What is the purpose of terminating resistors in a Communication Network?

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

JJS

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Reply to
John Speth

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Reply to
JW

"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.

Reply to
Magnum

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