Ethernet termination resistor

Question

Hello I have a question about the new Wiznet W5500. Why the Ethernet termination aren’t 50ohm? With the W5200, resistors were the same on both transmit and receive. Why is it different on the W5500? 800×532 9.3 KB

lpph · Answer

Ok, but on the W5100, the termination resistors are also 49.9 ohm. Why the W5500 has 82ohm termination instead of the standard 49.9 ohm on regular PHY? Why is it AC coupled on the RX line on the W5500 and not on the other chips? Why is there a RC filter on the centertap instead of a cap? If I put 49.9 ohm termination like on the W5200, will it works?

peter · Answer

You’re right. The termination resistor value is 49.9ohm. When using a transformer that TCT and RCT Center-Tab are connected each other (such as RB1-125BAG1A), AC coupling on the RX line is necessary for operating of the w5500.

lpph · Answer

[quote=“peter”]You’re right. The termination resistor value is 49.9ohm. When using a transformer that TCT and RCT Center-Tab are connected each other (such as RB1-125BAG1A), AC coupling on the RX line is necessary for operating of the w5500.[/quote] Ok, thank you. I will keep the 6.8nF on Rx and use a transformer with TCT and RCT with 49.9ohm.

lpph · Answer

Just to make sure I understood well. I post a picture of the schematic I’ve done. Thank you all!

peter · Answer

We recommend symmetric transformer as link below. wizwiki.net/wiki/lib/exe/fetch.p … 140108.pdf Your design has asymmetric transformer inside Magjack(0817-1a1t-11-F). And the wiring of the differencial pair line(TX+/-, RX+/-) is incorrect. TX+/- are connected to 3A3V through resistor and also TCT(Tx-Centertab of the transformer). Please change to symmetric transformer and redesign such as w5500 reference.