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

Logical Unit Numbers (LUNs) 1 VI  Speeds and Feeds

Understanding why termination is needed is critical to understanding SCSI. Let's take a moment to discuss the physics involved. As you saw in Chapter 0000, an electron is the basis for electricity. It was revealed that the electron is only a piece of an atom, and a single atom certainly does not have much weight. Further, you saw that an electron moves at nearly the speed of light (about 186,000 miles a second).

Finally, it became clear that an electron could move through some materials such as copper, very easily. Unless an electron is being pushed by very high voltage, it does not move through air. So to end or complete the signal it is necessary to terminate the SCSI bus. This is done either one of three ways. The first option is resistor packs that are installed on the SCSI device. The second option is auto termination, available on more intelligent devices. The third is by jumpers.

A resistor pack will completely impede the flow of electricity (remember our discussion in Chapter 0000 on electricity?) Simply put the signal is absorbed by the terminator preventing signal bounce.

So imagine being an electron, or to be more precise, a group of electrons carrying a command down a copper wire. So, here you are tooling along at a speed that would take you (literally) to the moon in only two seconds. Suddenly, you reach the end of the copper, and hit air.

Without much weight, and certainly a lot of velocity, each electron would bounce off the air much like a super ball would bounce off a wall. (Since the electron is already sub-atomic, it would be difficult to make it crash into smaller pieces.) And the group of electrons would come flying back down the copper wire, repeating the same command sequence, again, and again, and ... (you get the idea).

Can you see how the device on the SCSI bus would get quite confused, getting the same command repeated to it over and over and over? Actually, in real life, it is even a worse situation. The electron reflects and subtracts in weird combinations. The circuitry attached to the SCSI bus is simply responding to electrons. As you can see, complete havoc and data loss would be the result.

The solution lies in Termination. The idea is to absorb the signaling electrons at both ends of the transmission line (cable), making signal bounce a non-issue.

Active termination works by using a small voltage regulator, eliminating possible voltage fluctuation from Term Power, allowing termination resistance to be lowered to 110 ohms. Active termination is always recommended. Active termination is required for single ended Ultra SCSI, and above. Active termination does a better job of matching the impedance of the SCSI bus.

If you ever see the term Forced Perfect Termination, it was a great idea. It wasn't widely implemented, and died some time ago.

Logical Unit Numbers (LUNs) 1 VI  Speeds and Feeds