SCSI Basics

SCSI is based on an older, proprietary bus interface called Shugart Associates System Interface (SASI). SASI was originally developed in 1981 by Shugart Associates in conjunction with NCR Corporation. In 1986, the American National Standards Institute (ANSI) ratified SCSI (pronounced "scuzzy"), a modified version of SASI. SCSI uses a controller to send and receive data and power to SCSI-enabled devices, like hard drives and printers.

SCSI connector

SCSI has several benefits. It's fairly fast, up to 320 megabytes per second (MBps). It's been around for more than 20 years and it's been thoroughly tested, so it has a reputation for being reliable. Like Serial ATA and FireWire, it lets you put multiple items on one bus. SCSI also works with most computer systems.

However, SCSI also has some potential problems. It has limited system BIOS support, and it has to be configured for each computer. There's also no common SCSI software interface. Finally, all the different SCSI types have different speeds, bus widths and connectors, which can be confusing. When you know the meaning behind "Fast," "Ultra" and "Wide," though, it's pretty easy to understand. We'll look at these SCSI types next.

RAID SCSI is often used to control a redundant array of independent discs (RAID). Other technologies, like serial-ATA (SATA), can also be used for this purpose. Newer SATA drives tend to be faster and cheaper than SCSI drives. A RAID is a series of hard drives treated as one big drive. These drives can read and write data at the same time, known as striping. The RAID controller determines which drive gets which chunk of data. While that drive writes the data, the controller sends data to or reads it from another drive. RAID also improves fault tolerance through mirroring and parity. Mirroring makes an exact duplicate of one drive's data on a second hard drive. Parity uses a minimum of three hard drives, and data is written sequentially to each drive, except the last one. The last drive stores a number that represents the sum of the data on the other drives. For more information on RAID and fault tolerance, check out this page.