[SOLVED] Ethernet magnetics between embedded devices.. do i need them?

[thunderdantheman]

G'day,

I have two OEM Ethernet devices that I will embed on a motherboard. On the bench I connect the two devices together via the standard RJ45 crossover with magnetics on each device interfacing the PHY to the cable. When I embed these two devices, am I able to connect them PHY to PHY (cross over)? i.e. without the magnetics?

 

[FvM]

Possibly. It depends on the receiver and transmitter design details. In some cases, at least inductors may be needed.

 

[thunderdantheman]

G'day,

If anyone else is interested I found this app note from Texas Instruments - TLK100 Ethernet PHY Transformerless Operation and was available from https://www.ti.com/lit/an/slla310/slla310.pdf It suggests using capacitors as the interface. Whilst it is based on the TLK100 Phy, it should be applicable to others.

 

[FvM]

As said, some PHYs may require an output inductor to avoid output stage saturation, particularly in 10 MBPS operation. But I guess, the method will work with most PHYs.

As a problematic point, the application report doesn't address the risk of PHY damage by common mode voltages in non-embedded applications. Obviously it's not sufficient to use capacitors with high voltage rating, because the capacitors will be charged/discharged when plugging the network cable. The energy stored in a 33 to 100 nF capacitor is far beyond the handling capability of built-in chip ESD protection.

While the solution is O.K. for interchip communication, it shouldn't be suggested for other cases in my view.