[Moved]: Need help understanding CT pins in RJ-45 with magnetics

Hi,
I'm trying to reuse some old unused LAN cards to connect to some microcontroller evaluation boards with ethernet support. I chopped off the PHY while keeping the rest. I am treating the RJ-45 to PHY circuitry as blackbox, while dealing with the wires coming out the transformer connecting the PHY. Connecting TX+ , TX- , RX+ , RX- pins to MCU through 1,2,3,6 pins of RJ-45 holes seems fairly easy, but I can't get hold of the idea of TXCT, RXCT pins (4,5 pins of RJ-45 holes). What are these pins for? How to treat them in general? The RC values connected with TXCT and RXCT are wildly different in different schematics. How sensitive are they for error free connection?

While looking for reference schematic, I found that different circuits connects them very differently, so I am very confused. I couldn't find any reference material explaining these pins clearly. Please have a look at the following pics of LAN card, transformer and schematic I am trying to cope with-

The 75 ohm resistors act as a common mode termination and are optional.

FvM said:

The 75 ohm resistors act as a common mode termination and are optional.

Click to expand...

I understand. So far I can see, that part should be already implemented on the LAN card. So, should I just GND pins (TXCT, RXCT) going to PHY?

CT pins on the PHY side of the transformer have to be connected according to the PHY specification, they are rarely connected to ground. Typical variants are unconnected, grounded through capacitor, connected to Vdd (often used for 100-Mbit TX).

CT = Center Tap, they are the 'half way' point along the windings and used as the zero SIGNAL reference point for the differential data. On the LAN side they usually have an isolated DC supply added to them to alert the other end of the link that it is 'live'.

Brian.