Formally known as the Small Computer System Interface, SCSI as it is known today originated in the early 80s and was declared a standard in 1986. Like ATA, SCSI makes use of a bus topology. However, there the similarities end.
Using a bus topology means that every device on the bus must be uniquely identified somehow. While ATA supports only two different devices for each bus and gives each one a specific name, SCSI does this by assigning each device on a SCSI bus a unique numeric address or SCSI ID. Each device on a SCSI bus must be configured (usually by jumpers or switches) to respond to its SCSI ID.
Before continuing any further in this discussion, it is important to note that the SCSI standard does not represent a single interface, but a family of interfaces. There are several areas in which SCSI varies:
- Bus width
- Bus speed
- Electrical characteristics
The original SCSI standard described a bus topology in which eight lines in the bus were used for data transfer. This meant that the first SCSI devices could transfer data one byte at a time. In later years, the standard was expanded to permit implementations where sixteen lines could be used, doubling the amount of data that devices could transfer. The original "8-bit" SCSI implementations were then referred to as narrow SCSI, while the newer 16-bit implementations were known as wide SCSI.
Originally, the bus speed for SCSI was set to 5MHz, permitting a 5MB/second transfer rate on the original 8-bit SCSI bus. However, subsequent revisions to the standard doubled that speed to 10MHz, resulting in 10MB/second for narrow SCSI and 20MB/second for wide SCSI. As with the bus width, the changes in bus speed received new names, with the 10MHz bus speed being termed fast. Subsequent enhancements pushed bus speeds to ultra (20MHz), fast-40 (40MHz), and fast-80. Further increases in transfer rates lead to several different versions of the ultra160 bus speed.
By combining these terms, various SCSI configurations can be concisely named. For example, "ultra-wide SCSI" refers to a 16-bit SCSI bus running at 20MHz.
The original SCSI standard used single-ended signaling; this is an electrical configuration where only one conductor is used to pass an electrical signal. Later implementations also permitted the use of differential signaling, where two conductors are used to pass a signal. Differential SCSI (which was later renamed to high voltage differential or HVD SCSI) had the benefit of reduced sensitivity to electrical noise and allowed longer cable lengths, but it never became popular in the mainstream computer market. A later implementation, known as low voltage differential (LVD), has finally broken through to the mainstream and is a requirement for the higher bus speeds.
The width of a SCSI bus not only dictates the amount of data that can be transferred with each clock cycle, but it also determines how many devices can be connected to a bus. Regular SCSI supports 8 uniquely-addressed devices, while wide SCSI supports 16. In either case, you must make sure that all devices are set to use a unique SCSI ID. Two devices sharing a single ID causes problems that could lead to data corruption.
One other thing to keep in mind is that every device on the bus uses an ID. This includes the SCSI controller. Quite often system administrators forget this and unwittingly set a device to use the same SCSI ID as the bus's controller. This also means that, in practice, only 7 (or 15, for wide SCSI) devices may be present on a single bus, as each bus must reserve an ID for the controller.
Most SCSI implementations include some means of scanning the SCSI bus; this is often used to confirm that all the devices are properly configured. If a bus scan returns the same device for every single SCSI ID, that device has been incorrectly set to the same SCSI ID as the SCSI controller. To resolve the problem, reconfigure the device to use a different (and unique) SCSI ID.
Because of SCSI's bus-oriented architecture, it is necessary to properly terminate both ends of the bus. Termination is accomplished by placing a load of the correct electrical impedance on each conductor comprising the SCSI bus. Termination is an electrical requirement; without it, the various signals present on the bus would be reflected off the ends of the bus, garbling all communication.
Many (but not all) SCSI devices come with internal terminators that can be enabled or disabled using jumpers or switches. External terminators are also available.
One last thing to keep in mind about SCSI -- it is not just an interface standard for mass storage devices. Many other devices (such as scanners, printers, and communications devices) use SCSI. Although these are much less common than SCSI mass storage devices, they do exist. However, it is likely that, with the advent of USB and IEEE-1394 (often called Firewire), these interfaces will be used more for these types of devices in the future.
The USB and IEEE-1394 interfaces are also starting to make inroads in the mass storage arena; however, no native USB or IEEE-1394 mass-storage devices currently exist. Instead, the present-day offerings are based on ATA or SCSI devices with external conversion circuitry.
No matter what interface a mass storage device uses, the inner workings of the device has a bearing on its performance. The following section explores this important subject.
 Some storage hardware (usually those that incorporate removable drive "carriers") is designed so that the act of plugging a module into place automatically sets the SCSI ID to an appropriate value.
 Fast-80 is not technically a change in bus speed; instead the 40MHz bus was retained, but data was clocked at both the rising and falling of each clock pulse, effectively doubling the throughput.