purpose of termination resistor

Question

My question is not about a particular bus or line and the termination I should put on it.

I know some busses need termination resistors like CAN or address/data for memories.

If I understood well these resistor are needed to avoid reflection on the signal.

My questions are :

• Are termination resistors needed only to avoid reflection?

• What is the electrical phenomenon that lead to reflection? Why is there reflection on a signal if there is no resistor?

• Is the value of the resistor depending on the bus length or more on the frequency of the bus?

• If resistors are for reflection purposes what is the phenomenon that destroy the reflection by adding a resistor?

• Why are termination resistors needed sometimes in parallel and other times in series?

Answer 1

You may want to read about the impedance and capacity of a wire/transmission line. I will try my best to translate to English as I learned most of this in German ;)

Every wire not only has a resistance, but also and impedance and capacity. These add up to the [electrical impedance] (https://en.wikipedia.org/wiki/Electrical_impedance). If you look at the effects of impedance and capacity, you will notice that these scale with frequency.

You can always use a serial matching end, it just depends on the configuration. If you have a wire with a impedance of 50 Ohm (typical HF wire) or 100 Ohm (like CAT5 Network cable), you need a matching end. This end is a 'network', wich will also get the frequency, so a normal resistor 50Ohm will work at 1MHz, but will have a mismatch (and a reflection!) at 1Ghz (which is why extra expensive frequency tolerant resistors exist). To counter this, you can measure the values of your resistor (C and L) and attach additional resistors/inductors/capacitys to counter the effect.

So your network on the end of the cable has to match your cables impendance at the given frequency. How you archive this matching is your choice. A series resistor is the common choice for 'household' electronics in the sub GHz area, above there are special solutions.

Answer 2

When you throw the light switch in your home, current must flow into the cable before that current reaches the lamp. So you have a traveling wave front of voltage and current flowing down the cable and these wave fronts then meet the lamp.

Before they meet the lamp something must have defined the current wave front i.e. some impedance must have been immediately present so that a current could start to flow (after all the current doesn't meet the lamp for a few nanoseconds later).

The thing that defines the initial current is the cable - it has a characteristic impedance and that impedance defines the initial current flow.

So you have voltage and current traveling down this cable. Volts x amps = power and if the power reaching the lamp (or load) isn't impedance compatible with the load then some power is reflected back up the cable.

Of course, within a few more nanoseconds this resolves - the various waves are sent, returned, modified etc. and eventually settle down.

Now, as a thought experiment, imagine your cable was thousands of miles long - say 100,000 miles and, imagine it was lossless. You throw the switch and about one second later you see the lamp glow at about half brightness. One second later there is a reflected wave returned to the switch which causes a bigger current to flow and one second after that the lamp glows a bit more like it should. This continues back and forth until the lamp settles down to its normal constant brightness.

Now imagine you were transmitting high speed data and you didn't terminate the cable properly or you used the wrong cable. Can you imagine what would happen?

Answer 3

The termination resistor is selected to match the line to its characteristic impedence.This minimises reflections and is important at high bitrates and long cable lengths.The series termination resistor matches low impedences to the medium impedence line and the paralell termination resistor is used to match the line to high impedences.

Answer 4

The question demands a very vast answer & an in depth understanding of transmission line theory. I hope this will help :- http://www.ultracad.com/mentor.htm

In this link, start with - Propagation Times and Critical Length

You can always come back for any specific doubts.