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High Speed IDE: Serial ATA (II)

High Speed IDE: Serial ATA (II)

Serial ATA has just come to market as SATA II already makes its way to extend the standard. We explain the new high speed interface in detail and provide an outlook on further extensions.

Numerous interfaces and bus systems currently are switched to serial versions - such as PCI Express and Hyper Transport. This also applies to the parallel IDE interface, which has room for data transfer speeds of up to 133MBs. Due to technical issues, this is the top of the line and cannot be upped anymore.

Serial ATA technology allows for transfer speeds of up to 150MBs. It is planned to double this value twice until 2007. Back in 2000, APT Technologies, Dell, IBM, Intel, Maxtor, Seagate, and Quantum (now Maxtor), joined development efforts to develop a specification for Serial ATA. The new technology promises software compatibility, lower power consumption, increased speed, smaller and less expensive connects as well as thinner and longer cables. Results are optimized air streams to cool CPUs, memory and harddisks inside PC cases.

The specification 1.0 of Serial ATA was passed in August of 2001. Harddisks and controllers in conformance of the standard were available since the beginning of 2003.

Why Serial ATA?

In view of the achieved speeds, the technology of parallel data transfer can only be implemented across short distances. Greater distances require a lower transfer speed, as Bits can arrive with different speeds and different levels at their destination. Such run-time differences can only be leveled out through a decreased clock speed. Numerous line drivers also consume significant power; large connects incorporate several expensive and error-prone contacts. Additionally, flatband cables with 40 and 80 wires hinder air streams to cool single components inside the PC casing.

With serial data transfer, there are no run-time differences, since there is only one signal line. With appropriate choice of code, such as 8B/10B, there is also no need for a clock or strobe line. The only line driver consumes just a small amount of power. The plugs are small, feature less contacts and the cables are small enough to not affect air streams. To be able to work with serial data streams, parallel/serial converters are needed, which convert parallel processed data within harddisks to a serial format.

Thanks to today's high level of integration and high internal transfer speeds of CMOS chips, data streams are converted in real time. Conversion chips increase cost and complexity on the one hand. On the other hand, buffer amplifiers which are necessary in parallel technology are now obsolete. Also, mature LVDS technology calls for low voltages. This saves power and allows for fast circuit times.

Especially important is 100 per cent software compatibility between common parallel and the Serial ATA. Electric conversion and protocol adjustment are processed at the lowest logic level of the chips. The application and the software driver are not affected in this procedure. Thus, developers do not have to change, extend or recompile software.

Architecture

Parallel ATA is characterized through a bus structure with one master (computer) and two slaves (drives). Today's chipsets in most cases offer two ATA buses to manage four devices. In contrast, Serial ATA uses up to four direct point-to-point connections between the interface chipset on the mainboard or a card and one device with each connection line. As a result, there are no problems with jumpers and bus scheduling anymore, since each line is terminated at the end within the chip. This also ensures that a defect device does not influence other devices. Serial ATA is intended to be used only for mass storage devices (hard drives, CD-ROM, DVD, etc.) and short distances within the computer casing and not for peripheral devices such as a scanner or printer. Power control (low and with the ability to be turned off in several levels) is considered for mobile PC in battery mode within the Serial ATA specification.

Planning of Serial ATA envisions an application timeframe of approximately ten years, including tow performance enhancements. For the use of common devices there will be adapters (dongles) for both sides (computer, device) which will process the parallel/serial conversion. The transfer speed is expected to be increased when drives will offer more speed. That way, only small and inexpensive buffer memory is necessary to adjust the speed. To ensure low error rates with high transfer speeds, Serial ATA will offer features for error check and correction.

Serial ATA's specification calls for the support of Hot Plug. Hot Plug allows connecting and disconnecting a device while running the computer. The operating system has to support Hot Plug at the same time. Windows supports this feature in versions 98/ME, 2000, and XP. Linux also offers Hot Plug functionality.

Cabling and Connectors

Current Ultra-ATA/100/133-Systems use an 80-pin plug connector, which has substituted the antiquated 40-pin connector of the original ATA specification. Disadvantages of these cables are high prices and considerable consumption of space within the computer casing.

A cable with six or eight wires is sufficient for Serial ATA. Each drive receives its own cable to the interface of the computer.

Serial ATA is not a bus, but rather displays a star-shaped topology. Therefore there are only point-to-point connections without the selection of a bus (arbitration/negotiation). Sending and receiving is separately processed on a wire pair for each direction. LVDS requires a wire pair for each signal.

In the case of the parallel ATA version, signal and current cables are implemented through different plug connectors. The specification of Serial ATA calls for multiple configurations. For example, signals and power source can run in a common plug connector or use an own connector alternatively. Also the combination of a Serial ATA signal connector with the general power connector used with parallel ATA drives is mentioned within the specification.

Standards

The main requirement of Serial ATA is absolute software compatibility to the parallel ATA interface used up until now. Therefore there are no plans for any extras. The ATA interface - as it is seen by the user and the operating system - will not be changed.

Originally, there was no ATA standard. The specification is a result of multiple suggestions by different corporations. Over time, these suggestions were copied and used for production by device and mainboard manufacturers. Later, the ANSI T13 committee took charge of the standardization process. At this time, an extended version is published every year, since this market segment is characterized through frequent changes

The Small Form Factor Committee (SFF) additionally has defined the extension of ATA to the ATAPI interface and has handed respective standardization to the ANSI T13 committee. ATAPI defines the connection of CD-ROM and tape devices to an ATA cable. In general, this refers to a SCSI protocol which builds on the ATA interface for data and instruction transfer.

The specification of ATA and ATAPI, which has merged together, boasts a document of more than 400 pages. In large parts it deals with later implemented technical optimizations of the IDE/E-IDE specification. For example, bus scheduling which is necessary for every bus structure because of physical reasons, was introduced at a later time.

The cables for the parallel ATA interface were changed to 80 lines (before 40 lines). The additional wires are ground connections for shielding and current back-transfer through the cable. The grpound connections are especially necessary, if the high data speeds reached today are considered. Problems of this kind most likely will not occur with Serial ATA.

Protocols of the parallel transfer are not changed for Serial ATA. The specification only refers to the exchange of a parallel through a serial transfer section and their internal protocol.

Serial-ATA-II-Specification

The Serial ATA II working group was announced at the Intel Developer Forum in spring of 2002. Serial-ATA II does not only offer an increased transfer speed, but also features for server employment and network storage.

Serial ATA II Working Group consists of the same members, who also have had a leading role during the development of version 1.0: APT Technologies, Dell, Intel, Maxtor, and Seagate. The new working group also counts 15 additional firms.

Serial ATA II is the second generation of the serial interface and is to be considered as extension. The focus of Serial ATA II is on more functions for professional applications and compatibility to the Serial ATA 1.0 standard. The new features are supposed to pave the way into servers and network-based storage solutions, for example extended RAID functions. Also, Serial ATA II is supposed to integrate efficient command queuing and support more than just four devices. The Hot Plug support of Serial ATA 1.0 will be extended by features which are necessary for storage sub-systems.

The Serial ATA II specification will be passed in two steps: Features targeted at server employment and network-based storage were passed by the working group in October 2002. Development of products for the faster Serial ATA II interface (300MBs) will not arrive before mid to tlate 2004, according to the working group.

Planned technical development of Serial ATA
Parameter Generation 1 Generation 2 Generation 3
Serial data rate (GBit/s) 1.5 3.0 6.0
Data rate at 8B/10B (MByte/s) 150 300 600
Introduction 2002 2004 2007
Source: Serial ATA Working Group

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