Serial ATA

In computer hardware, Serial ATA (SATA, IPA: /ˈsata/ or /ˈseiˌtə/) is a computer bus technology primarily designed for transfer of data to and from a hard disk. It is the successor to the legacy Advanced Technology Attachment standard (ATA, also known as IDE). This older technology was retroactively renamed Parallel ATA (PATA) to distinguish it from Serial ATA.

SATA 1.5 Gb/s

First-generation Serial ATA interfaces, also known as SATA/150, run at 1.5 Gigahertz (GHz). Serial ATA uses 8B/10B encoding at the physical layer. This encoding scheme has an efficiency of 80%, resulting in an actual data transfer rate of 1.2 Gigabits per second (Gb/s), or 150 megabytes per second (MB/s). The relative simplicity of a serial link and the use of LVDS allow both the use of longer drive cables and an easier transition path to higher speeds.

SATA 3.0 Gb/s

Soon after SATA's introduction, enhancements were made to the standard. A 3Gb/s signalling rate was added to the PHY layer, offering up to twice the data throughput. To ensure seamless backward compatibility between older SATA and the newer faster SATA/3Gbs devices, the latter devices are required to support the original 1.5Gb/s rate. In practice, some older SATA systems that do not support SATA speed negotiation require the peripheral drive's speed be manually hardlimited to 150 MB/s with the use of a jumper for a 300 MB/s drive. ^*

Like SATA 1.5Gb/s, SATA 3Gb/s uses 8B/10B encoding resulting in an actual data transfer rate of 2.4 Gb/s, or 300 MB/s.

The 3.0 Gb/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 3Gb/s specification being only one of many that the former SATA II defined, and suggests that “SATA 3Gb/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-IO plans to further increase the maximum throughput of Serial ATA to 600 MB/s around the year 2007.

SATA 3Gb/s is sometimes also referred to as SATA/300 or SATA II, continuing the line of PATA/100, PATA/133 and SATA/150.

SATA 6.0 Gb/s

SATA-IO plans to make a 6.0 Gb/s standard. Although the theoretical thoroughput would be doubled, conventional hard disks can't approach saturating this speed.

Serial ATA innovations

SATA drops the master/slave shared bus of PATA, giving each device a dedicated cable and dedicated bandwidth. While this requires twice the number of host controllers to support the same number of SATA devices, at the time of SATA's introduction this was no longer a significant drawback. Another controller could be added into a controller ASIC at little cost beyond the addition of the extra seven signal lines and printed circuit board (PCB) space for the cable header.

Features allowed for by SATA but not by PATA include hot-swapping and native command queueing.

To ease their transition to SATA, many manufacturers have produced drives which use controllers largely identical to those on their PATA drives and include a bridge chip on the logic board. Bridged drives have a SATA connector, may include either or both kinds of power connectors, and generally perform identically to native drives. They may, however, lack support for some SATA-specific features. As of 2004, all major hard drive manufacturers produce either bridged or native SATA drives.

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

Physically, the SATA power and data cables are the most noticeable change from Parallel ATA. The SATA standard defines a data cable using seven conductors 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 46 cm (18 in) in length. The reduction in conductors makes SATA connectors and cables much narrower than those of PATA, thus making them more convenient to route within tight spaces and reducing obstructions to air cooling. Unlike early PATA connectors, SATA connectors are keyed — it is not possible to install cable connectors upside down without considerable force.

The SATA standard also specifies a power connector sharply differing from the four-pin Molex connector used by PATA drives and many other computer components. Like the data cable, it is wafer-based, but its wider 15-pin shape should prevent confusion between the two. The seemingly large number of pins are used to supply three different voltages if necessary — 3.3 V, 5 V, and 12 V. Each voltage is supplied by three pins ganged together (and 5 pins for ground). This is because the small pins cannot supply sufficient current for some devices, so they are combined. One pin from each of the three voltages is also used for hotplugging. The same physical connections are used on 3.5-in (90mm) and 2.5-in (70mm) (notebook) hard disks. Some SATA drives include in PATA style four-pin Molex connector for use with power supplies that lack the SATA power connector. Also, adaptors are available to convert a PATA style power connector to SATA power connector.

External SATA

eSATA was standardized in mid-2004, with specifically defined cables, connectors, and signal requirements for external SATA drives. eSATA is characterized by:

Full SATA speed for external disks (115MB/s have been measured with external RAID enclosures)
No protocol conversion from IDE/SATA to USB/Firewire, all disk features are available to the host
Cable length is restricted to 2m, USB and Firewire span longer distances.
Minimum and maximum transmit voltage decreased to 400mV - 500mV
Minimum and maximum receive voltage decreased to 240mV - 500mV

USB and Firewire require conversion of all communication with the external disk, so external USB/Firewire enclosures include an IDE or SATA bridge chip that translates from the ATA protocol to USB or Firewire. Drive features like S.M.A.R.T. cannot be exploited that way and the achiveable transfer speed with USB/Firewire is only about half of the entire bus data rate of about 50MB/s. This limited effective data transfer rate becomes very visible when using an external RAID array and also with fast single disks which may yield well over 70MB/s during real use.

Currently, most PC motherboards do not have an eSATA connector. eSATA may be enabled through the addition of an eSATA host bus adapter (HBA) or bracket connector for desktop systems or with a Cardbus or ExpressCard for notebooks.

Note: Prior to the final specification for eSATA, 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 IEEE 1394. These products are not eSATA compliant.

eSATA does not provide power, which means that external 2.5" disks which would otherwise be powered over the USB or Firewire cable need a separate power cable when connected over eSATA.

eSATA compared to other buses

	eSATA	PATA	Fire Wire 1394b	USB 2.0
Actual Speed	2.4 Gib/s	1064 Mib/s	786 Mib/s	~375 Mib/s
Max. cable length	2 meters	46 centimetres	4.5 meters 16 cables can be daisy chained up to 72 meters	5 meters
Power cable required?	Yes	Yes	No	No
Devices per Channel	1 (5 with multiplier)	3 (3rd device read only)	63	127

Backward compatibility

The backward compatibility of SATA hard discs is virtually non-existent in the sense that SATA drives will not work with the same connectors that IDE, SCSI, or any other format of hard drive connect to. It is, however, possible to purchase convertors that attach to the rear of the SATA hard disc and will allow it to function as an IDE drive. This can prove useful in situations where one wishes to use their SATA drive on older motherboards that may not have SATA connections, etc.

SATA vs SCSI

SCSI currently offers transfer rates higher than SATA, but is a more complex bus usually resulting in higher costs to the user. 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.

External links