Everything You Need to Know About Serial ATA
By Gabriel Torres on June 27, 2008


Introduction

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).

Serial ATA Cable
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Figure 1: Serial ATA cable.

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.

Serial ATA
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Figure 3: Serial ATA ports (in red) and standard parallel IDE ports (in lime green).

Installation

Installation of Serial ATA devices differs a little bit from standard IDE devices, as Serial ATA is a point-to-point connection, i.e., you can connect only one device per port (parallel IDE allows two devices per port using a master/slave configuration). So installing Serial ATA devices is easier than installing parallel IDE ones: connect one end of the cable on the Serial ATA port (usually located on the motherboard) and the other end of the cable on the device you wish to connect (a hard disk drive, for instance). As this connector has a notch, it can’t be installed in the wrong position.

Serial ATA standard also defines a new power 15-pin supply connector. This connector was made standard from ATX12V 1.3 specification on. So if your computer has an ATX12V 1.3 or above power supply, it will have this connector. Even though a 15-pin connector is used, this power connection uses only five wires (one +12 V, one +5 V, one +3.3 V and two grounds).

SATA-300 hard disk drives may have a configuration jumper to force the drive to work as a SATA-150 device. The problem is that this jumper comes installed on the SATA-150 position, limiting your disk drive performance if it is installed on a motherboard with SATA-300 ports. The correct configuration of this jumper is very important and we will describe it in details in the next page.

So installing a SATA hard disk drive is rather simple: remove or change the position of the SATA-150/SATA-300 jumper (if available, detailed information in the next page), connect the Serial ATA cable and the power cable with your PC turned off and that’s it.

SATA HDD
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Figure 4: Connectors found on a SATA hard disk drive.

Serial ATA Power Connector
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Figure 5: Serial ATA power connectors found on ATX12V power supplies revision 1.3 and above.

SATA Hard Disk Drive Installation
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Figure 6: SATA hard disk drive connected to the motherboard.

Serial ATA
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Figure 7: Standard IDE hard disk drive "converted" to Serial ATA through an adapter.

The SATA-150/SATA-300 Jumper

Because some SATA-300 hard disk drives don’t work correctly on motherboards with SATA-150 ports, some SATA-300 hard disk drives have a SATA-150/SATA-300 jumper (also known as 1.5 Gbps/3 Gbps jumper). The problem is that by default this jumper comes configured in the “SATA-150” position, limiting your hard disk drive performance if it is installed on a motherboard with SATA-300 ports. We will show you the performance impact of having a SATA-300 wrongly configured in a moment.

So before installing a SATA-300 hard disk drive you should check if it has a SATA-150/SATA-300 jumper and if it is configured on the correct position: if you have an old motherboard with SATA-150 ports you should maintain this jumper on the SATA-150 position, but if your motherboard has SATA300 – as it occurs with almost all motherboards present on the market today – you should move its position to “SATA-300.” This information can be usually found on the hard disk drive label, as shown in Figure 8.

SATA-150 SATA-300 Jumper
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Figure 8: Detail on the hard disk drive label explaining about the SATA-150/SATA-300 jumper.

This particular hard disk drive (a Seagate Barracuda 7200.10 160 GB) comes with a jumper limiting the hard disk drive performance to 150 MB/s (1.5 Gbps), see Figure 9. To make it properly work as a SATA-300 device, we must remove this jumper (see the diagram in Figure 8). In this case the jumper can be removed with a small flat tip screwdriver, small pliers or tweezers. Note that depending on the hard disk drive model you may need to move the position of the jumper instead of having it removed. So it is very important to pay attention on what is written on the hard disk drive label.

SATA-150 SATA-300 Jumper
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Figure 9: Hard disk drive with its SATA-150/SATA-300 jumper on the “SATA-150” position.

What is the performance impact of having a SATA-300 hard disk drive wrongly configured? We made some tests in our lab to show you this. We measured the transfer rate of our Seagate Barracuda 7200.10 160 GB hard disk drive with three different programs, SpeedDisk32, HD Tach and HD Tune, first with the jumper on its default configuration (“SATA-150”) and then removing the jumper and thus making the hard disk drive a truly SATA-300 device. You can see the results below (click here for a full description of the system we used on this benchmarking).

SATA-150 vs. SATA-300

SATA-150 vs. SATA-300

SATA-150 vs. SATA-300

As you can see the results on the three programs show the same thing. Even though the maximum, average and minimum transfer rates remained the same, with the jumper in the SATA-300 position the burst transfer rate increased between 60% and 69%.

In summary, don’t forget to check for the existence and the correct position of this jumper when installing SATA-300 hard disk drives!

Port Multiplier

Port Multiplier is a device that expands the number of devices to be installed on a single SATA port to 15.

Port multiplier has several applications, like allowing a home user to install more than one hard disk drive per SATA port and allowing storage racks to use fewer cables.

With Serial ATA it is easier to connect hard disk drives located outside the computer case at a high-speed rate because of the cable that is used (which is thinner than the traditional 80-wire flat cable). But if we need to install a rack containing 16 hard disk drives to a server, there will be 16 Serial ATA cables from the rack to the server, and the server must have 16 SATA ports. We illustrate this scenario in Figure 10.

Port Multiplier
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Figure 10: Connecting a server to 16 hard disk drives.

Using port multiplier it is possible to connect them using fewer cables. For example, one port multiplier connected to one SATA port allows you to connect up to 15 hard disk drives to it. And you would have only one cable connecting the rack to the server.

But there is a huge performance issue here. If a SATA-150 port were used, the 150 MB/s bandwidth would have to be split between 15 devices, creating a huge bottleneck.

To solve this issue another approach may be used. Instead of using only one port multiplier chip, you could use four of them, connecting the rack to the server using four cables (instead of 16). The maximum transfer rate between the server and the rack would be of 600 MB/s (4x 150 MB/s) if SATA-150 ports were used or of 1,200 MB/s (4x 300 MB/s) if SATA-300 were used. Inside the rack, you could install up to 60 hard disk drives (15 x 4), but for optimal performance you should install four hard disk drives to each port multiplier chip, matching your 16 drives. We illustrate this scenario in Figure 11. “PM” there is the port multiplier chip.

Port Multiplier
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Figure 11: Connecting a server to 16 hard disk drives using port multiplier.

Pinout

Here are the pinouts for both Serial ATA data cable and Serial ATA power supply cable. As mentioned before, Serial ATA uses two separated data channels, named A and B, using differential transmission, hence the + and symbols below. On the wires marked with the minus signal, the data is an inverted copy of what is being transmitted on the corresponding wires with plus signal. Click here to learn how differential transmission works.

Serial ATA Data Connector

Pin

Function

1

Ground

2

A+

3

A-

4

Ground

5

B-

6

B+

7

Ground

Serial ATA Power Connector

Pin Function
1 +3.3 V
2 +3.3 V
3 +3.3 V
4 Ground
5 Ground
6 Ground
7 +5 V
8 +5 V
9 +5 V
10 Ground
11 Reserved/Ground
12 Ground
13 +12 V
14 +12 V
15 +12 V

Originally at http://www.hardwaresecrets.com/article/Everything-You-Need-to-Know-About-Serial-ATA/27


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