SATA Types

SATA 1.5 Gbit/s

First-generation SATA interfaces, also known as SATA/150 or SATA 1, run at 1.5 gigabits per second (Gbit/s). 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 Gbit/s, or 150 megabytes per second (MB/s) (or 146.48 MiB/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 Gbit/s

Soon after SATA's introduction, enhancements were made to the standard. A 3 Gbit/s signaling rate was added to the PHY layer, offering up to twice the data throughput. Like SATA 1.5 Gbit/s, SATA 3.0 Gbit/s uses 8B/10B encoding, resulting in a maximum data transfer rate of 2.4 Gbit/s or 300 MB/s for the wire. However, hard drives cannot supply data nearly at these speeds, so the actual speed depends on the hard disk.

To ensure seamless backward compatibility between SATA 1.5 Gbit/s controllers and SATA 3.0 Gbit/s devices, the latter devices are required to support the original 1.5 Gbit/s rate. In practice, some older SATA controllers do not support SATA speed negotiation, and require that SATA 3.0 Gbit/s peripherals be manually hardlimited to 1.5 Gbit/s with the use of a jumper. [1] Chipsets which exhibit this problem 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.

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.) Most SATA drive and controller manufacturers also do not use the term “SATA II”.  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.

Future design: SATA 6.0 Gbit/s

SATA-IO plans to make a 6.0 Gbit/s standard. Although the theoretical throughput would be doubled, conventional hard disks cannot approach saturating this speed. The 6.0 Gbit/s standard will however be useful in combination with port multipliers, which allow multiple drives to be connected to one Serial ATA port, as well as with solid-state drives such as RAM disks.

In actual use in modern personal computers both SATA 3 Gbit/s and SATA 1.5 Gbit/s hard disk drives run at non-burst speeds comparable to earlier IDE interfaces (under 50 MB/s). 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 drops the 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 queuing.

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 -- but they do tend to come loose much more often.[citation needed]

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 power connector is known to be quite flimsy, as the thin plastic tops of the connectors (see power connector picture at right) will often break off when even the slightest force is used to wiggle it whilst it is plugged in (as is often required in tight spaces), rendering the connector useless.

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. The same physical connections are used on 3.5-in (90 mm) and 2.5-in (70 mm) (notebook) hard disks. Some SATA drives include a PATA-style 4-pin Molex connector for use with power supplies that lack the SATA power connector.

Color Function Yellow +12V Black Ground Red +5V Black Ground Orange +3.3V

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 hot plugging as mentioned in the previous paragraph.

External SATA

Standardized in mid-2004, eSATA defined cables, connectors, and signal requirements for external SATA drives:

- Full SATA speed for external disks (115 MB/s have been measured with external RAID enclosures)

- Identical logical signalling (link/transport-layer and above), allowing native SATA traffic from end-to-end, all disk features are available to the host

- Maximum cable length less than 2 metres (USB and Firewire allow longer distances)

- Minimum and maximum transmit voltage increased to 500 mV - 600 mV (from 400 mV - 600 mV)

- Minimum and maximum receive voltage decreased to 240 mV - 600 mV (from 325 mV - 600 mV)

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 (up to 16 or even 127) whereas SATA only allows one per cable.

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 per bus.

SATA 2 devices are generally compatible with SAS enclosures and adapters, while SCSI devices cannot be directly connected to a SAS bus.

Another difference between ATA and SCSI is reliability,