2
\$\begingroup\$

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. Ethernet Front End 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.

\$\endgroup\$
  • \$\begingroup\$ 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. \$\endgroup\$ – tomnexus Sep 8 '15 at 11:15
1
\$\begingroup\$

Yes it would be totally acceptable to use 49.9 ohms instead of 51. Theoretically the value should be 50 ohms I think so 49.9 is even closer to that.

The terminations are there to terminate the transmission line which in this case is the ethernet cable. By terminating it properly you make sure any signals that are on the line (cable) dissipate in the resistor instead of reflecting back into the cable and distorting the signal.

For unused ethernet pairs it is probably not that critical but it can be that due to capacitive coupling there could be some signals on the unused pairs. By aso terminating these properly also these signals cannot reflect and disturb the other signals. Also it's just good practice.

\$\endgroup\$
3
\$\begingroup\$

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.

\$\endgroup\$
1
\$\begingroup\$

I agree that 49.9 ohm termination resistors are suitable to use instead of the 51 ohm. Ideally it should match the cable exactly, but I doubt you are going to find an exact cable to match. The 75 ohm pair terminations (Bob Smith terminations) are apparently sub optimal. There is an interesting article on termination of unused Ethernet pairs here: https://www.eetimes.com/document.asp?doc_id=1277940 This suggests a 52.3 (read 49.9) ohm star connection for CAT5, or a 66 ohm between pairs gives a better match to the inter-pair impedance. Note that there is no evidence that this better matching would make noticeable reduction to electro magnetic emissions. (I expect that the Bob Smith terminations will absorb sufficient energy to prevent ringing). Common Mode Terminations Here is a rough sketch of what the Ethernet terminations may look like if you were to include the inter-pair matching as suggested in this article. C1 to C4 are required for PoE systems. Theoretical, rather than real-world resistance values have been shown.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.