# Termination of unused PoE pairs

How important are the specific termination values used for PoE pairs (pins 4&5, 7&8)?

Below is the recommended Ethernet front end for Micrel’s KSZ8841/42. http://www.micrel.com/_PDF/Ethernet/app-notes/an-139.pdf

Would it be acceptable to use 49.9 ohm resistors in place of the 51ohm resistors? Why did they specify 49.9 on one side, and 51 ohms on the other? I’d like to make them the same in order to reduce the number of component types. This would not only help drive down the price of the 49.9 resistors but also mean there is one less component type to manage in inventory and load into the pick n place machine.

There must be a good reason for specifying slightly different resistor. I'm sure they didn't do it just for fun. How critical are the terminations of the PoE pairs? In my mind they are not critical because there is no data being transmitted over them, but I’m not sure how much their termination influences the characteristics of the cable for the other conductors.

• The impedance of the cable is, depending where you look, 100+-13 ohms. And you are terminating the unused pairs. So a few ohms either way won't matter at all. – tomnexus Sep 8 '15 at 11:15

Make sure you place caps (~ 22nF) in series with the $75\Omega$ if using PoE as Bob Smith did not envisage having PoE power on the data lines. It is also very important to consider saturation of the choke magnetics which also is not an issue on non-PoE. This took up months of my time trying to resolve.