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From: janii on 30 Apr 2010 11:27
What do terminating resistors do in a Communication network?
Can someone please explain why terminating resistors are needed in a
Communication network?

For example, consider RS-485 network with one Master board and Two Slave
boards connected to each other with 3 pins. Pins A and B are differential
signal. And there is a ground pin.

Why would it be necessary to attach terminating resistor to both sides
instead of just one side.

Please assume I have some basic electronics knowledge but no transmission
Line Theory.

Thank you!

---------------------------------------
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From: John Speth on 30 Apr 2010 11:36

"janii" <amernaseem(a)n_o_s_p_a_m.rocketmail.com> wrote in message
news:zuWdnSnmxKlDa0fWnZ2dnUVZ_oOdnZ2d(a)giganews.com...
> What do terminating resistors do in a Communication network?
> Can someone please explain why terminating resistors are needed in a
> Communication network?
>
> For example, consider RS-485 network with one Master board and Two Slave
> boards connected to each other with 3 pins. Pins A and B are differential
> signal. And there is a ground pin.
>
> Why would it be necessary to attach terminating resistor to both sides
> instead of just one side.
>
> Please assume I have some basic electronics knowledge but no transmission
> Line Theory.

A perfect candidate for a simple Google search. Try that first.

JJS



--- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: JW on 30 Apr 2010 11:39
On Fri, 30 Apr 2010 10:27:26 -0500 "janii"
<amernaseem(a)n_o_s_p_a_m.rocketmail.com> wrote in Message id:
<zuWdnSnmxKlDa0fWnZ2dnUVZ_oOdnZ2d(a)giganews.com>:

>What do terminating resistors do in a Communication network?
>Can someone please explain why terminating resistors are needed in a
>Communication network?
>
>For example, consider RS-485 network with one Master board and Two Slave
>boards connected to each other with 3 pins. Pins A and B are differential
>signal. And there is a ground pin.
>
>Why would it be necessary to attach terminating resistor to both sides
>instead of just one side.
>
>Please assume I have some basic electronics knowledge but no transmission
>Line Theory.
>
>Thank you!

http://www.ni.com/support/serial/resinfo.htm
http://www.lammertbies.nl/comm/info/RS-485.html
From: Magnum on 30 Apr 2010 11:49
"janii" <amernaseem(a)n_o_s_p_a_m.rocketmail.com> wrote in message
news:zuWdnSnmxKlDa0fWnZ2dnUVZ_oOdnZ2d(a)giganews.com...
> What do terminating resistors do in a Communication network?
> Can someone please explain why terminating resistors are needed in a
> Communication network?
> For example, consider RS-485 network with one Master board and Two Slave
> boards connected to each other with 3 pins. Pins A and B are differential
> signal. And there is a ground pin.
> Why would it be necessary to attach terminating resistor to both sides
> instead of just one side.
> Please assume I have some basic electronics knowledge but no transmission
> Line Theory.

"Simple" circuit theory is a small part of the bigger picture in which
waves travel along lines.

Clearly from your comment above you already have some inkling of that.

The line has what is known as the Characteristic Impedance (or the Surge
Impedance from the way that it is calculated).

That impedance determines the voltages and currents on the line in
much the same way as does Ohm's Law for simple circuits. (Although
Ohm's law seems to work in this case, it is strictly a misapplication, BTW)

When the wave gets to the other end, if the line is terminated by a resistor
whose value exactly matches the Characteristic impedance, then the
wave is absorbed by the resistance. Hopefully, that matching resistance
will be part of the sensing amplifier used to receive the signal.

If, however, the terminating impedance is not a match, or is a short
or open circuit, then some of the wave will be reflected back down the
line where it will distort other pulses on the line.

This is why you need to have the terminating resistance, to ensure that
it matches the impedance of the line. If your pulses are quite slow, such
that the reflections die out before the middle of the pulse where you
might be expected to be doing the measurement, then you could get away
with no termination. This is the world of the simple circuit!

Similar examples of waves being reflected at an impedance mismatch
are light waves being reflected off a mirror, and sea-side waves being
reflected off the rocks.


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