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{{Short description|Transmission line impedance matching}}
{{Other uses|Termination (disambiguation){{!}}Termination}}
{{distinguish|Electrical terminal}}
[[File:SCSI-terminator-exposed-hdr-0a.jpg|thumb|right|[[SCSI]] terminator]]

In [[electronics]], '''electrical termination''' is the practice of ending a [[transmission line]] with a device that matches the [[characteristic impedance]] of the line. [[Signal reflection]]s occur where there is an [[impedance mismatch]]. Termination prevents signals from reflecting off the end of the transmission line. Reflections at the ends of unterminated transmission lines cause [[distortion]], which can produce ambiguous [[digital signal]] levels and misoperation of digital systems. Reflections in [[analog signal]] systems cause such effects as [[ghosting (television)|video ghosting]], or [[Power margin|power loss]] in radio transmitter transmission lines.

==Transmission lines==
Signal termination often requires the installation of a terminator at the beginning and end of a wire or cable to prevent an [[RF signal]] from being reflected back from each end, causing [[Interference (communication)|interference]], or power loss. The terminator is usually placed at the end of a [[transmission line]] or [[Daisy chain (electrical engineering)|daisy chain]] [[Bus (computing)|bus]] (such as in [[SCSI]]), and is designed to [[impedance matching|match]] the [[Characteristic impedance|AC impedance]] of the cable and hence minimize signal [[Reflection (electrical)|reflections]], and power losses. Less commonly, a terminator is also placed at the driving end of the wire or cable, if not already part of the signal-generating equipment.<ref name="CTS">{{cite web |title=Clock Termination Techniques and Layout Considerations |url=https://www.ctscorp.com/wp-content/uploads/2015/10/AN1025.pdf |access-date=2021-01-03 |publisher=CTS |archive-date=2021-10-24 |archive-url=https://web.archive.org/web/20211024085900/https://www.ctscorp.com/wp-content/uploads/2015/10/AN1025.pdf |url-status=dead }}</ref>

Radio frequency currents tend to reflect from discontinuities in the cable, such as [[Electrical connector|connectors]] and joints, and travel back down the cable toward the source, causing interference as primary reflections. Secondary reflections can also occur at the cable starts, allowing interference to persist as repeated echoes of old data. These reflections also act as bottlenecks, preventing the signal power from reaching the destination.

Transmission line cables require [[impedance matching]] to carry electromagnetic signals with minimal reflections and power losses. The distinguishing feature of most transmission line cables is that they have uniform cross-sectional dimensions along their length, giving them a uniform electrical [[characteristic impedance]]. Signal terminators are designed to specifically match the characteristic impedances at both cable ends. For many systems, the terminator is a [[resistor]], with a value chosen to match the characteristic impedance of the transmission line and chosen to have acceptably low parasitic [[inductance]] and [[capacitance]] at the frequencies relevant to the system. Examples include 75-ohm resistors often used to terminate 75-ohm video transmission coaxial cables.

Types of transmission line cables include balanced line such as [[ladder line]], and [[twisted pair]]s ([[Category 6 cable|Cat-6 Ethernet]], [[Parallel SCSI]], [[ADSL]], [[Plain old telephone service|Landline Phone]], [[Balanced line#Balanced and differential|XLR audio]], [[USB]], [[Firewire]], [[RS-485|Serial]]); and unbalanced lines such as [[coaxial cable]] (Radio antenna, [[CATV]], [[10BASE5 Ethernet]]).

== Types of electrical and signal terminators ==

===Passive===
[[Electronic component#Passive components|Passive]] terminators often consist of a single [[resistor]]; however, significantly [[Electrical reactance|reactive]] loads may require other passive components such as inductors, capacitors, or transformers.

===Active===
[[Electronic component#Active components|Active]] terminators consist of a [[voltage regulator]] that keeps the voltage used for the terminating resistor(s) at a constant level.

====Forced perfect termination====
[[File:Electrical Termination, Forced Perfect Termination. or diode-clamped.svg|thumb|right|Forced perfect termination]]
Forced perfect termination (FPT) can be used on [[Single-ended signalling|single ended]] buses where [[diode]]s remove [[Overshoot (signal)|over and undershoot]] conditions. The signal is locked between two actively regulated voltage levels, which results in superior performance over a standard active terminator.<ref>{{citation |url=http://www.t10.org/ftp/x3t9.2/document.91/91-037r0.pdf |title=Forced Perfect Termination |publisher=[[IBM]] |date=1990-08-03 |access-date=2016-12-19}} "FPT is a terminator that [dynamically] self-terminates. One that matches itself to the line perfectly, thereby removing all reflections at the endpoints</ref>

== Signal termination applications ==

===SCSI===
All parallel [[SCSI]] units use terminators. SCSI is primarily used for storage and backup. An '''active terminator''' is a type of single-ended SCSI terminator with a built-in [[voltage regulator]] to compensate for variations in terminator power.{{citation needed|date=March 2013}}

===Controller Area Network===
[[Controller area network]], commonly known as CAN Bus, uses terminators consisting of a 120 ohm resistor.

===Dummy load===
{{Main|Dummy load}}
Dummy loads are commonly used in HF to EHF circuits.

===Ethernet coaxial 50 ohm===

[[Image:10base2 BNC Terminator.jpg|thumb|right|10BASE2 cable end signal terminator]]
[[10BASE2]] networks absolutely must have proper termination with a 50 ohm [[BNC connector|BNC terminator]]. If the bus network is not properly terminated, too much power will be reflected, causing all of the computers on the bus to lose network connectivity.

===Antenna network 75 ohm===
A terminating resistor for a television [[coaxial cable]] is often in the form of a cap, threaded to screw onto an [[F connector]]. Antenna cables are sometimes used for internet connections; however, RG-6 should not be used for 10BASE2 (which should use RG-58) as the impedance mismatch can cause phasing problems with the baseband signal.

===Unibus===
[[File:Unibus terminator card.jpg|thumb|[[Unibus]] terminator-and-bootstrap card from a [[PDP-11|PDP-11/34]]]]
The [[Digital Equipment Corporation]] minicomputer [[Unibus]] systems used terminator cards with 178 Ω [[pull-up resistor]]s on the multi-drop address and data lines and 383 Ω on the single-drop signal lines.<ref>{{cite web
| url = http://textfiles.com/bitsavers/pdf/dec/unibus/UnibusSpec1979.pdf
| title = Unibus Specification
| date = 1979
| author = Digital Equipment Corporation
| access-date = 2014-06-06
| archive-date = 2016-03-04
| archive-url = https://web.archive.org/web/20160304002031/http://textfiles.com/bitsavers/pdf/dec/unibus/UnibusSpec1979.pdf
| url-status = dead
}}</ref>

===MIL-STD-1553===
'''Terminating''' resistor values of 78.7 ohms 2 watt 1% are used on the [[MIL-STD-1553]] bus. At the two ends of the bus, resistors connect between the positive (high) and negative (low) signal wires either in internally terminated bus couplers or external connectorized terminators.

The MIL-STD-1553B bus must be terminated at both ends to minimize the effects of signal reflections that can cause waveform distortion and disruption or intermittent communications failures.

Optionally, a high-impedance terminator (1000 to 3000 ohms) may be used in vehicle applications to simulate a future load from an unspecified device.

Connectorized terminators are available with or without safety chains.

== See also ==
* [[Electrical connector]]
* [[Electrical network]]
* [[MIL-STD-1553]]
* [[Telecommunications pedestal]]

==References==
{{reflist}}

[[Category:Electronic circuits]]
[[Category:SCSI]]

Electrical termination - Revision history

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