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SCSI Duplex Terminator

Posted on 1998-11-30
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What is SCSI Duplex Terminator? All I know it is used in some Compaq servers and Storage Systems.
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Question by:Yaroslav
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by:rosefire
ID: 1130003
I extracted the following from http://www.bobjohnson.com/glosdefs.htm
It sounds as though Duplex Terminator is referring to something that runs in two directions (duplex) hence, Force Perfect Termination may be involved.  Do you have any more data other than the name "SCSI Duplex Terminator"  perhaps some key words in this definition will connect to some other info you have and you will have your answer.  Note Force Perfect Termination limitiations which also may be relevant. The fact that this type of termination may not be compatible would explain why they might make up a name like SCSI Duplex Terminator.

Terminators: Electrical circuitry at the end of a cable to prevent 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 five basic types of SCSI termination: Active, active negation, FPT, LVD (including LVD/SE) and passive.

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.
Active negation - A method of providing single-ended termination that uses voltages to drive the bus signal not just high as in active termination, but high and low. Results in faster switching which is necessary for FAST and Ultra-SCSI speeds.
Force Perfect Termination (FPT) - 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.
HVD - High Voltage Differential - Differential SCSI scheme that has been in use for years. Terminators run of 5 VDC. See also LVD.
LVD - Low Voltage Differential - A method of driving SCSI cables that will be formalized in the SCSI-3 specifications. Lower power consumption than the current differential drive (HVD), is less expensive and will allow the higher speeds of Ultra-2 SCSI. Requires 3.3 VDC instead of 5 VDC for HVD.
LVD/SE - LVD that uses "Universal" transceivers. Depending on the voltage level appearing on the DIFFSENSE pin of the cable, the Universal transceivers of LVD/SE will be automatically configured for LVD or single-ended (SE). Most new SCSI designs will include Universal transceivers.
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 SCSI uses only passive terminators.
Universal - See LVD/SE.

Hope that helps.
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jrhelgeson earned 100 total points
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