Serial ATA - or simply SATA - is the hard disk standard created to replace the parallel ATA interface, a.k.a. IDE. SATA provides a transfer rate of 150 MB/s or 300 MB/s against of a 133 MB/s maximum using the previous technology. In this tutorial we will explain everything you need to know about Serial ATA.
The conventional IDE port (now called parallel ATA or simply PATA) transfers data in parallel. The advantage of parallel transmission over serial transmission is the higher speed of the former mode, seeing that several bits are sent at the same time. Its major disadvantage, however, relates to noise. As many wires have to be used (at least one for each bit to be sent per turn), one wire generates interference in another. This is why ATA-66 and higher hard disks require a special, 80-wire cable. The difference between this 80-wire cable and the normal 40-wire IDE cable is that it includes a ground wire between each original wire, providing anti-interference shielding. In our tutorial Everything you Need to Know About ATA-66, ATA-100 and ATA-133 Hard Disks we provide an in-depth explanation on this issue. The current maximum transference rate of the parallel IDE standard is 133 MB/s (ATA/133).
Serial ATA, on the other hand, transmits data in serial mode, i.e., one bit per time. Traditional thinking makes us to think that serial transmission is slower than parallel transmission. This is only true if we are comparing transmissions using the same clock rate. In this case parallel transmission will be at least eight times faster, as it transmits at least eight bits (one byte) per clock cycle, compared to serial transmission where only one bit is transmitted per clock cycle. However, if a higher clock rate is used on serial transmission, it can be faster than parallel. That’s exactly what happens with Serial ATA.
The problem in increasing parallel transmission transfer rate is increasing the clock rate, as the higher the clock rate, more problems with electromagnetic interference show up. Since serial transmission uses just one wire to transmit data it has fewer problems with noise, allowing it to use very high clock rates, achieving a higher transfer rate.
Serial ATA standard transfer rate is of 1,500 Mbps. As it uses 8B/10B coding – where each group of eight bits is coded into a 10-bit number – its effective clock rate is of 150 MB/s. Serial ATA devices running at this standard speed are also known as SATA-150. Serial ATA II provides new features such as Native Command Queuing (NCQ), plus a higher speed rate of 300 MB/s. Devices that can run at this speed are called SATA-300. The next standard to be released will be SATA-600.
It is important to note that SATA II and SATA-300 are not synonyms. One can build a device that runs only at 150 MB/s but using new features provided by SATA II such as NCQ. This device would be a SATA II device, even though it doesn’t run at 300 MB/s.
Native Command Queuing (NCQ) increases the hard disk drive performance by reordering the commands send by the computer. Read our tutorial NCQ (Native Command Queuing) and TCQ (Tagged Command Queuing) Explained to a full explanation on this technology. In summary, if your motherboard has SATA II ports supporting NCQ, prefer buying an NCQ-enabled hard disk drive.
It is also very important to notice that Serial ATA implements two separated datapaths, one for transmitting and another for receiving data. On parallel design only one datapath is available, which is shared for both data transmission and reception. Serial ATA cable consists in two pair of wires (one for transmission and the other for reception) using differential transmission (click here to learn how differential transmission works). Three ground wires are also used, so Serial ATA cable has seven wires.
Another advantage of using serial transmission is that fewer wires need to be used. Parallel IDE ports use a 40-pin connector and 80-wire flat cables. Serial ATA ports use a seven-pin connector and seven-wire cable. This helps a lot on the thermal side of the computer, as using thinner cables makes air to flow easier inside the PC case.
On the figures below you can compare Serial ATA to parallel IDE: how does Serial ATA cable looks like, its size compared to an 80-wire IDE flat cable and a comparison between the physical aspect of a Serial ATA port (in red in Figure 3) to a parallel IDE port (in lime green in Figure 3).
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Figure 1: Serial ATA cable.
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Figure 2: Comparison between a Serial ATA cable and a standard 80-wire cable used by parallel IDE devices.
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Figure 3: Serial ATA ports (in red) and standard parallel IDE ports (in lime green).