drives have a SATA connector, may include either or both kinds of power connectors, and, in general, perform identically to their native-SATA equivalents.

As of April 2010, the fastest 10,000 rpm SATA hard disk drives could transfer data at maximum (not average) rates of up to 157 MB/s, which is beyond the capabilities of the older PATA/133 specification and also exceeds the capabilities of SATA 1.5 Gbit/s.

SATA revision 2.0 (3 Gbit/s, 300 MB/s, Serial ATA-300)

SATA revision 2.0 was released in April 2004, introducing Native Command Queuing (NCQ). It is backward compatible with SATA 1.5 Gbit/s.

Second-generation SATA interfaces run with a native transfer rate of 3.0 Gbit/s that, when accounted for the 8b/10b encoding scheme, equals to the maximum uncoded transfer rate of 2.4 Gbit/s (300 MB/s). The theoretical burst throughput of the SATA revision 2.0, which is also known as the SATA 3 Gbit/s, doubles the throughput of SATA revision 1.0.

All SATA data cables meeting the SATA spec are rated for 3.0 Gbit/s and handle modern mechanical drives without any loss of sustained and burst data transfer performance. However, high-performance flash-based drives can exceed the SATA 3 Gbit/s transfer rate; this is addressed with the SATA 6 Gbit/s interoperability standard.

Announced in August 2005, SATA revision 2.5 consolidated the specification to a single document.

Announced in February 2007, SATA revision 2.6 introduced the following features:

Slimline connector

Micro connector (initially for 1.8” HDD)

Mini Internal Multilane cable and connector

Mini External Multilane cable and connector

NCQ Priority

NCQ Unload

Enhancements to the BIST Activate FIS

Enhancements for robust reception of the Signature FIS

SATA revision 3.0 (6 Gbit/s, 600 MB/s, Serial ATA-600)

Serial ATA International Organization (SATA-IO) presented the draft specification of SATA 6 Gbit/s physical layer in July 2008, and ratified its physical layer specification on August 18, 2008. The full 3.0 standard was released on May 27, 2009.

Third-generation SATA interfaces run with a native transfer rate of 6.0 Gbit/s; taking 8b/10b encoding into account, the maximum uncoded transfer rate is 4.8 Gbit/s (600 MB/s). The theoretical burst throughput of SATA 6.0 Gbit/s is double that of SATA revision 2.0. It is backward compatible with earlier SATA implementations.

The SATA 3.0 specification contains the following changes:

6 Gbit/s for scalable performance.

Continued compatibility with SAS, including SAS 6 Gbit/s, as per "a SAS domain may support attachment to and control of unmodified SATA devices connected directly into the SAS domain using the Serial ATA Tunneled Protocol (STP)" from the SATA Revision 3.0 Gold specification.

Isochronous Native Command Queuing (NCQ) streaming command to enable isochronous quality of service data transfers for streaming digital content applications.

An NCQ management feature that helps optimize performance by enabling host processing and management of outstanding NCQ commands.

Improved power management capabilities.

A small low insertion force (LIF) connector for more compact 1.8-inch storage devices.

A 7 mm optical disk drive profile for the slimline SATA connector (in addition to the existing 12.7 mm and 9.5 mm profiles).

Alignment with the INCITS ATA8-ACS standard.

Cables, connectors, and ports

Connectors and cables present the most visible differences between SATA and parallel ATA drives. Unlike PATA, the same connectors are used on 3.5-inch SATA hard disks (for desktop and server computers) and 2.5-inch disks (for portable or small computers).

Standard SATA connectors for both data and power have a conductor pitch of 1.27 mm (0.050 inches). Low insertion force is required to mate a SATA connector. A smaller mini-SATA or mSATA connector is used by smaller devices such as 1.8-inch SATA drives, some DVD and Blu-ray drives, and mini SSDs.

A special eSATA connector is specified for external devices, and an optionally implemented provision for clips to hold internal connectors firmly in place. SATA drives may be plugged into SAS controllers and communicate on the same physical cable as native SAS disks, but SATA controllers cannot handle SAS disks.

Female SATA ports (on motherboards for example) are for use with SATA data cables that have locks or clips to prevent accidental unplugging. Some SATA cables have right- or left-angled connectors to ease connection to circuit boards.

Data connector

The SATA standard defines a data cable with seven conductors (three grounds and four active data lines in two pairs) and 8 mm wide wafer connectors on each end. SATA cables can have lengths up to 1 meter (3.3 ft), and connect one motherboard socket to one hard drive. PATA ribbon cables, in comparison, connect one motherboard socket to one or two hard drives, carry either 40 or 80 wires, and are limited to 45 centimeters (18 in) in length by the PATA specification; however, cables up to 90 centimeters (35 in) are readily available. Thus, SATA connectors and cables are easier to fit in closed spaces and reduce obstructions to air cooling. Some cables even include a locking feature, whereby a small (usually metal) spring holds the plug in the socket.

SATA connectors may be straight, upward-angled, downward-angled, leftward-angled, or rightward-angled. Angled connectors allow lower-profile connections. Downward-angled connectors lead the cable immediately away from the drive, on the circuit-board side. Upward-angled connectors lead the cable across the drive towards its top.

One of the problems associated with the transmission of data at high speed over electrical connections is described as noise, which is due to electrical coupling between data circuits and other circuits. As a result, the data circuits can both affect other circuits and be affected by them. Designers use a number of techniques to reduce the undesirable effects of such unintentional coupling. One such technique used in SATA links is differential signaling. This is an enhancement over PATA, which uses single-ended signaling. The use of fully shielded, dual coax conductors, with multiple ground connections, for each differential pair improves isolation between the channels and reduces the chances of lost data in difficult electrical environments.

There are SATADOM variants of the connector merging power supply into the 7-pin connector, saving space in industrial setups. The "pin 7" variation repurposes pin 7 for +5V power. The "pin 8" variation repurposes two unnumbered shielding pins for power GND and +5V.

SATA power connectors

SATA specifies a different power connector than the four-pin Molex connector used on Parallel ATA (PATA) devices (and earlier small storage devices, going back to ST-506 hard disk drives and even to floppy disk drives that predated the IBM PC). It is a wafer-type connector, like the SATA data connector, but much wider (fifteen pins versus seven) to avoid confusion between the two. Some early SATA drives included the four-pin Molex power connector together with the new fifteen-pin connector, but most SATA drives now have only the latter.

The new SATA power connector contains many more pins for several reasons:

3.3 V is supplied along with the traditional 5 V and 12 V supplies. However, very few drives actually use it.

Pin 3 in SATA revision 3.3 has been redefined as PWDIS and is used to enter and exit the POWER DISABLE mode in line with SAS-3. If Pin 3 is driven HIGH (2.1–3.6 V max), power to the drive circuitry is cut. Pins 1 and 2 are redefined as retired and no longer used for any purpose. Drives with this feature enabled do not power up in systems designed to SATA revision 3.1 or earlier, because Pin 3 driven HIGH prevents the drive from powering up. Workarounds include using a Molex adapter without 3.3 V, putting thin low-friction thermal-resistant insulating tape (e.g. kapton tape) over the PWDIS pin or physically removing the pin.

To reduce resistance and increase current capability, each voltage is supplied by three pins in parallel, though one pin in each group is intended for precharging (see below). Each pin should be able to carry 1.5 A.

Five parallel pins provide a low-resistance ground connection.

Two ground pins and one pin for each supplied voltage support hot-plug precharging. Ground pins 4 and 12 in a hot-swap cable are the longest, so they make contact first when the connectors are mated. Drive power connector pins 3, 7, and 13 are longer than the others, so they make contact next. The drive uses them to charge its internal bypass capacitors through current-limiting resistances. Finally, the remaining power pins make contact, bypassing the resistances and providing a low-resistance source of each voltage. This two-step mating process avoids glitches to other loads and possible arcing or erosion of the SATA power-connector contacts.

Additional pins

Some SATA drives, in particular mechanical ones, come with an extra 4 or more jumper pin interface which isn't uniformly standardized but nevertheless serves similar purpose defined by each drive manufacturer. As IDE drives used those extra pins for setting up Master and Slave drives, on SATA drives, those pins are generally used to select different Power modes for use in USB-SATA bridges or enables additional features like Spread Spectrum Clocking, SATA Speed Limit or Factory Mode for Diagnostics and Recovery, by the use of a jumper.

eSATA

Standardized in 2004, eSATA (e standing for external) provides a variant of SATA meant for external connectivity. It uses a more robust connector, longer shielded cables, and stricter (but backward-compatible) electrical standards. The protocol and logical signaling (link/transport layers and above) are identical to internal SATA. The differences are:

Minimum transmit amplitude increased: Range is 500–600 mV instead of 400–600 mV.

Minimum receive amplitude decreased: Range is 240–600 mV instead of 325–600 mV.

Maximum cable length increased to 2 meters (6.6 ft) from 1 meter (3.3 ft).

The eSATA cable and connector is similar to the SATA 1.0a cable and connector, with these exceptions:

The eSATA connector is mechanically different to prevent unshielded internal cables from being used externally. The eSATA connector discards the L-shaped key and changes the position and size of the guides.

The eSATA insertion depth is deeper: 6.6 mm instead of 5 mm. The contact positions are also changed.

The eSATA cable has an extra shield to reduce EMI to FCC and CE requirements. Internal cables do not need the extra shield to satisfy EMI requirements because they are inside a shielded case.

The eSATA connector uses metal springs for shield contact and mechanical retention.

The eSATA connector has a design-life of 5,000 matings; the ordinary SATA connector is only specified for 50.

Aimed at the consumer market, eSATA enters an external storage market served also by the USB and FireWire interfaces. The SATA interface has certain advantages. Most external hard-disk-drive cases with FireWire or USB interfaces use either PATA or SATA drives and "bridges" to translate between the drives' interfaces and the enclosures' external ports; this bridging incurs some inefficiency. Some single disks can transfer 157 MB/s during real use, about four times the maximum transfer rate of USB 2.0 or FireWire 400 (IEEE 1394a) and almost twice as fast as the maximum transfer rate of FireWire 800. The S3200 FireWire 1394b specification reaches around 400 MB/s (3.2 Gbit/s), and USB 3.0 has a nominal speed of 5 Gbit/s. Some low-level drive features, such as S.M.A.R.T., may not operate through some USB or FireWire or USB+FireWire bridges; eSATA does not suffer from these issues provided that the controller manufacturer (and its drivers) presents eSATA drives as ATA devices, rather than as SCSI devices, as has been common with Silicon Image, JMicron, and Nvidia nForce drivers for Windows Vista. In those cases SATA drives do not have low-level features accessible.

Mini-SATA (mSATA)

Mini-SATA (abbreviated as mSATA), which is distinct from the micro connector, was announced by the Serial ATA International Organization on September 21, 2009. Applications include netbooks, laptops and other devices that require a solid-state drive in a small footprint.

The physical dimensions of the mSATA connector are identical to those of the PCI Express Mini Card interface, but the interfaces are electrically incompatible; the data signals (TX±/RX± SATA, PETn0 PETp0 PERn0 PERp0 PCI Express) need a connection to the SATA host controller instead of the PCI Express host controller.

The M.2 specification has superseded both mSATA and PCI Express Mini.

SFF-8784 connector

Slim 2.5-inch SATA devices, 5 mm (0.20 inches) in height, use the twenty-pin SFF-8784 edge connector to save space. By combining the data signals and power lines into a slim connector that effectively enables direct connection to the device's printed circuit board (PCB) without additional space-consuming connectors, SFF-8784 allows further internal layout compaction for portable devices such as ultrabooks.

Pins 1 to 10 are on the connector's bottom side, while pins 11 to 20 are on the top side.

SATA Express

SATA Express, initially standardized in the SATA 3.2 specification, is an interface that supports either SATA or PCI Express storage devices. The host connector is backward compatible with the standard 3.5-inch SATA data connector, allowing up to two legacy SATA devices to connect. At the same time, the host connector provides up to two PCI Express 3.0 lanes as a pure PCI Express connection to the storage device, allowing bandwidths of up to 2 GB/s.

Instead of the otherwise usual approach of doubling the native speed of the SATA interface, PCI Express was selected for achieving data transfer speeds greater than 6 Gbit/s. It was concluded that doubling the native SATA speed would take too much time, too many changes would be required to the SATA standard, and would result in a much greater power consumption when compared to the existing PCI Express bus.

In addition to supporting legacy Advanced Host Controller Interface (AHCI), SATA Express also makes it possible for NVM Express (NVMe) to be used as the logical device interface for connected PCI Express storage devices.

As M.2 form factor, described below, achieved much larger popularity, SATA Express is considered as a failed standard and dedicated ports quickly disappeared from motherboards.

M.2 (NGFF)

M.2, formerly known as the Next Generation Form Factor (NGFF), is a specification for computer expansion cards and associated connectors. It replaces the mSATA standard, which uses the PCI Express Mini Card physical layout. Having a smaller and more flexible physical specification, together with more advanced features, the M.2 is more suitable for solid-state storage applications in general, especially when used in small devices such as ultrabooks or tablets.

A M.2 SSD is "keyed" to prevent insertion of a card connector (male) to an incompatible socket (female) on the host. Typically, M.2 SSDs with a B key or B+M key are SATA, while M.2 SSDs with M key only are mostly NVMe only and incompatible with SATA.

The M.2 standard is designed as a revision and improvement to the mSATA standard, so that larger printed circuit boards (PCBs) can be manufactured. While mSATA took advantage of the existing PCI Express Mini Card form factor and connector, M.2 has been designed to maximize usage of the card space, while minimizing the footprint.

Supported host controller interfaces and internally provided ports are a superset to those defined by the SATA Express interface. Essentially, the M.2 standard is a small form factor implementation of the SATA Express interface, with the addition of an internal USB 3.0 port.

U.2 (SFF-8639)

U.2, formerly known as SFF-8639. Like M.2, it carries a PCI Express electrical signal, however U.2 uses a PCIe 3.0 ×4 link providing a higher bandwidth of 32 Gbit/s in each direction. In order to provide maximum backward compatibility the U.2 connector also supports SATA and multi-path SAS.

Topology

SATA uses a point-to-point architecture. The physical connection between a controller and a storage device is not shared among other controllers and storage devices. SATA defines multipliers, which allows a single SATA controller port to drive up to fifteen storage devices. The multiplier performs the function of a hub; the controller and each storage device is connected to the hub. This is conceptually similar to SAS expanders.

Modern PC systems have SATA controllers built into the motherboard, typically featuring two to eight ports. Additional ports can be installed through add-in SATA host adapters (available in variety of bus-interfaces: USB, PCI, PCIe).

Backward and forward compatibility

SATA and PATA

At the hardware interface level, SATA and PATA (Parallel AT Attachment) devices are completely incompatible: they cannot be interconnected without an adapter.

At the application level, SATA devices can be specified to look and act like PATA devices.

Many motherboards offer a "Legacy Mode" option, which makes SATA drives appear to the OS like PATA drives on a standard controller. This Legacy Mode eases OS installation by not requiring that a specific driver be loaded during setup, but sacrifices support for some (vendor specific) features of SATA. Legacy Mode often if not always disables some of the boards' PATA or SATA ports, since the standard PATA controller interface supports only four drives. (Often, which ports are disabled is configurable.)

The common heritage of the ATA command set has enabled the proliferation of low-cost PATA to SATA bridge chips. Bridge chips were widely used on PATA drives (before the completion of native SATA drives) as well in standalone converters. When attached to a PATA drive, a device-side converter allows the PATA drive to function as a SATA drive. Host-side converters allow a motherboard PATA port to connect to a SATA drive.

The market has produced powered enclosures for both PATA and SATA drives that interface to the PC through USB, Firewire or eSATA, with the restrictions noted above. PCI cards with a SATA connector exist that allow SATA drives to connect to legacy systems without SATA connectors.

SATA 1.5 Gbit/s and SATA 3 Gbit/s

The designers of SATA standard as an overall goal aimed for backward and forward compatibility with future revisions of the SATA standard.

To prevent interoperability problems that could occur when next generation SATA drives are installed on motherboards with standard legacy SATA 1.5 Gbit/s host controllers, many manufacturers have made it easy to switch those newer drives to the previous standard's mode.

Examples of such provisions include:

Seagate/Maxtor has added a user-accessible jumper-switch, known as the "force 150", to enable the drive switch between forced 1.5 Gbit/s and 1.5/3 Gbit/s negotiated operation.

Western Digital uses a jumper setting called OPT1 enabled to force 1.5 Gbit/s data transfer speed (OPT1 is enabled by putting the jumper on pins 5 and 6).

Samsung drives can be forced to 1.5 Gbit/s mode using software that may be downloaded from the manufacturer's website. Configuring some Samsung drives in this manner requires the temporary use of a SATA-2 (SATA 3.0 Gbit/s) controller while programming the drive.

The "force 150" switch (or equivalent) is also useful for attaching SATA 3 Gbit/s hard drives to SATA controllers on PCI cards, since many of these controllers (such as the Silicon Image chips) run at 3 Gbit/s, even though the PCI bus cannot reach 1.5 Gbit/s speeds. This can cause data corruption in operating systems that do not specifically test for this condition and limit the disk transfer speed.

SATA 3 Gbit/s and SATA 6 Gbit/s

SATA 3 Gbit/s and SATA 6 Gbit/s are compatible with each other. Most devices that are only SATA 3 Gbit/s can connect with devices that are SATA 6 Gbit/s, and vice versa, though SATA 3 Gbit/s devices connect with SATA 6 Gbit/s devices only at the slower 3 Gbit/s speed.

SATA 1.5 Gbit/s and SATA 6 Gbit/s

SATA 1.5 Gbit/s and SATA 6 Gbit/s are compatible with each other. Most devices that are only SATA 1.5 Gbit/s can connect with devices that are SATA 6 Gbit/s, and vice versa, though SATA 1.5 Gbit/s devices only connect with SATA 6 Gbit/s devices at the slower 1.5 Gbit/s speed.

Comparison to other interfaces

SATA and SCSI

Parallel SCSI uses a more complex bus than SATA, usually resulting in higher manufacturing costs. SCSI buses also allow connection of several drives on one shared channel, whereas SATA allows one drive per channel, unless using a port multiplier. Serial Attached SCSI uses the same physical interconnects as SATA, and most SAS HBAs also support 3 and 6 Gbit/s SATA devices (an HBA requires support for Serial ATA Tunneling Protocol).

SATA 3 Gbit/s theoretically offers a maximum bandwidth of 300 MB/s per device, which is only slightly lower than the rated speed for SCSI Ultra 320 with a maximum of 320 MB/s total for all devices on a bus. SCSI drives provide greater sustained throughput than multiple SATA drives connected via a simple (i.e., command-based) port multiplier because of disconnect-reconnect and aggregating performance. In general, SATA devices link compatibly to SAS enclosures and adapters, whereas SCSI devices cannot be directly connected to a SATA bus.

SCSI, SAS, and fibre-channel (FC) drives are more expensive than SATA, so they are used in servers and disk arrays where the better performance justifies the additional cost. Inexpensive ATA and SATA drives evolved in the home-computer market, hence there is a view that they are less reliable. As those two worlds overlapped, the subject of reliability became somewhat controversial. Note that, in general, the failure rate of a disk drive is related to the quality of its heads, platters and supporting manufacturing processes, not to its interface.

Use of serial ATA in the business market increased from 22% in 2006 to 28% in 2008.

Comparison with other buses

SCSI-3 devices with SCA-2 connectors are designed for hot swapping. Many server and RAID systems provide hardware support for transparent hot swapping. The designers of the SCSI standard prior to SCA-2 connectors did not target hot swapping, but in practice, most RAID implementations support hot swapping of hard disks.