Chapter 5. Managing Storage

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electrical and mechanical aspects of the ATA interface but uses the communication protocol from the

next interface discussed   SCSI.

5.3.2.2. SCSI

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

4

) 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. Sub 

sequent enhancements pushed bus speeds to ultra (20MHz), fast 40 (40MHz), and fast 80

5

. 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 will cause 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

4. 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.

5. 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, effectivly doubling the throughput.