End of line resistors

[Pitt123]

What is the purpose of end of line resistors on communication network trunks? For example DeviceNet has end of line resistors on both ends. Is this to match the characteristic impedance of the trunk to avoid reflection?

 

[steve66]

Yes.

Although in some other systems like fire alarms, EOL's sometimes only serve to let the control panel verify the field wiring is intact without any shorts or opens.

 

[gar]

100914-1600 EST

Pitt123:

Try calling it a terminating resistor.

See if this site helps explain what happens:
http://web.cecs.pdx.edu/~greenwd/xmsnLine_notes.pdf

If the reflection components die out quickly enough relative to when you need to sample the data, than a perfect match at the far end may not be the best choice because it reduces the low frequency output voltage. The die out time is determined by cable length.

A cable I have measured has a propagation delay of about 1.5 microseconds per 1000 ft, 7.92 microseconds/mile, and a velocity of 126,000 miles/second. The velocity of light is about 186,000 miles per second. So the signal propagation velocity in my cable is about 126/186 = 68% the speed of light.

.