Why does terminating resistor do in a Communication network?

[naseeam]

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!

 

[Nigel Goodwin]

To stop reflections - just like SWR on a radio transmitter.

 

[crutschow]

Do you mean both sides of the differential signal?

Termination resistors are normally to prevent signal reflections which can cause false signals. Any line has a characteristic impedance and, depending upon the frequency of the signal being sent and the length of the line, the line may need to be terminated with a resistor equal to its characteristic impedance to avoid problem reflections.

Reflections are caused when the energy in the pulse propagating down the line is not completely absorbed by the load at the end of the line. For example if you have a 5V pulse traveling down a line with a 50Ω characteristic impedance, the power contained in the pulse is 5²/50 = 0.5W. If that pulse sees any load impedance other than 50Ω at the end of the line, then not all the energy in the pulse is absorbed. The excess will be reflected back down the line.

 

[naseeam]

Thank you for great and quick reply.

>> Do you mean both sides of the differential signal?

I have slave1 board connected to master board and slave2 board. Would I need terminating a resistor on Slave1 board, Slave2 board, and Master board? That's 3 terminating resistors.

 

[JimB]

See the attached sketch.

The devices 1, 2 and 3 can be masters* or slaves.

The terminators are connected between the A and B wires at the two ends of the RS485 buss.

Where the buss length is short (less than 2 metres at a guess), it will work with only one terminator, but for best results terminate at both ends as I have shown.

JimB

* Usually there is only one master, however...
I once built a system which had two masters and three slaves on an RS485 network.
Only one of the masters could be active at any one time, due to what was driving the masters.
As far as I know it is still working 15+ years later.