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I was wondering what do the 49.9 ohm resistors do in this circuit? Are they absolutely required since I don't have any that size.

Schematic

JYelton
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Ageis
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  • Near duplicate: http://electronics.stackexchange.com/questions/10884/how-termination-resistors-work-what-happens-if-i-use-lower-values – The Photon Jan 31 '13 at 17:14
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    Those resistors are termination resistors for the incoming transmission lines. If the lines are longer than a couple of inches, you should include them. If you don't have 49.9, then 51.1 should work. Even 47 Ohm 5% is better than nothing. – The Photon Jan 31 '13 at 17:15
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    ah ok. I didn't know even "modern" ethernet has termination resistors. I know thick ethernet had one at the very end bus topology. by that do you mean if the micro is more than a few inches away or the actual ethernet cable from the wall outlet? – Ageis Jan 31 '13 at 17:39
  • I mean the total transmission path from wherever it comes from to the TPOUT+/- and TPIN+/- nodes. – The Photon Jan 31 '13 at 18:03

1 Answers1

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Yes, they are required. Your circuit shows a twisted pair ethernet connection. Each pair must be terminated with its characteristic impedance to maintain signal integrity. The resistors you mention effectively put 100 Ω load on the inboard side of the pulse transformer. In this case, it is a 1:1 transformer, so that is also the load seen on the cable end.

If you leave them off, the voltage levels will be higher than expected, and reflections will occur at the ends of the twisted pair at the transformer. How much this matters depends on the length of the cable and the speed you are running the ethernet at. In any case, leaving off the terminating resistors is a bad idea.

If you don't have 49.9 Ω, use something as close as possible for short term testing with short cables while you're waiting for the right resistors to arrive. I wouldn't ship product with these resistors a few percent off, but for bench testing and debugging other parts of the system, especially if you keep the ethernet cable short, you'll probably get away with it. Either of the common values of 47 Ω or 51 Ω will probably be fine for bench testing under controlled conditions.

Olin Lathrop
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    For bench testing, two parallel 100ohm resistors are the best. Anyway, many PHYs now have those resistors already inside the chip silicon. – Tomas D. Feb 01 '13 at 13:26