Serial ATA History
First Serial ATA controllers and drives were first introduced in late 2002. Different from the 40-wire UltraATA protocol (which is a parallel bus for up to 100/133 MB/s), Serial ATA is based on two low-voltage high-speed signal pairs with 8/10 bit encoding (8 bit net transfer rate at 10 bit gross transfer rate). Pins 2, 3, 5 and 6 are used for data, 1, 4 and 7 are ground pins. Serial ATA/150 provides a maximum net bandwidth of 150 MB/s based on a gross transfer speed of 1.5 Gb/s. Serial ATA/300 doubles the transfer speed to 3.0 Gb/s or 300 MB/s net. The Serial ATA interface lowers cost and is highly scalable by simply speeding up the link speed. Also, SATA cables are easy to handle and can extend up to 39" (1 m) in length as opposed to 18" with UltraATA. SATA cables also don't obstruct the airflow inside the case. The downside is somewhat increased power consumption, which can be observed with all sorts of high-speed serial point-to-point interconnects (think of PCI Express).
Serial ATA also introduced a 15-pin power connector with hot-plugging support, offering 3.3 V, 5 V and 12 V power to hard drives in 3.5" and 2.5" form factors. The traditional Molex connector (which you know from hard drives or optical drives) doesn't offer 3.3 V power, and it doesn't fit on 2.5" hard drives. SATA also introduced a feature called Native Command Queuing (known as NCQ), which allows the drive to queue up incoming commands, analyze them, and process them in an efficient order. This means that the drive should not have to reposition read/write heads more than necessary. This way, accelerating and slowing down the actuator can be minimized, and the drive can spend most of its time efficiently reading or writing blocks sequentially.
In 2004, the SATA specification was revised to support external devices, which is known as eSATA. The goal was to maintain all SATA features and to improve flexibility. Transmission and reception voltages were increased to double the cable length to two meters. At the same time, the connectors were revised as well to make them more robust. As a consequence, eSATA connectors are not compatible with internal SATA cabling.
| SATA 2.5 Specification | SATA/150 | SATA/300 | eSATA |
|---|---|---|---|
| Gross Bandwidth | 1.5 Gb/s | 3.0 Gb/s | 3.0 Gb/s |
| Net Bandwidth | 150 MB/s | 300 MB/s | 300 MB/s |
| Max. Cable Length | 1 m (39") | 1 m (39") | 2 m (78") |
| Signaling Voltage | Send: 400-600 mVReceive: 325-600 mV | Send: 400-600 mVReceive: 325-600 mV | Send: 500-600 mVReceive: 240-600 mV |
| Max. Devices per Port | 1 | 1 | Up to 15 with port multipliers |
| NCQ Support | Yes | Yes | Yes |
