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Dell Storage Technology Foundations Online Training Course

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Storage Interfaces

SCSI Signaling

As with all electronic devices, SCSI devices communicate by sending signals. Digital data is represented by either a one or a zero; these representations can take many different forms:

A hard drive encodes ones and zeroes magnetically.
CD-ROM drives encode using "pits and lands".
Electronic circuits use voltage levels.

SCSI devices use two different types of voltage signals: Single Ended and Differential.

Single-Ended Signaling

Single-ended (SE) signaling is fairly straightforward; a positive voltage representing a "one" and zero or ground voltage representing a "zero" are transmitted over one wire.

Single-Ended

Single-ended SCSI Logo graphic Of the SCSI device types, single-ended devices are the least costly and easiest to implement. All early SCSI devices used this method of signaling. SE devices illustrate their signal type with the symbol at left.
Single Ended devices do have limitations. As the SCSI bus becomes faster and faster, signals lose their integrity through interference, crosstalk, signal bouncing, and signal degradation. As a result, SE cables must be short in length. The faster the SCSI bus, the shorter the cables become.

Differential Signaling

Differential signaling is an improvement over SE signaling in that it is much more resistant to signal noise and allows for longer cable lengths. Differential signaling uses two wires to transmit signals. The "signal" is the voltage difference between the two wires.

For example, the "logical zero" signal is transmitted over both wires, while the "logical one" signal is represented by a positive signal being transmitted over one wire in the two-wire set and its electrical opposite being transmitted over the second wire. The difference between the two represents the signal.

High Voltage

Differential Signaling SCSI logo Differential signaling, in the form of High Voltage Differential (HVD), has existed since the original SCSI specification was written. HVD was comparatively very expensive to implement, however, so there was little reason to use HVD until SE devices reached their "speed limit".
HVD devices have the symbol at left displayed on them. SE devices cannot be mixed with HVD controllers. The higher voltage required by HVD is not compatible with SE devices.

Dell uses the Adaptec AHA-3944 and AHA-2944 controllers as its high-voltage controllers.

Low Voltage


			Low Voltage Differential (LVD) uses the same differential signaling techniques that HVD uses, but at a much lower voltage. This means that LVD controllers and devices are compatible with SE devices and controllers. LVD controllers and devices have the symbol to the left attached. LVD is more cost effective than HVD, and it has become the signaling method of choice. The only drawback to LVD is that if SE devices are used on an LVD controller, all devices will operate as though they were SE—causing a performance degradation.

	The Adaptec AIC-7899 is the most commonly-used low voltage controller shipped with Dell systems.