terminating resistor

E

Thread Starter

engineer

hi,

i have some conflicting information regarding this.

when exactly is a terminating resistor needed? how much? for example- its used in profibus connectors.

thanks in advance.
 
C

Curt Wuollet

A terminating resistor is needed anytime a controlled impedance transmission line ends with a load that doesn't match it's characteristic impedance. At least if you'ld like to prevent line reflections and maintain signal integrity.

Almost all distance communications involve controlled impedance transmission lines. Often terminators are built into the end devices. For ProfitBus the resistors are only needed at the ends and switched off on intermediate nodes.

Regards
cww
 
W
To add to CWW, when you terminate a cable that is acting like a transmission line with the cable's characteristic impedance, the line appears to be infinitely long, hence no reflections. If you don't, you get a dead end which will have reflections that can cause interference and standing waves. You can some times get away without having a cable terminator if the line is short and/or the transmission speed is slow, but in general it is not a good engineering practice. See the following links for more details.
http://en.wikipedia.org/wiki/Characteristic_impedance
http://www.allaboutcircuits.com/vol_2/chpt_14/3.html
http://www.epanorama.net/documents/wiring/cable_impedance.html

William (Bill) L. Mostia, Jr. PE Fellow

SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.
 
This 7 minute video on youtube, Reflected waves on a cable, shows the distorted waveform on an oscilloscope as energy is reflected back from an open or a shorted length of cable, which helps explain the phenomenon of reflections.

A resistance inserted which matches the load clean ups the square wave. That resistance would be a terminating resistor.

 
C

Curt Wuollet

And if you want to understand why comms are so picky about wiring as to use transmission lines, borrow an oscilloscope and look at the signals on wires in a trough or coming through a length of conduit. Especially the edges. The reason PLCs don't seem to get faster is that the inputs must be very heavily filtered to use typical open wiring. The relatively fast signals used for comms would very seldom be recognizable or intelligible sent across typical open wiring. A wire is a wire only up to a few hundred Hz. After that it becomes a complex network as reactances become a factor. What a transmission line does is simply keep the reactances in balance so the line appears resistive.

Regards
cww
 
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