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I've noticed that most ethernet physical layers (i.e. the interface between the MAC/network and e.g. an RJ45 connector with magnetics) have pull-up or pull-down resistors on the TX and RX lines. For example, the reference schematic for the Wiznet W5500 has a pull-up on the differential TX line and a pull-down on the RX line. These have the effect of making the "virtual ground" between the TX and RX differential channels either +3.3V or GND. In addition, the central tap of the TX line into the RJ45 magnetics is pulled up, and the RX line is pulled down:

Wiznet W5500 connected to ethernet RJ45

I've seen other schematics that do the pulling up or down differently - for instance this design pulls both the TX and RX lines down.

What purpose do these resistors serve? And why in the case of the W5500 is the RX line ac coupled (by the capacitor in series) and not also the TX line? I guess the answer is something to do with signal to noise, but I don't understand the reasoning.

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  • \$\begingroup\$ I don't see any point in pulling Tx pair up (Initially thought could be to drive status LEDs but does not look like that is the case here). Also in the other reference schematic Tx/Rx pairs are not pulled down , rather have a noise filters implemented. \$\endgroup\$ – Duster2503 Jan 23 '17 at 9:42
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What purpose do these resistors serve?

Those resistors are line terminators and also double-up as phantom power providers should phantom power be needed somewhere.

For the transmit connection, the likely explanation is that the output driver is a constant current (high compliance) driver and therefore needs pull-up resistors and a DC connection.

For the receiver, it is likely to be a diff amp that has its own biasing arrangement that should not be disturbed by an external dc connection hence, capacitors are used.

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  • \$\begingroup\$ They can't put phantom power onto the line, they're on the wrong side of the magnetics surely? \$\endgroup\$ – pjc50 Jan 23 '17 at 10:41
  • \$\begingroup\$ @pjc50 doesn't it depend how the magnetics are configured - some have internal connections that join up the centre-taps. \$\endgroup\$ – Andy aka Jan 23 '17 at 10:48
  • \$\begingroup\$ Ah - that must be what R3 10R to "TCT" is, the center tap. \$\endgroup\$ – pjc50 Jan 23 '17 at 10:59
  • \$\begingroup\$ @Andy aka The magnetics in the reference schematic have separate centre-taps that are on the same side of the transformer as the TX and RX connections to the W5500. There are no components attached to these lines inside the RJ45 housing. What do you mean by "phantom power"? \$\endgroup\$ – Sean Jan 23 '17 at 11:02
  • \$\begingroup\$ @Sean Phantom power is the same as power over ethernet. \$\endgroup\$ – Andy aka Jan 23 '17 at 11:13

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