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Serial ATA
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Serial Advanced Technology Attachment (SATA, , or ) is a computer bus primarily designed for transfer of data between a computer and mass storage devices such as hard disk drives and optical drives.
The main advantages over the older parallel ATA interface are faster data transfer, ability to remove or add devices while operating, thinner cables that let air cooling work more efficiently, and more reliable operation with tighter data integrity checks.
It was designed as a successor to the Advanced Technology Attachment standard (ATA), and is expected to eventually replace the older technology. Serial ATA adapters and devices communicate over a high-speed serial cable.
Advanced Host Controller Interface
The standard interface for SATA controllers is Advanced Host Controller Interface (AHCI), which allows advanced features of SATA such as hot plug 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 features of devices to be accessed that are not supported by the ATA/IDE standard. Windows device drivers that are labeled as SATA are usually running in IDE emulation mode unless they explicitly state that they are AHCI. While the drivers included with Windows XP do not support AHCI, AHCI has been implemented by proprietary device drivers. Windows Vista and the current versions of Mac OS X and Linux have native support for AHCI.
Features The current SATA specification can support data transfer rates as high as 3.0 Gbit/s per device. SATA uses only 4 signal lines; cables are more compact and cheaper than for PATA. SATA supports hot-swapping and NCQ. There is a special connector (eSATA) specified for external devices, and an optionally implemented provision for clips to hold internal connectors firmly in place. SATA drives may be plugged into Serial Attached SCSI (SAS) controllers and communicate on the same physical cable as native SAS disks, but SATA controllers cannot handle SAS disks.
Throughput
SATA 1.5 Gbit/s First-generation SATA interfaces, also known as SATA/150 or unofficially as SATA 1, communicate at a rate of 1.5 gigabits per second. Taking into account 8b10b coding overhead, the actual uncoded transfer-rate is 1.2 Gbit/s, or 1,200 megabits per second. The theoretical burst throughput of SATA/150 is similar to that of PATA/133, but newer SATA devices offer enhancements such as NCQ which improve performance in a multitasking environment. Data transfer rates are limited by mechanical hard drives themselves, not the interfaces: the fastest modern desktop hard drives transfer data at a maximum of about 118 MB/s, which is well within the capabilities of even the older PATA/133 specification.
During the initial period after SATA/150'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 generally perform identically to their PATA equivalents. Most 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 were observed. At the application level SATA could only handle one pending transaction at a time like PATA. The SCSI interface has long been able to accept multiple outstanding requests and service them in the order which minimizes response time. This feature, Native Command Queuing (NCQ), was adopted as an optional supported feature for SATA 1.5 Gbit/s and SATA 3.0 Gbit/s devices.
First-generation SATA devices were at best a little faster than parallel ATA/133 devices. A 3 Gbit/s signaling rate was added to the Physical layer (PHY layer), effectively doubling maximum 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. A SATA data cable rated for 1.5 Gbit/s will handle current second-generation SATA 3.0 Gbit/s drives without any loss of sustained and burst data transfer 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 fall back 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 the user to set the SATA 3.0 Gbit/s peripherals to 1.5 Gbit/s mode, generally through the use of a jumper. Chipsets known to have this fault include the VIA VT8237 and VT8237R south bridges, and the VIA VT6420 and VT6421L standalone SATA controllers. SiS's 760 and 964 chipsets also initially exhibited this problem, though it can be rectified with an updated SATA controller ROM.
This table shows the real speed of SATA 1.5 Gbit/s and SATA 3 Gbit/s (note the bottom row shows megabytes per second (MB/s, not Mbit/s):
| | SATA 1.5 Gbit/s | SATA 3 Gbit/s |
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| Frequency | 1500 MHz | 3000 MHz |
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| Bits/clock | 1 | 1 |
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| 8b10b encoding | 80% | 80% |
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| bits/Byte | 8 | 8 |
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| Real speed | 150 MB/s | 300 MB/s |
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SATA II Misnomer The 3.0 Gbit/s specification has been widely referred to as "Serial ATA II" ("SATA II" or "SATA2"), contrary to the wishes of the Serial ATA International Organization (SATA-IO) which defines the standard. SATA II was originally the name of a committee defining updated SATA standards, of which the 3 Gbit/s standard was just one. However since it was among the most prominent features defined by the former SATA II committee, the name SATA II became synonymous with the 3 Gbit/s standard, so the group has since changed names to the Serial ATA International Organization, or SATA-IO, to avoid further confusion.
SATA 6.0 Gbit/s SATA-IO has already come up with the draft specification of 6GBPS Phy Layer in July 2008. In current PCs, SATA 3.0 Gbit/s already greatly exceeds the sustainable (non-burst) transfer rate of even the fastest hard disks. The 6.0 Gbit/s standard is useful right now 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. Solid-state drives may also one day make use of the faster transfer rate.
Cables and connectors Connectors and cables are the most visible difference between SATA and Parallel ATA drives. Unlike PATA, the same connectors are used on 3.5-in (90 mm) SATA hard disks for desktop and server computers and 2.5-in (70 mm) disks for portable or small computers; this allows 2.5" drives to be used in desktop computers without the need for wiring adapters (a mounting adaptor is still likely to be needed to securely mount the drive).
SATA power connectors and data connectors have been criticized for their fragility and poor robustness — the thin plastic tops of the connectors (see power connector picture at right) can easily break due to shearing force when the user pulls the plug at a non-orthogonal angle, as can the connectors on drives they connect to. In the case of a broken connector on a hard drive, this could result in a complete loss of access to all data stored on the drive.
Data| Pin # | Function |
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| 1 | Ground | | 2 | A+ (Transmit) | | 3 | A− (Transmit) | | 4 | Ground | | 5 | B− (Receive) | | 6 | B+ (Receive) | | 7 | Ground | | - | coding notch | | | A 7-pin Serial ATA right-angle data cable. |
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, and connect one motherboard socket to one hard drive. PATA ribbon cables, in comparison, connect one motherboard socket to up to two hard drives, carry either 40- or 80-conductor wires, and are limited to 45 cm (18 in) in length by the PATA specification (however, cables up to 90 cm (36 in) are readily available). Thus, SATA connectors and cables are easier to fit in closed spaces and reduce obstructions to air cooling. They are more susceptible to accidental unplugging and breakage than PATA, but cables can be purchased that have a 'locking' feature, whereby a small (usually metal) spring holds the plug in the socket.
Parallel ATA uses single-ended signalling. In this system, the noise combines with the data signal during the signal transmission. Noise causes significant interference with the data signal at higher speeds. In order to reduce the noise interference, the driving voltage of Parallel ATA is as high as 5 V. Although the higher voltage can reduce the noise interference, the 5 V is too high for modern high speed silicon devices. Thus the fabrication cost of driving ICs is higher, and the speed is limited in comparison to low voltage silicon ICs.
In comparison, SATA systems use differential signaling. In this system, it is easy to filter out the noise from the data signal at the receiving end. The higher noise rejection allows the SATA system to use only 500 mV peak-to-peak differential voltage to carry the signal at higher speeds without distortion or noise interference.
Compared with the 5 V driving voltage in PATA ribbon cables, the 0.5 V in SATA cables in theory make the SATA system much more power efficient. However most SATA chipsets need significantly more power than PATA chipsets, due to the faster required encoding per wire.
Power| Pin # | Function |
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| | 1–3 | 3.3Volts | | | 4–6 | Ground | | | 7–9 | 5V | | | 10 | Ground | | | 11 | Staggered spinup
(in supporting drives) | | | 12 | Ground | | | 13–15 | 12V | | | A 15-pin Serial ATA power connector. |
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 in addition to the SATA power connector.
There are more pins than the traditional connector for several reasons:
- A third voltage is supplied – 3.3 V – in addition to the traditional 5 V, and 12 V.
- Each voltage is supplied by three pins ganged together – because the small pins by themselves cannot supply sufficient current for some devices. (Each pin should be able to provide 1.5 A.)
- For each of the three voltages, one of the three pins is used for hotplugging.
- Ground is provided by five pins ganged together.
- Pin 11 is used in newer drives for staggered spinup.
Adaptors are available to convert a 4-pin Molex connector to a 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 unconnected. 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.
Topology
SATA is a point to point architecture. The connection between the controller and the storage device is direct.
In a modern PC system, the SATA controller is usually found on the motherboard, or installed in a PCI or PCI Express slot. Some SATA controllers have multiple SATA ports and can be connected to multiple storage devices. There are also port expanders which allow multiple storage devices to be connected to a single SATA controller port.
Encoding These high-speed transmission protocols use a logic encoding known as 8b10b. The signal is sent using Non-return to Zero (NRZ) encoding with Low Voltage Differential Signaling (LVDS).
In the 8b10b encoding the synchronizing signal is included in the data sequence. This technique is known as Clock Data Recovery, because it doesn't use a separate synchronizing signal. Instead, it uses the serial signal's 0 to 1 transitions to recover the clock signal.
External SATA
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
- The external cable connector is a shielded version of the connector specified in SATA 1.0a with these basic differences:
- The External connector has no “L” shaped key, and the guide features are vertically offset and reduced in size. This prevents the use of unshielded internal cables in external applications.
- To prevent ESD damage, the insertion depth is increased from 5 mm to 6.6 mm and the contacts are mounted farther back in both the receptacle and plug.
- To provide EMI protection and meet FCC and CE emission requirements, the cable has an extra layer of shielding, and the connectors have metal contact points.
- There are springs as retention features built into the connector shield on both the top and bottom surfaces.
- The external connector and cable are designed for over five thousand insertions and removals while the internal connector is only specified to withstand fifty.
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. Some single disks can transfer almost 120 MB/s during real use, more than twice the maximum transfer rate of USB 2.0 or FireWire 400 (IEEE 1394a) and well in excess of the maximum transfer rate of FireWire 800, though the S3200 FireWire 1394b spec reaches ~400 MB/s (3.2Gb/s). Finally, some low-level drive features, such as S.M.A.R.T., may not be available through USB or FireWire bridging. eSATA does not suffer from these issues.
It is likely that eSATA co-exists with USB 2.0 and FireWire external storage for several reasons. As of early 2008 the vast majority of mass-market computers have USB ports and many computers and consumer electronic appliances have FireWire ports, but few devices have external SATA connectors. For small form-factor devices (such as external 2.5" (70 mm) 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.
Desktop computers that lack a built-in eSATA interface can 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 may be of interest to the enterprise and server market, which has already standardized on the Serial Attached SCSI (SAS) interface, because of its hotplug capability and low price.
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 handling, 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 yanking or wiggling which would break a male SATA connector (the hard drive or host adapter, usually fitted inside the computer). With an eSATA connector considerably more force is needed to damage the connector, and if it does break it is likely to be the female side, on the cable itself, which is relatively easy to replace.
There were a number of disadvantages in using SATA drives as external hard disks. An independent IT specialist attempted to redesign and enhance the interface in 2002; which would have provided for an external SATA interface with power, USB functionality, and have the interface positioned on the front of a desktop PC. The patent of the "ATA Device Attachment System" described a number of disadvantages and remedies to the interface; written by It has only taken 5 years since, for SATA-IO to announce suitable power for the external SATA BUS.
Backward and forward compatibility
SATA and PATAAt 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 to 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, 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/sSATA is designed to be backward and forward compatible with future revisions of the SATA standard.
According to the hard drive manufacturer 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 3 Gbit/s drives. To address interoperability problems, the largest hard drive manufacturer Seagate/Maxtor have added a user-accessible jumper-switch known as the Force 150, to switch between 150 MB/s and 300 MB/s operation. Users with a SATA 1.5 Gbit/s motherboard with one of the listed chipsets should either buy an ordinary SATA 1.5 Gbit/s hard disk, buy a SATA 3 Gbit/s hard disk with the user-accessible jumper, or buy a PCI or PCI-E card to add full SATA 3 Gbit/s capability and compatibility. Western Digital uses jumper setting called "OPT1 Enabled" to force 150 MB/s data transfer speed.
Comparisons with other interfaces
SATA and 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 allows one drive per channel, unless using a port multiplier.
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, SAS and FC drives are typically more expensive so they are traditionally used in servers and disk arrays where the added cost is justifiable. Inexpensive ATA and SATA drives evolved in the home computer market, hence the general opinion is that they are less reliable. As those two worlds started to overlap, the subject of reliability became somewhat controversial. It is worth noting that generally a disk drive has a low failure rate because of increased quality of its heads, platters and supporting manufacturing processes, not because of having a certain interface.
See also
External links
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