Serial ATA (SATA) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. SATA is the serial version of the parallel Advanced Technology Attachment (ATA) interface. It transmits data in serial fashion rather than parallel fashion. This allows for cabling that is thinner, more flexible, and easier to work with.
The SATA specification defines three distinct protocol layers: physical, link, and transport. This separation allows SATA to be highly compatible with legacy ATA software drivers and applications. SATA can operate at data transfer speeds ranging from 1.5 Gbit/s (first generation) up to 16 Gbit/s for the latest generation.
Some key advantages of Serial ATA over Parallel ATA include:
– Thinner, more flexible cables for improved airflow and routing
– Point-to-point connections instead of shared bus architecture for improved scalability
– Hot-swapping support for increased uptime and replaceability
– Native command queuing for increased performance
– Lower voltage requirements for reduced power consumption
While nearly all modern computers use SATA instead of Parallel ATA, many continue to use the traditional ATA/PATA drive size terminology. For example, a common hard drive form factor is referred to as a 3.5″ drive bay, even though it is designed for a SATA drive.
Brief History of SATA
The SATA specification was originally conceived in 2001 by the Serial ATA International Organization, a group led by major storage companies like Intel, Dell, Seagate, Western Digital, and others. The new standard was deemed Serial ATA 1.0 and provided capabilities up to 1.5 Gbit/s.
In 2003, the revisions SATA 1.0a and SATA 1.1 were released to make some minor updates for improved stability and consistency across vendors.
The much more significant update came in 2004 with SATA 2.0. This increased maximum bandwidth to 3.0 Gbit/s. A further “SATA 2.6” update increased maximum speed to 3.2 Gbit/s.
In July 2006, SATA 3.0 or SATA 6G was announced with a maximum theoretical transfer rate doubled again to 6.0 Gbit/s. The full 3.0 standard was released in May 2009.
August 2008 saw the release of the SATA 3.1 specification. This made only minor changes for better integration with the existing SATA 2.6 standard.
SATA 3.2 was released in August 2013 to enable device speeds of up to 16 Gbit/s, though usable bandwidth is lower due to 8b/10b encoding overhead. The increase to 16 Gbit/s bandwidth also facilitates transfers with the PCI Express 3.0 bus, bringing them to parity.
SATA Cables and Connectors
Unlike PATA ribbon cables, SATA cables are thin and flexible for easier installation and improved interior airflow. They use shielded twisted pair conductors, similar to other standards like USB and Gigabit Ethernet.
Original SATA cables use 7 conductors and can reach up to 1 meter in length. Later SATA 3.0 cables contain an extra pair of conductors to enable higher speeds, though they maintain backward compatibility with older SATA ports.
The SATA connector itself features a compact L-shaped design. It is about half the height of the larger PATA connector, yet robust enough for up to 50 insertion cycles. The smaller dimensions allow SATA ports to be positioned closer together on printed circuit boards.
Data Connector
The standard SATA data connector is typically described as a 7-pin connector, containing seven conductive contacts for data transmission purposes. The shape consists of an “L” next to a vertical rectangle.
The thinner L-shaped section contains side-by-side logic power and ground pins. The thicker vertical section contains two differential signal pairs for transmit and receive, along with ground lines located between each pair.
Power Connector
Along with the data connector, most SATA devices also have a 15-pin power connector to deliver required voltage. It consists of three grounds, three +3.3V lines, and three +5V lines.
This compact power connector allows SATA devices to be designed without onboard power circuitry, helping reduce costs. The connector features staggered pins to prevent improper insertion.
Not all SATA devices require the supplemental power connector. Many adapters and solid-state drives can draw adequate power through the data cable.
SATA Revisions
Many revisions of the SATA specification have been created over the years to improve performance, enhance features, and add support for new capabilities.
SATA 1.0 (1.5 Gbit/s)
The first generation SATA interface ran at speeds up to 1.5 Gigabit per second (Gbit/s). Though not nearly as fast as some later versions, it still provided a huge boost over Parallel ATA, Firewire 400, and USB 1.1 connections common at the time.
Actual usable bandwidth was up to 150 Megabytes per second (MB/s) due to 8b/10b encoding overhead.
SATA 2.0 (3.0 Gbit/s)
SATA 2.0 increased maximum bandwidth to 3.0 Gbit/s. This allowed for drives fast enough to fully saturate the first generation SATA 1.5 Gbit/s bus speed. Backward compatibility allowed mixing SATA 1.0 and SATA 2.0 devices on the same controller.
SATA 3.0 (6.0 Gbit/s)
This third generation SATA interface increased theoretical transfer rates to 6.0 Gbit/s. A further “SATA 3.2” update increased this to 6.4 Gbit/s. Real bandwidth also essentially doubled compared to SATA 2.0, providing up to 600 MB/s transfer speeds.
Larger 19-pin data connectors were introduced to enable the faster speeds. However, these remained backward compatible with the original 7-pin SATA connectors.
SATA 3.3 (16 Gbit/s)
Released in 2016, SATA 3.3 bumped maximum line speed up to 16 Gbit/s, though usable bandwidth peaks around 1969 MB/s. This interface helps facilitate parity in speed with PCI Express 3.0 bus transfers.
SATA Device Types
Many varieties of storage devices come equipped with a SATA interface. The most common examples include:
Hard Disk Drives (HDDs)
One of the original goals of SATA was to replace Parallel ATA connections on internal hard disk drives. Nearly all modern HDDs now use either SATA or the newer NVMe interfaces. Many are backwards compatible and can function on either SATA or NVMe controller connections.
Solid State Drives (SSDs)
Solid state drives commonly use the SATA interface, especially on older SATA 2.0 and SATA 3.0 generation controllers. Newer SSDs increasingly utilize PCIe NVMe connections instead for improved bandwidth, but may retain SATA compatibility.
Optical Drives
CD, DVD, and Blu-Ray disc drives transitioned from Parallel ATA to SATA connections as optical storage declined in usage. However, nearly all modern laptops and desktops no longer include built-in optical drives. Externally-attached optical drives typically use USB instead of SATA.
RAID Cards
Many RAID cards and RAID enclosures provide SATA connectivity for attaching HDDs and SSDs. This allows assembling external storage arrays with redundancy features like mirroring, parity, and striping. RAID over faster NVMe is also increasingly common.
SATA Host Bus Adapters
SATA requires host bus adapter (HBA) controllers to provide the SATA ports used to connect devices. These adapters contain the hardware interfaces, drivers, and logic required for communication between SATA devices and the rest of the computer system.
On PCs, SATA HBAs typically take one of two forms:
Integrated SATA Controllers
Most modern motherboards have integrated SATA controllers built directly into the chipset or as a discrete chip on board. This provides between 2 to 8 native SATA ports, though availability varies by model. Integrated SATA runs at the full speed of the particular SATA generation supported.
Add-In SATA Cards
For additional ports, PCI or PCI Express add-in cards can be installed to expand SATA capabilities. These low profile cards commonly provide 1-8 extra ports, depending on model. They function similar to integrated SATA, but will typically not boot by default.
SATA vs. NVMe
NVMe (Non-Volatile Memory Express) is a newer interface that is replacing SATA for connecting high-speed storage devices. Compared to SATA, key advantages of NVMe include:
– Much higher bandwidth – NVMe runs up to 5GB/s on PCIe 4.0 x4 lanes compared to SATA’s 0.6GB/s peak.
– Lower latency – The streamlined NVMe protocol has less processing overhead than SATA, leading to faster response times.
– More queues – NVMe supports up to 65,535 queues and 64K commands per queue for increased parallelism.
– Improved scalability – The NVMe point-to-point architecture connects each device directly to the PCIe bus instead of shared bus cables.
However, SATA retains some advantages over NVMe:
– Lower cost – SATA drives remain far less expensive than NVMe models. The SATA interface itself has largely been amortized.
– Hardware support – SATA ports and cables remain nearly universally supported, while NVMe requires PCIe lanes and slots.
– Maturity – SATA is a mature, refined standard that will still remain viable into the foreseeable future. NVMe continues to evolve.
In newer systems, NVMe tends to be used for boot drives and other high-performance storage roles. SATA handles more cost-sensitive secondary storage and external devices. The two are complimentary rather than exclusive.
Conclusion
The SATA interface helped usher in the era of high-speed serial connections for storage devices. Its increased speeds, smaller form factor cables, and hot-swap capabilities accelerated the transition away from Parallel ATA.
Though increasingly supplanted by NVMe for top-tier storage performance, Serial ATA remains widely used today for HDDs, SSDs, optical drives, and RAID arrays across consumer and enterprise environments. SATA will continue playing a role in connecting lower-cost and legacy storage devices into computer systems for years to come.
