Serial ATA

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 Hard drive and optical storage devices for the personal computer market have been using variations of the IDE and ATA standards for over 10 years now. It has undergone a great many revisions over the years and is finally reaching the limits of its capabilities. To address the limitations of the ATA interface, the new Serial ATA interface has been under development for several years and is now finally starting to become available to consumers in the forms of controllers and more importantly hard drives. So lets take a look at this new interface and what its capabilities are compared to the older ATA format.

Serial vs. Parallel

The fundamental difference between the two formats is how the data is transferred between the device and the processors. Traditional ATA devices and controllers use a parallel data transfer mechanism. Parallel processing is a fairly common technique where multiple channels of data are sent simultaneously to try and increase the amount of data transferred in a single clock cycle. In the case of the ATA/100 standards used by today's IDE drives and controllers, they send the data across a 16-bit channel. The problem with this type of mechanism is the number of wire required to transfer that data. This is why the ATA cables are so wide. It is necessary to have the 40 or 80 wires required to transfer the data. The problem with this is the interference caused between these wires. At higher clock speeds necessary for faster speeds, the interference between the wires is too great to allow for reliable transmission.

Over the last couple of years, many advances have been developed in serial transmission techniques. Specifically through the development of the Universal Serial Bus interfaces. Serial transmissions run across a single control channel compared to the multiple channels of a parallel interface. This means that at the same clock speeds, the serial line will carry less data, but because the serial method requires fewer wires, less interference is generated to cause data integrity problems. This allows for serial transmission methods to run at much higher speeds than the equivalent parallel methods. In the case of the first Serial ATA standard, the clock runs at 1500 MHz compared to a clock rate of 50 MHz of the ATA/100 standard.

Cables and Connectors

As mentioned above, one of Serial ATA's advantages comes from the cabling compared to that of the older ATA standard. The reduced amount of wires required for data transmission allows for a greatly reduced cable size. Below is a picture comparing an 80-pin ATA/100 device cable to a Serial ATA cable.

80-pin ATA/100 cable (top) and SATA cable (bottom)

This smaller cable should allow for a greater flexibility of installation of Serial ATA devices compared to current parallel ATA devices.

Not only will it be easier to route the cables through the interior of systems, but the cable specification allows for the SATA cables to be upwards of 1 meter in length compared to the 18" of the maximum recommended length of an older ATA cable. This means that the drive can be approximately twice as far away from the controller as an equivalent parallel ATA setup. The reduced width of the SATA cable will also allow for greater airflow within a computer system. This should help reduce the amount of radiant heat that gets trapped within a computer case.

To utilize the new features of the Serial ATA standard and cables, a new method of connectors also needed to be developed. Below is a picture showing the older parallel ATA connectors on a hard drive compared to the Serial ATA connectors on a new hard drive.

ATA/100 Drive (top) and SATA Drive (bottom)

The new connector design was necessary to reduce the amount of crosstalk between the transmission wires. The design also allows the cabling to be easily connected properly as they are designed with an L shape key bit. The major difference comes from the power connector for Serial ATA drives which could cause problems for initial Serial ATA device adoption.

In order to support newer designs that utilize lower voltage and power consumption devices, it was necessary to support a 3.3V line for the interface. Similarly, the designers wanted the Serial ATA connectors to be laid out identically between a 2.5" laptop size drive as a 3.5" desktop drive. This necessitated a smaller power connector design from the traditional 4-pin molex connector used in desktop PCs. At this point, no power supply manufacturer has started to include the Serial ATA connectors onto their units. This means that users will need to have a Serial ATA power adapter in order to connect a Serial ATA drive into existing computer systems. Some drive manufacturers have opted to include the traditional 4-pin molex connectors onto desktop drives in addition to the Serial ATA power connector to ease migration.

Other Advantages

Serial ATA does have other developed and intended design advantages built into the interface as well. Older ATA/IDE devices were developed around the concept of a master/slave relationship that allows for 2 devices to reside on the same controller. In order to do this, the two devices had to be set in a master and slave relationship to allow the controller to know which device could potentially be used as a boot device. In addition, the two devices when running simultaneously would have to contend for the bandwidth along the data path. This effectively halved the maximum transfer rates for each device when running two devices simultaneously (which is also the reason to put hard drives and optical drives on separate ATA contollers or cables). Serial ATA removes this by giving each device on a controller a full dedicated point-to-point connector with 150 MB/s bandwidth that does not have to be shared.

Another great feature that will be implemented into the specification is the ability to hot swap drives. This is something that manufacturer's will have to implement into both the controllers and the storage devices in order for it to work. Don't expect the first generation or even many second or third generation devices to be able to be plugged into the computer or out of it during operation and not have massive data problems. This could be a real advantage for those looking for small sized storage units with RAID and hot swap capability in the future instead of large and expensive SCSI drive arrays.

The developers of Serial ATA did not forget about future bandwidth either. The current roadmap for Serial ATA technology has planned for a second generation of the technology to run at a 300 MB/s transfer rate in 2-3 years and eventually 600 MB/s. This is very surprising considering that current drive technology can barely support burst transfer rates of even 100 MB/s. The other problem is that the current PCI bus standard used to interconnect Serial ATA controllers onto a motherboard or via an expansion card is restricted to 133 MB/s which is lower than the initial standard. Until the new PCI-X interface is adopted or Serial ATA controllers are integrated into the chipsets for the motherboard, there is no reason to update the standard to a higher speed.

And don't be too concerned about not being able to use your old ATA devices with the new Serial ATA controllers. Serial ATA controllers have been designed to function with older ATA drives through an appropriate Parallel to Serial ATA adapters. These small circuit boards or blocks connect into the back of existing ATA drives and the Serial ATA cables in turn are connected to the adapter. The adapter then does the necessary conversions of the Parallel ATA signals into a Serial ATA signal to function with the drive controller. This is in fact how many early adopters of Serial ATA technology have begun to use the Serial ATA controllers due to the absence of early SATA drives.

Article courtesy of: About

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