Inelectronics,electrical termination is the practice of ending atransmission line with a device that matches thecharacteristic impedance of the line.Signal reflections occur where there is animpedance mismatch. Termination prevents signals from reflecting off the end of the transmission line. Reflections at the ends of unterminated transmission lines causedistortion, which can produce ambiguousdigital signal levels and misoperation of digital systems. Reflections inanalog signal systems cause such effects asvideo ghosting, orpower loss in radio transmitter transmission lines.
Signal termination often requires the installation of a terminator at the beginning and end of a wire or cable to prevent anRF signal from being reflected back from each end, causinginterference, or power loss. The terminator is usually placed at the end of atransmission line ordaisy chainbus (such as inSCSI), and is designed tomatch theAC impedance of the cable and hence minimize signalreflections, 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.[1]
Radio frequency currents tend to reflect from discontinuities in the cable, such asconnectors 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 requireimpedance 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 electricalcharacteristic impedance. Signal terminators are designed to specifically match the characteristic impedances at both cable ends. For many systems, the terminator is aresistor, with a value chosen to match the characteristic impedance of the transmission line and chosen to have acceptably low parasiticinductance andcapacitance 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 asladder line, andtwisted pairs (Cat-6 Ethernet,Parallel SCSI,ADSL,Landline Phone,XLR audio,USB,Firewire,Serial); and unbalanced lines such ascoaxial cable (Radio antenna,CATV,10BASE5 Ethernet).
Passive terminators often consist of a singleresistor; however, significantlyreactive loads may require other passive components such as inductors, capacitors, or transformers.
Active terminators consist of avoltage regulator that keeps the voltage used for the terminating resistor(s) at a constant level.
Forced perfect termination (FPT) can be used onsingle ended buses wherediodes removeover and undershoot conditions. The signal is locked between two actively regulated voltage levels, which results in superior performance over a standard active terminator.[2]
All parallelSCSI units use terminators. SCSI is primarily used for storage and backup. Anactive terminator is a type of single-ended SCSI terminator with a built-involtage regulator to compensate for variations in terminator power.[citation needed]
Controller area network, commonly known as CAN Bus, uses terminators consisting of a 120 ohm resistor.
Dummy loads are commonly used in HF to EHF circuits.
10BASE2 networks absolutely must have proper termination with a 50 ohmBNC 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.
A terminating resistor for a televisioncoaxial cable is often in the form of a cap, threaded to screw onto anF 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.
TheDigital Equipment Corporation minicomputerUnibus systems used terminator cards with 178 Ωpull-up resistors on the multi-drop address and data lines and 383 Ω on the single-drop signal lines.[3]
Terminating resistor values of 78.7 ohms 2 watt 1% are used on theMIL-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.