What is Serial ATA

By Loyd Case  |  Posted 2003-01-09 Email Print this article Print

?"> The first ATA hard drives appeared on the scene in the late 1980s, and have evolved into the latest ATA100 and ATA133 standards. Also known as IDE (for integrated drive electronics), the ATA standard has gone through a number of iterations. Todays ATA hard drives max out at 133MB/sec (Maxtor) and 100MB/sec (everyone else). The original ATA standard specified a connection speed of 3.3MB/sec. Early ATA drives offered 10-40MB of storage -- a staggering amount at the time, but completely useless for most PC applications today. Capacities have evolved along with connection speeds, and we now have 320GB ATA drives available. However, todays hard drives still use an interconnect standard thats over fifteen years old, even as capacities and drive technologies have progressed. The ATA standard is a 16-bit, parallel connection. Parallel ATA uses source-synchronous (non-interlocked) clocking, which means that the clock signal is actually sent with the data. This can create problems as data rates -- and hence, clock rates -- increase. Because of potential signal reflection and signal skew issues, the ATA100 standard reduced the voltage for ATA100 signaling to 3.3v. The high clock rates also require 80-conductor cables, with alternating ground and signal wires. The net result is a maximum cable length of 18 inches for reliable operation in a wide variety of environments.
Serial ATA Defined: Serial ATA is, as the name implies, a serial link. A single Serial ATA (S-ATA) cable consists of a minimum of four wires, with differential pairs for transmitting and receiving data. The standard also allows for additional ground wires as deemed necessary. Maximum cable length for the S-ATA 1.0 standard is 1 meter (roughly 3.1 feet). This makes external S-ATA drives possible.
S-ATA is also point-to-point. Each S-ATA connection supports a single drive, so the days of figuring out which jumper to set for master or slave will become an historic artifact. Making S-ATA point-to-point also makes termination much easier, as opposed to parallel ATAs requirement to have a device attached to the middle of the cable. Todays systems typically only support two S-ATA connections. This is partly because current systems still require parallel ATA connections and partly because all of todays Serial ATA implementations work through PCI host adapter cards or chips. Being bound to PCI adds additional overhead and potentially limits throughput. S-ATA also offers "first party" DMA support, meaning that devices arent dependent on a host controller for DMA. The standard also has hot-swapping designed in, which means you can (in theory) swap drives while the system is running.
S-ATA uses a 7-pin connector (to accommodate any additional ground wires), and is considerably more compact than the parallel ATA plug. As you can see, four S-ATA cables and connectors take up roughly the same room as a single parallel ATA cable. In the future, when motherboard core logic directly supports S-ATA, well probably see as many as four S-ATA connections on a motherboard. Unfortunately, parallel ATA wont vanish overnight. If nothing else, optical drive makers will transition to S-ATA more slowly, since they view the additional bandwidth as more a luxury than a necessity for their applications.

Loyd Case came to computing by way of physical chemistry. He began modestly on a DEC PDP-11 by learning the intricacies of the TROFF text formatter while working on his master's thesis. After a brief, painful stint as an analytical chemist, he took over a laboratory network at Lockheed in the early 80's and never looked back. His first 'real' computer was an HP 1000 RTE-6/VM system.

In 1988, he figured out that building his own PC was vastly more interesting than buying off-the-shelf systems ad he ditched his aging Compaq portable. The Sony 3.5-inch floppy drive from his first homebrew rig is still running today. Since then, he's done some programming, been a systems engineer for Hewlett-Packard, worked in technical marketing in the workstation biz, and even dabbled in 3-D modeling and Web design during the Web's early years.

Loyd was also bitten by the writing bug at a very early age, and even has dim memories of reading his creative efforts to his third grade class. Later, he wrote for various user group magazines, culminating in a near-career ending incident at his employer when a humor-impaired senior manager took exception at one of his more flippant efforts. In 1994, Loyd took on the task of writing the first roundup of PC graphics cards for Computer Gaming World -- the first ever written specifically for computer gamers. A year later, Mike Weksler, then tech editor at Computer Gaming World, twisted his arm and forced him to start writing CGW's tech column. The gaming world -- and Loyd -- has never quite recovered despite repeated efforts to find a normal job. Now he's busy with the whole fatherhood thing, working hard to turn his two daughters into avid gamers. When he doesn't have his head buried inside a PC, he dabbles in downhill skiing, military history and home theater.

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