John Beetem said:
From looking at the photos of the bare RasPi PCB (there are links at http://elinux.org/Rpi_Hardware
), it seems RasPi uses Enet jack pins 1+3 and 4+6 for the TX and RX differential pairs. This is consistent with some magnetic jacks. A non-magnetic jack uses 1+2 and 3+6 for the pairs, same as an Ethernet cable. Magnetic jacks have all sorts of different pinouts, so I can see why it's difficult to get the correct one.
Looking at http://www.raspberrypi.org/wp-.....rbers2.png
in addition to the yellow tracks running off towards the ethernet chip there is a brown track heading off to a couple of passive components.
Comparing with the reference design schematic I linked in my previous post I'm pretty sure that said pair of passive components are the resistor and capacitor connected to the center tap.
Yes, those Enet jack pins 2 and 5 are center taps. I expect the RasPi physical Enet is the same as the LAN9512 reference design, except that they use magnetic jacks with a different pinout. I'm not sure, but I think the transmit drivers on most Enet chips are open drain, i.e., they only pull down. The center tap provides a common pull-up. The transmitter pulls down one side for a positive pulse and pulls down the other for a negative pulse. From the schematic, the LAN9512 supports Auto MDIX which means that the chip can automatically swap the TX and RX pairs so you don't need to find a cross-over cable. So the TX and RX circuits are exactly the same, with both TX and RX termination.