SATA

SATA
Serial ATA
Year created	2003
No. of devices	1
Speed	1.5 Gbit/s, 3.0 Gbit/s
Style	Serial
Hotplugging interface	Yes
External interface	Yes
Website	sata-io.org

In computer hardware, Serial ATA (SATA, /ˈseɪ.tə/ or /ˈsæ.tə/) is a computer bus technology primarily designed for transfer of data to and from hard disks and optical drives. It was designed as a successor to the legacy Advanced Technology Attachment standard (ATA), and is expected to eventually replace the older technology (retroactively renamed Parallel ATA or PATA). Serial ATA adapters and devices communicate over a high-speed serial link.

Architecture

At the physical layer of the Serial ATA architecture, the data-connection is formed by two pairs of (unidirectional) signal wires. Over these wires, SATA uses Low Voltage Differential Signaling (LVDS), enabling much higher (per-wire) signalling rates (1.5 Gbit/s and up) than traditional parallel ATA. Byte data is encoded and transmitted using 8B/10B encoding, which is also used in Ethernet, Fibre Channel, PCI Express, etc. This encoding scheme has an efficiency of 80%, but the 8B/10B encoding embeds the information necessary to distinguish bit and character boundaries, eliminating the need for a separate clock signal. The switch from a parallel to serial electrical scheme facilitates future upgrades to performance, and lowers costs (compared to a comparably fast parallel-interface.) Above the SATA physical level are the link level and transport level. These higher levels convert configuration and data-operations into discrete ordered packets, for transmission over the SATA link. At the application level, SATA inherits ATA's operational model, with the 'task register file' (for generating read/write PIO and DMA requests) a mandatory part of all SATA host implementations. Vendor implementations may include additional functionality (such as simulated RAID) above and beyond the SATA specification, but require device-specific drivers to exploit. The final major difference between SATA and PATA, from a user standpoint, is the point-to-point link between host and drive. Each SATA/device has a link to a SATA host-port, with no sharing of cable or bandwidth between devices.

Many vendors have implemented SATA in conjunction with Serial Attached SCSI (SAS). Since the connectors are universal between the protocols, it allows them to create an entry level product which connects to SATA storage while providing an upgrade path for users to move to the higher end SAS technology.

SATA 1.5 Gbit/s

First-generation SATA interfaces, also known as SATA/150 or (erroneously) as SATA 1, communicate at a rate of 1.5 gigabits per second (Gbit/s). Taking into account 8b10b coding overhead, the actual uncoded transfer-rate is 1.2 Gbit/s, or 150 megabytes per second (MB/s). In actual operation, SATA/150 and PATA/133 are comparable in terms of their theoretical burst-throughput. However, newer SATA devices offer enhancements (such as native command queuing) to SATA's performance in a multitask environment.

During the initial period after SATA/150's finalization, both adapter and drive manufacturers used a "bridge chip" to convert existing designs with the PATA-interface to the SATA-interface. Bridged drives have a SATA connector, may include either or both kinds of power connectors, and generally perform identically to native drives. They generally lack support for some SATA-specific features (such as NCQ). Bridged products gradually gave way to native SATA products.

SATA 3.0 Gbit/s

Soon after SATA/150's introduction, a number of shortcomings in the original SATA were observed. First and foremost, at the application-level, SATA's operational model emulated PATA in that the interface could only handle one pending transaction at a time. SCSI disks have long benefited from the SCSI-interface's support for multiple outstanding requests, allowing the drive targets to re-order the requests to optimize response-time. Native command queuing (NCQ) adds this capability to SATA. NCQ is an optional feature, and may be used in both SATA 1.5 Gbit/s or SATA 3.0 Gbit/s devices.

First-generation SATA devices were scarcely faster than legacy parallel ATA/133 devices. So a 3 Gbit/s signaling rate was added to the Physical layer (PHY layer), effectively doubling data throughput from 150 MB/s to 300 MB/s. SATA/300's transfer rate is expected to satisfy drive throughput requirements for some time, as the fastest desktop hard disks barely saturate a SATA/150 link. This is why a SATA data cable rated for 1.5 Gbit/s will currently handle second generation, SATA 3.0 Gbit/s sustained and burst data transfers without any loss of performance.

Backward compatibility between SATA 1.5 Gbit/s controllers and SATA 3.0 Gbit/s devices was important, so SATA/300's autonegotiation sequence is designed to fallback to SATA/150 speed (1.5 Gbit/s rate) when in communication with such devices. In practice, some older SATA controllers do not properly implement SATA speed negotiation. Affected systems require user-intervention to manually set the SATA 3.0 Gbit/s peripherals to 1.5 Gbit/s mode, generally through the use of a jumper. [1] Known faulty chipsets include the VIA VT8237 and VT8237R south bridges, and the VIA VT6420 and VT6421L standalone SATA controllers. [2] SiS's 760 and 964 chipsets also initially exhibited this problem, though it can be rectified with an updated SATA controller ROM. ^{[citation needed]}

The 3.0 Gbit/s specification has been very widely referred to as “Serial ATA II” (“SATA II”), contrary to the wishes of the Serial ATA standards organization that authored it. The official website notes that SATA II was in fact that organization's name at the time, the SATA 3.0 Gbit/s specification being only one of many that the former SATA II defined, and suggests that “SATA 3.0 Gbit/s” be used instead. (The Serial ATA standards organization has since changed names, and is now “The Serial ATA International Organization”, abbreviated SATA-IO.)

SATA 3.0 Gbit/s is sometimes also referred to as SATA 3.0 or SATA/300, continuing the line of ATA/100, ATA/133 and SATA/150. The term “SATA II” is still in very common use, however, for retail of both drives[3] and cables and interfaces[4].

The next step: SATA 6.0 Gbit/s

SATA's roadmap includes plans for a 6.0 Gbit/s standard. In current PCs, SATA 3.0 Gbit/s already greatly exceeds the sustainable (non-burst) transfer-rate of even the best hard-disks. The 6.0 Gbit/s standard is right now[5] useful in combination with port multipliers, which allow multiple drives to be connected to a single Serial ATA port, thus sharing the port's bandwidth with multiple drives; terms in use for this standard are "multilane" and "infiniband". Solid-state drives such as RAM disks may also one day exploit the faster transfer-rate. Since the theoretical burst speeds marketed by drive manufacturers are rarely achieved, a smaller power and interface cable plus the ability to hot-plug are the most practical SATA benefits to everyday computing.

Serial ATA innovations

SATA uses only 4 signal-lines, allowing for much more compact (and less-expensive) cables compared to PATA. It also offers new features such as hot-swapping and native command queuing.

SATA drives may be plugged into Serial Attached SCSI (SAS) controllers and communicate on the same physical cable as native SAS disks. SAS disks, however, may not be plugged into a SATA controller.

Cables and connectors

The SATA power and data cables are the most noticeable change from Parallel ATA. Unlike Parallel ATA, the same physical connectors are used on 3.5-in (90 mm) desktop hard disks and 2.5-in (70 mm) notebook disks; this eliminates the need for a mechanical adapter when using a notebook drive in a desktop computer.

The SATA standard defines a data cable with seven conductors (3 grounds and 4 active data lines in two pairs) and 8 mm wide wafer connectors on each end. SATA cables can be up to 1 m (39 in) long. PATA ribbon cables, in comparison, carry either 40- or 80-conductor wires and are limited to 152 cm (5 feet) in length. Thus, SATA connectors and cables are easier to fit in closed spaces and reduce obstructions to air cooling. However, they are more susceptible to accidental unplugging.

Pin #	Function
1	Ground
2	A+
3	A-
4	Ground
5	B-
6	B+
7	Ground

The SATA standard also specifies a new power connector. Like the data cable, it is wafer-based, but its wider 15-pin shape prevents accidental misidentification and forced insertion of the wrong connector type. Native SATA devices favor the SATA power-connector over the old four-pin Molex connector (found on all PATA equipment), although some SATA drives retain older 4-pin Molex. The SATA/power connector has been criticized for its poor robustness -- the thin plastic tops of the connectors (see power connector picture at right) can break due to shearing force when the user pulls the plug at a non-orthogonal angle. The seemingly large number of pins are used to supply three different voltages: 3.3 V, 5 V, and 12 V. Each voltage is supplied by three pins ganged together, 5 of the remaining pins are for ground. The last pin, pin 11, is used in newer drives for staggered spinup. The supply pins are ganged together because the small pins by themselves cannot supply sufficient current for some devices. One pin from each of the three voltages is also used for hotplugging.

Pin #		Function
	1-3	3.3V
	4-6	Ground
	7-9	5V
	10	Ground
	11	Staggered spinup (in supporting drives)
	12	Ground
	13-15	12V

Adaptors are available to convert a 4-pin Molex connector to SATA power connector. However, because the 4-pin Molex connectors do not provide 3.3 V power, these adapters provide only 5 V and 12 V power and leave the 3.3 V lines disconnected. This precludes the use of such adapters with drives that require 3.3 V power. Understanding this, drive manufacturers have largely left the 3.3 V power lines unused. However, without 3.3 V power, the SATA device may not be able to implement hotplugging as mentioned in the previous paragraph.

External SATA

The official eSATA logo

Standardized in mid-2004, eSATA defined separate cables, connectors, and revised electrical requirements for external applications:

Minimum transmit potential increased: Range is 500–600 mV instead of 400–600 mV.
Minimum receive potential decreased: Range is 240–600 mV instead of 325–600 mV.
Identical protocol and logical signaling (link/transport-layer and above), allowing native SATA devices to be deployed in external enclosures with minimal modification
Maximum cable length of 2 m (USB and FireWire allow longer distances.)

Aimed at the consumer market, eSATA enters an external storage market already served by the USB and FireWire interfaces. 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, and this bridging incurs some inefficiency. In the case of USB 2.0, protocol overhead limits the maximum effective bandwidth to a fraction of USB's physical signalling rate. With modern hard disk drives, USB's transfer rate is a bottleneck. FireWire, with its isochronous transfer mode, is more efficient, such that even in its slower variant, FireWire 400 (IEEE 1394a), the effective transfer rate is significantly faster than that of USB 2.0, but this can still be a bottleneck for fast drives or RAIDs. Some single disks can transfer well over 70 MB/s during real use, well in excess of USB 2.0's or the older FireWire 400's abilities. Finally, some low-level drive features, such as S.M.A.R.T., are not usable through USB or FireWire bridging. eSATA does not suffer from these issues.

eSATA will likely co-exist alongside USB 2.0 and FireWire storage for several reasons. The ubiquity of USB ports on all mass-market computers, and FireWire ports on many consumer electronic appliances, guarantee a large market for USB and FireWire storage. For small form-factor devices (such as external 2.5" disks), a PC-hosted USB or FireWire link supplies sufficient power to operate the device. Where a PC-hosted port is concerned, eSATA connectors cannot supply power, and would therefore be more cumbersome to use.

As of 2007, few computer motherboards have eSATA connectors. An eSATA external drive enclosure will typically ship with a passive eSATA-to-SATA bracket/cable-adapter to install on desktops. Desktops can also be upgraded with the installation of an eSATA host bus adapter (HBA), while notebooks can be upgraded with Cardbus or ExpressCard versions of an eSATA HBA. With passive-adapters, the maximum cable length is reduced to 1 meter, due to the absence of compliant eSATA signal levels. Full SATA speed for external disks (115 MB/s) have been measured with external RAID enclosures.

eSATA is likely to be ignored by the enterprise and server market, which has already standardized on the separately-developed Serial Attached SCSI (SAS) interface, which has extra features for remote management, link redundancy, and link monitoring.

Note: Prior to the final eSATA specification, there were a number of products designed for external connections of SATA drives. Some of these use the internal SATA connector or even connectors designed for other interface specifications, such as FireWire. These products are not eSATA compliant. The final eSATA specification features a specific connector designed for rough manipulation. It's similar to the regular SATA connector, but with reinforcements in both the male and female sides, inspired by the USB connector. It's harder to unplug and can withstand a cable being yanked or wiggled. On a SATA connector, this kind of action will break the male side of the connection (the hard drive or host adapter), rendering the device unusable. With an eSATA connector, considerably more force is needed to damage the connector, and even in this situation, only the female side (the cable itself) will break, possibly leaving the male usable.^{[citation needed]}

eSATA compared to other buses

	eSATA	SATA 300	SATA 150	PATA 133	FireWire 800	FireWire 400	USB 2.0	Ultra-320 SCSI
Speed (Mbit/s)	2400	2400	1200	1064	786	393	480 (burst)	2560
Max. cable length (m)	2	1	1	0.46	4.5 (16 cables can be daisy chained up to 72 m)	4.5 (16 cables can be daisy chained up to 72 m)	5 (USB hubs can be daisy chained up to 25 m)	12
Power provided	No	No	No	No	Yes (12-25 V, 15 W)	Yes (12-25 V, 15 W)	Yes (5 V, 2.5 W)	No
Devices per Channel	1 (5 with port multiplier)	1 per line	1 per line	2	63	63	127	16

Unlike PATA, both SATA and eSATA are designed to support hot-plugging. However, this feature requires proper support at the host, device (drive), and operating-system level. In general, all SATA/devices (drives) support hot-plugging (due to the requirements on the device-side), but requisite support is less common on SATA host adapters.

USB allows hot-plugging; this is supported by virtually every current operating system. However, USB-based storage hardware can infrequently sustain data loss when disconnected. This problem exists with media players and digital cameras using flash memory as well as mobile 2.5-inch USB hard drives. Firmware damage and data loss can occasionally result from unclean spin-downs and power loss when the drive or device is removed from the USB port without first initiating a device shutdown via the computer's operating system. ^{[citation needed]}

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

Serial Attached SCSI (SAS) is designed for hot-plugging.

Backward and forward compatibility

SATA and PATA

At the device level, SATA and PATA devices are completely incompatible -- they cannot be interconnected. At the application level, SATA devices are specified to look and act like PATA devices. In early motherboard implementations of SATA, backward compatibility allowed SATA drives to be used as drop-in replacements for PATA drives, even without native (driver-level) support at the operating system level.

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

Powered enclosures are available for both PATA and SATA drives, which interface to the PC through USB or Firewire, 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-150 and SATA-300

SATA is designed to be backwards and forwards compatible with future revisions of the SATA standard [6]. Unfortunately, there are already documented incompatibilities between particular first-generation SATA/150 controllers and later-generation drives.

According to the hard drive manufacture Maxtor, motherboard host controllers using the VIA and SIS chipsets VT8237, VT8237R, VT6420, VT6421L, SIS760, SIS964 found on the ECS 755-A2 which was manufactured in 2003, do not support SATA II 300 drives. To address interoperability problems, the largest hard drive manufacture Seagate\Maxtor have added a user-accessible jumper-switch known as the Force 150, to select between SATA-150 and SATA/300 operation [7]. Users with a SATA-150 motherboard should either buy an ordinary SATA-150 hard disk, or buy a SATA-300 hard disk with the user-accessible jumper or buy a PCI or PCI-E card to add full SATA II capability and compatibility..

SATA vs SCSI

SCSI currently offers transfer rates higher than SATA, but is a more complex bus usually resulting in higher manufacturing cost. Some drive manufacturers offer longer warranties for SCSI devices, however, indicating a possibly higher manufacturing quality control of SCSI devices compared to PATA/SATA devices. SCSI buses also allow connection of several drives (using multiple channels, 7 or 15 on each channel) whereas SATA only allows one per channel.

SATA 3.0 Gbit/s offers a maximum bandwidth of 300 MB/s per device compared to SCSI with a maximum of 320 MB/s. Also, SCSI drives provide greater sustained throughput than SATA drives because of disconnect-reconnect and aggregating performance. SATA devices are generally compatible with SAS enclosures and adapters, while SCSI devices cannot be directly connected to a SATA bus.

SCSI hardware is used in enterprises for server purposes. The MTBF of SATA drives is usually about 600,000 hours, while SCSI drives are rated for upwards of 1,500,000 hours.

External links