Purpose of a Terminating Resistor?

Thread Starter

brightnight1

Joined Jan 13, 2018
75
In some communication wires such as a CAN bus you need a terminating resistor between the two wires used for communication (120Ω), otherwise the signal can be overwhelmed by noise. What does the terminating resistor actually do and why is it so important? Thanks in advance :)
 

Thread Starter

brightnight1

Joined Jan 13, 2018
75
Thanks for the reply, that was a very helpful article.

So the resistor doesn't actually attenuate the signal reflections, it just makes it so the wires are matched and therefore there are no reflections?

I assume it works be even though it's just short wire should have almost no impedance, it's more about having the wires matched then having no impedance? The 120 Ohms in comparison to the short wire makes any inconsistencies in the wire negligible and therefore "matched". That sounds right? Thanks again!
 

MrChips

Joined Oct 2, 2009
23,277
Reflections down a piece of rope is a classic physics demonstration.


You can get reflections anytime you send an electrical signal down any piece of electrical conducting wire.

http://www.physicsclassroom.com/mmedia/waves/free.cfm

Every electrical conductor has what is known as characteristic impedance. Reflections are minimized when the end of the conductor is terminated with a resistance equal to the conductor's characteristic impedance.

Hence the terminating resistor is to minimize signal reflection.
 

MrChips

Joined Oct 2, 2009
23,277
Why is the length of wire an important factor?

As the signal propagates along the wire, reflections occur at both ends of the wire.
Hence by the time the signal makes three trips along the wire, reflections can combine with the original signal and distort the shape of the signal.
The speed of the signal along the wire is about 60% to 75% of the speed of light ( 3 x 10^8 m/s).
Knowing the speed of the signal propagation and the length of the wire, one can predict the effect of multiple reflections on the integrity of the signal.

Thus, the length of the wire and the frequency of the signal (along with the amount of termination) are key factors in understanding the severity of improper cable termination.
 

MrChips

Joined Oct 2, 2009
23,277
A signal cable is not purely resistance. It has inductance and capacitance.
The characteristic impedance of a cable is the same regardless of the length of the cable.
Note that we are referring to impedance which is the opposition to the flow of AC, and not resistance which applies to DC.

The difference with a short cable is that the propagation time along the cable is so short that any reflections have very little effect at relatively low frequencies.

With propagation speed at 66% the speed of light, signal travels approx. 5ns/m.
Hence if your cable is less than 1m long and your signal transitions are much longer that 15ns then you will not observe much reflections.
 

crutschow

Joined Mar 14, 2008
27,021
The reason 120Ω is used in this case is that the characteristic impedance of twisted wire or a pcb trace over a ground plane is typically close to that value.
Coax lines are typically 50Ω (RF) or 75Ω (video), with some being 93Ω.
 

panic mode

Joined Oct 10, 2011
1,881
you don't need terminating resistor if cable is infinitely long (but of course that is not possible).
if we had infinitely long cable and you introduce signal, signal would travel in both cable directions.
with distance signal will get weaker (lower amplitude) but not distorted. it means we just need to amplify it to get correct wave form.

but since infinite cable lengths are impossible we need to work with cable segment.
signal reaching end of the cable will not just vanish, part of the signal will be reflected.
reflection will then interfere with original wave and we got ourselves a mess...

to prevent reflection, cables need to be terminated at each end with an impedance that is equal to characteristic impedance of the cable.

manufacturers of various fieldbuses choose cable and that determines type of terminator.
for CAN bus and DeviceNEt that is 121 Ohm 1%
for DH+ it is 82 Ohm
same goes for CC-Link and ProfiBus etc
 

nsaspook

Joined Aug 27, 2009
8,292
Nice, neat bus devices with matched terminating resistors and matched transmissions lines are great but there are times when it's not practical (SPI driver load limitations for a proper termination or a random length of wiring on a prototype board to many device) to match to the transmission lines characteristic impedance so there will be reflections and a need to reduce those reflections but not the wanted component signal. In these cases we can use a series (series termination) resistor to improve signal (the SCK clock edges are critical at high speed) integrity at the device.

http://www.the-signal-and-power-int...te_Source_Termination_Resistor_Location_2.pdf

http://www.ni.com/white-paper/3854/en/
5. Types of Termination

There are several forms of line termination, including parallel, series, and differential. Parallel termination matches the characteristic impedance of the medium at the end of the line, squelching the wavefront at the destination (Zt = Z0). Differential termination is a variation of parallel termination used for differential transmission lines. Many electrical standards, such as emitter-coupled logic (ECL) and LVDS, require that traces are routed differentially. As such, parallel termination is used between the two modes of the differential trace. Series termination places series impedance equal to the characteristic impedance at the source of the transmission line. This termination prevents the source from re-reflecting any reflections from an unterminated transmission line. It also prevents reflections from the transmission line to the source at the entry (ZS = Z0). Practically, termination at only one end of the transmission line is often adequate and is more commonly used.
 

panic mode

Joined Oct 10, 2011
1,881
In some cases, termination at only one end is sufficient.
yes, if cable is short enough, or delays between telegrams are long enough for ringing to die out, or when other side of the cable is long enough so that reflected signal is bellow noise threshhold - single terminator may be ok.

if there is a fieldbus specification that requires termination but tolerates one sided termination, i would need to see it.
 
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