SCSI (usually pronounced “scuzzy”) stands for Small Computer System Interface, and is the oldest type of interface used for attaching peripheral devices to computers. Almost all PCs, Apple Macintosh computers and other UNIX systems used these connectors to connect server motherboards with the hard drives and transfer data to and from them. Note that SCSI and many other terms used in this article such as SAS and SATA, are frequently used to describe the hard drives attached to these connectors as well.
SCSIs were parallel interfaces that used a 50-pin flat ribbon connector. They were mounted physically and allowed 7 to 15 devices to be connected. Modern SCSIs can transfer up to 80 megabytes/second, but are quite expensive to buy. Eventually, this technology was surpassed by the more modern SAS (Serial Attached SCSI), which improved the performance by allowing multiple devices to be connected simultaneously with longer but thinner cables. SAS devices are also capable of full-duplex signal transmission with a much higher transfer speed of up to 3.0 gigabytes/second.
The next generation of connectors were the IDEs (Integrated Drive Electronics), another parallel interface used to support the ATA (Advanced Technology Attachment) drives. Launched in 1986 by Western Digital Electronics, the first generation of IDE controllers used 40-pin and 80-ribbon cables, although modern ones use just 28 pins working on a plug-and-play basis. Data transfer peaks at 8.3 megabytes/second for ATA-2 and up to 100 megabytes/second for ATA-6.
ATA drives are much cheaper than SCSI ones since they use a single processor for both executing the commands and to control the head positioning through servos. However, for this same reason, ATA hard disks have a shorter life, incur wear much more quickly, and their performance is slightly inferior. However, their price-performance ratio was so high, that by the late '90s, the ATA-connected drives almost completely eclipsed the old SCSI devices.
The last and most modern interface is the evolution of the parallel ATA: the Serial ATA (SATA), launched in 2003. Today, it has captured 98% of the market, actually standing as the standard interface used by almost every consumer. SATA effectively extended the capabilities of ATA while keeping their cheap overall cost. Similar to SAS, they use a serial link to create a point-to-point connection between devices, thus removing the limitations of the parallel interface on the number of devices per port connection. Transfer rates for SATA begin at 150 megabytes/second, but can reach up to 6 gigabytes/second. Most modern hard disks usually average a 1.5 to 3 gigabytes/second top speed.
Another great advantage of SATA-connected drives is that they offer hot plugging, a function that allows one to replace components in a computer without having to shut down the system. A SATA data cable has 9 pins and is short enough to fit in small devices and help with heat management. However, Universal Storage Modules allow for cable-less support of peripherals and devices.