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.