Electrical circuitry at the end of a cable designed to match impedances for the purpose of preventing the reflection of electrical signals when they reach the end of the cable. In SCSI systems, this electrical circuitry is called a terminator. It should be noted that any SCSI bus segment requires two terminators and only two terminators. Not one, not three, but two terminators. Also, the terminators must be installed at the very ends of the SCSI cable, not at devices in the middle of the bus. Terminators require power that is usually provided by the host adapter on the TERMPWR line(s) on the bus. Many SCSI devices power their own terminators. There are four basic types of SCSI termination: Passive, active, FPT, and LVD (including LVD/MSE)
* Passive Termed "Alternative 1" in SCSI-2
The simplest form of terminator consisting of a 220 ohm resistor from TERMPWR to the signal line and a 330 ohm resistor from the signal line to ground. Low cost but has the disadvantage that any fluctuations in the TERMPWR voltage will show up on the signal lines of the bus which may cause data errors. SCSI-2 recommends the use of active terminators whenever possible for single-ended SCSI. Differential (HVD) SCSI uses only passive terminators.
* Active Termed "Alternative 2" in SCSI-2 Because fluctuations in the TERMPWR voltage supplied to passive terminators shows up as fluctuations in signal levels, active terminators include a voltage regulator to reduce the effect of fluctuations in TERMPWR to insignificance. Uses only a 110 ohm resistor from the regulator to the signal line which is a much closer match to the SCSI cable impedance. Results in more stable SCSI signals, less signal reflection and fewer data errors.
* Force Perfect Termination (FPT) Not recognized in the SCSI specifications. Single-ended termination method utilizing diode switching and biasing to actively compensate for impedance mismatches between the SCSI cabling and the peripheral device. You should be aware that there are several designs of FPT that may not be totally compatible. Also, our customers have found that, generally speaking, FPT likes to "talk" only to FPT.
* LVD Terminator A form of active termination necessary for the LVD SCSI bus. Defined in the SPI-2 and SPI-3 documents of the SCSI-3 specifications. There are LVD only terminators and "multimode LVD" or "LVD/MSE" terminators. The latter should be used if there is a possibility that a single-ended device will be connected to the multimode LVD bus.
* LVD/MSE LVD that uses "Multimode" transceivers. Depending on the voltage level appearing on the DIFFSENSE pin of the cable, the Multimode transceivers of LVD/MSE will be automatically configured for LVD or single-ended (SE). Most new SCSI designs will include Multimode transceivers.

This definition is in context to SCSI. See more contextual defintions for Terminator (Terminations).