How SCSI Works

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 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:

Different combinations of doubled bus speed, doubled clock speed and SCSI-3 specifications have led to lots of SCSI variations. The chart on this page 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.

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 devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device.

The cable itself typically consists of three layers:

Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors.

Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be closed. We'll look at how to do this next.

If the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between devices and the SCSI controller. The solution is to terminate the bus, closing each end with a resistor circuit. If the bus supports both internal and external devices, then the last device on each series must be terminated.

Types of SCSI termination can be grouped into two main categories: passive and active. Passive termination is typically used for SCSI systems that run at the standard clock speed and have a distance of less than 3 feet (1 m) from the devices to the controller. Active termination is used for Fast SCSI systems or systems with devices that are more than 3 feet (1 m) from the SCSI controller.

SCSI also employs three distinct types of bus signaling, which also affect termination. Signaling is the way that the electrical impulses are sent across the wires.

Both HVD and LVD normally use passive terminators, even though the distance between devices and the controller can be much greater than 3 ft (1 m). This is because the transceivers ensure that the signal is strong from one end of the bus to the other.

For more information on SCSI and other busses, check out the links on the following page.