SCSI Technology

SCSI Technology

SCSI (pronounced “scuzzy”) stands for Small Computer Systems Interface and is a standard for communication between a subsystem of peripheral devices and the system bus. The SCSI bus is a closed system that can contain, and be used by, up to seven or 15 devices, depending on the SCSI standard. The gateway from this bus to the system bus is an adapter card inserted into an expansion slot on the system board.

The adapter card, called the host adapter, is responsible for managing all the devices on the SCSI bus. When one of these devices must communicate with the system bus, the data passes through the host adapter The following figure illustrates the concept of SCSI as a bus. On the left, the CPU communicates with the hard drive controller, which is contained in the hard drive case, through the system bus. On the right, the CPU communicates over the system bus to the SCSI host adapter, which communicates over the SCSI bus to the SCSI adapter in the hard drive case. The SCSI adapter communicates with the hard drive controller, which, in turn, communicates with the hard drive.

Each device on the SCSI bus is assigned a number from zero to seven called the SCSI ID, using DIP switches, dials on the device, or software settings. The host adapter is assigned a number larger than all other devices, either 7 or 15. Cables connect the devices physically in a straight chain. The devices can be either internal or external, and the host adapter can be at either end of the chain or somewhere in the middle.

The SCSI ID identifies the physical device, which can have several logical devices embedded in it. For example, a CD-ROM changer or jukebox device might have seven CD trays. Each tray is considered a logical device and is assigned a Logical Unit Number, or (LUN), to identify it, such as 1 through 7 or 0 through 6. The ID and LUN are written as two numbers separated by a colon. For instance, if the SCSI ID is 5, the fourth tray in the jukebox is device 5:4. A SCSI device such as a hard drive, tape drive, or CD-ROM drive, interfaces with the host adapter rather than directly with the CPU. The technology of a SCSI device can be the same as the technology of a similar device that is not SCSI, with the added ability to use the SCSI bus and communicate with the host adapter. A device is a SCSI device not because of its technology, but because of the bus it uses.

To reduce the amount of electrical interference on a SCSI cable, each end of the SCSI chain has a terminating resistor. The terminating resistor can be a hardware device plugged into the last device on each end of the chain, or the chain can have software-controlled termination resistance, which makes installation simpler.

SCSI standards use different names. The two general categories of all SCSI standards used on PCs have to do with the number of bits that travel on the SCSI bus, either 8 bits (narrow SCSI) or 16 bits (wide SCSI). Narrow SCSI uses a 50-pin cable, and wide SCSI uses a 68-pin cable. A SCSI cable can be built in two different ways, depending on the method by which the electrical signal travels on the cable: single -ended and differential. A single -ended cable is less expensive than a differential cable, but the maximum cable length cannot be as long because data integrity is not as great. Cables for both narrow SCSI and wide SCSI can be either single -ended or differential. Single -ended cables and differential cables look the same, but single -ended cable is more popular because it is cheaper. The table below summarizes the different SCSI standards, including the three major standards: SCSI-1, SCSI-2 and SCSI-3 that are more commonly known as Regular SCSI, Fast SCSI, and Ultra SCSI, respectively.

Ultra SCSI is backward compatible with SCSI-1 and SCSI-2, all three can coexist on the same SCSI bus. Note that the only connection this subsystem has to the overall computer system and the CPU is through the host adapter.