CertiGuide to A+ (Core Hardware)  9  Chapter 0101:  SCSI

II  SCSI 1 IV  SCSI IDs

What is SCSI? The first thing to keep in mind about SCSI is it does not have to be a drive. SCSI is a device. In theory, your household toaster could be a SCSI device. A popular myth about SCSI is thinking of it as an interface. A more accurate description would be to call it an I/O bus, because you can attach more than one device to the bus. Lets take a brief tour of history to see what makes SCSI unique.

The idea allowed changes in drives to occur, leaving the interface intact. In 1980, Shugart approached the American National Standards Institute (ANSI), which wasn't too interested. Another firm, National Cash Register (NCR) added functionality to the SASI standard, and in 1982 approached ANSI to formalize a standard. About this time, the Write Once Read Many (WORM) optical drives appeared, expanding the standard to more than magnetic media.

In 1986 ANSI adopted SCSI, which used a 50-pin interface.

Although SCSI follows standards set by ANSI, a clue that life is not a bed of roses with SCSI is the little 's' added to the end of the word standard. Yes, there are multiple standards in the SCSI world, which can lead to incompatibility within the SCSI standards. Currently, there are 7 generations of SCSI, and some variations within each generation.

SCSI has been available for many types of computers, and was a standard interface for many Macintosh systems. The standard is quite popular in PC servers, and high-end PC workstations. You'll see why this is a little later in this chapter.

Before tackling SCSI, lets look at an IDE drive. Let's use an analogy. Suppose I ask you to go the store and get a quart of milk. For the purpose of this analogy this is like getting a file. While I am waiting for you to come back with milk, that is exactly what I am doing, waiting. While I ask you to get a quart of milk, Matt asks if you would get him a 6-pack of iced tea. With IDE, you have to come back from the store (bringing me my file) before you can go back out and get Matt’s’ iced tea (his file).

Contrast this to SCSI. In this case, it is possible to combine the errands of getting the milk and the iced tea (both file requests). This is done using Tagged Command Queuing (TCQ). The commands to get the different files are sent to the drive for processing, and kept in the command buffer, and sorted into an optimal sequence. Because the drive knows where both files are, it executes the commands in a sequence using the least amount of thrashing around the drive surface. Additionally, TCQ cuts down on overhead by not having to complete the first request before working on the second (or third) request. TCQ was introduced in SCSI-2 (1995).

In most cases, when a single user is not doing many different file accesses at the same time, IDE makes the most sense. A PC server that needs to access different files for different users at the same time benefits greatly by having a modern SCSI sub-system.

II  SCSI 1 IV  SCSI IDs