SCSI Terminators Terminology

Introduction

SCSI terminators are needed to make a SCSI bus operate correctly. Good termination is equally important as good cabling products. Different types of terminators have been developed during the evolution of the SCSI technology.
Please select the terminator depending upon the requirements of your installation.

Passive Terminator
A  resistor type terminator with a 220 and 330 Ohm resistor per bus line. The oldest type of terminator and the least expensive. Used for slow transmission speed (below 5 MHz) and shorter bus lengths.

Active Terminator
A terminator with a built in voltage regulator (2.85 Volt) and 110 Ohm resistor per bus line. The terminator has superior performance compared to a  passive terminator. Should be used for higher speed transmissions (5 MHz and higher) and when bus cabling is long. Recently active terminators have been reduced in price; therefore they are now very much affordable.

High Voltage Differential Terminator
Used solely with a differential SCSI bus (where all devices are of the High Voltage Differential type). A  three resistor type network (330-150-330 Ohm) per bus line. A differential terminator should not be used on a single ended bus.

F(orced) P(erfect) T(erminator)
This type of terminator was introduced to eliminate/reduce unwanted oscillations in bus signals; the principle is to clamp voltages when they otherwise would rise too high and to feed  current into the line to eliminate undershooting of a signal. This terminator has lost its importance because the quality of cabling has greatly been improved. Moreover, with certain modern drives now using active negation drivers the FPT type is highly undesirable in these situations.

Active Negation Terminator
In order to improve the signal quality at ULTRA SCSI transmission speeds some disk drive suppliers use line drivers which force current into the bus lines when the signal goes high; this to improve the speed and signal integrity. Therefore a terminator must not only be able to supply current into the line (when the line goes low), but also be able to absorb current (supplied by a driver when the driver makes the line go high). This technology, which is recently introduced will gain importance. An Active Negation Terminator is for a single ended bus and can be used also in place of a "regular active" terminator.

Low Voltage Differential (LVD)
Terminators using Low Voltage Differentials offer auto selection Multi-Mode Single Ended or Low Voltage Differential Termination. The Low Voltage Differential is a requirement for the higher speeds at a  reasonable cost and is the only way to have adequate skew budgets. Low Voltage Differential is specified for 80MB and 160MB SCSI, but has the higher potential speeds.

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