A computer is full of busses -- highways that take information and power from one place to another. For example, when you plug an MP3 player or digital camera into your computer, you're probably using a universal serial bus (USB) port.

Your USB port is good at carrying the data and electricity required for small electronic devices that do things like create and store pictures and music files.But that bus isn't big enough to support a whole computer, a server or lots of devices simultaneously.

SCSI devices usually connect to a controller card like this one.

For that, you'd need something more like SCSI. SCSI originally stood for Small Computer System Interface, but it's really outgrown the "small" designation. It's a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD-ROM/RW drives, printers and tape drives.

Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use. This article will review SCSI basics and give you lots of information on SCSI types and specifications.

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.

SCSI has three basic specifications:

• SCSI-1 The original specification developed in 1986, SCSI-1 is now obsolete. It featured a bus widthof 8 bits andclock speedof 5 MHz.



• SCSI-2 Adopted in 1994, this specification included the Common Command Set (CCS) -- 18 commands considered an absolute necessity for support of any SCSI device. It also had the option to double the clock speed to 10 MHz (Fast)double the bus width from to 16 bits and increase the number of devices to 15(Wide), or do both(Fast/Wide).SCSI-2 also addedcommand queuing, allowing devices to store and prioritize commands from the host computer.



• SCSI-3 his specification debuted in 1995 and included a series of smaller standards within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other, has continued to evolve within SCSI-3. Most SCSI-3 specifications begin with the term Ultra, such as Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations. The Fast and Wide designations work just like their SCSI-2 counterparts. SCSI-3 is the standard currently in use.

Different combinations of doubled bus speed, doubled clock speed and SCSI-3 specifications have led to lots of SCSI variations. The chart below compares several of them. Many of the slower ones are no longer in use -- we've included them for comparison.

In addition to the increased bus speed, Ultra320 SCSI uses packeted data transfer, increasing its efficiency. Ultra2 was also the last type to have a "narrow," or 8-bit, bus width.

All of these SCSI types are parallel -- bits of data move through the bus simultaneously rather than one at a time. The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second, and each SAS port can support up to 128 devices or expanders.

SCSI controller

All the different SCSI varieties use controllers and cables to interface with devices. We'll look at this process next.

A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus.

Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.

Internal SCSI devices connect to a ribbon cable.