I wanted to test if its possible to have a magnetic-less ethernet connection for an upcoming project. Why magnetic-less? you might ask, well space & weight is critical for my application and since both devices will be present on the same PCB, why not skip the magnetics and reduce space & weight. I have to admit it was allot easier than I imagined it would be. When I first thought about it I imagined I would need to simulate the impedance of the transformer with some inductors, this way before I did any reading on the subject.
Thankfully I had a friend who helped me with some network switches from his junk box. The switches were running ok but its clear that they once had some reliability issues which got them into the junk box, but it was enough for my little experiment. I decided to use the following configuration to test the magnetic-less connection:
- both switches will have the transformers removed from one of the ports.
- computer1 connects to switch1 with standard connection.
- switch1 connects to switch2 with magnetic-less connection(no transformer).
- computer2 connects to switch2 with standard connection.
After the setup is made and link is up I would ping computer2 from computer1 and if the link is ok the result should be obvious.
Now, at this step I still haven’t done any reading on magnetic-less ethernet connections, so I took the bad choice of connecting the two magnetic-less ports wire to wire, “wire coupling” . I knew i had slim chances to make it work, but I said nothing can go wrong. I hooked the two ports, and nothing happened, no link, no LED turned on. Probably somewhere in this process I damaged the two network switches(RTL8309SB) because they were both working before I did surgery on them, and they were both not working when I finished the surgery So directly coupling the two interfaces is a bad idea, it should of ringed a bell the first time I thought about it but it didn’t, I learned it the hard way.
This is where I knew I had to do some reading on the subject. Luckily every manufacturer of ethernet interfaces has an application note on how to couple them magnetic-less for exactly the same situation that I have: both chips on the same pcb. I picked up another 2 switches and opened them up. Since they both had Realtek controllers I used this application note from Realtek. The app note provides a simple solution, capacitive coupling.
I got to work, once again I removed 1 transformer from each network switch but this time I used capacitive coupling. I soldered wires to where the transformer used to be and I connected those wires to a mini breadboard. The resistors+capacitor pair which sit between the lines on the schematic are still on the board, they’re needed with or without the magnetic transformer. All I had to add was one 0.1uF capacitor on each line. I didn’t even used the pull-up 1.8V as suggested by the app-note. I did the connections as mentioned above, the link activity LED’s signaled immediately, I assigned the computers some ip addresses and success I had the link up & working in no time. I only did a ping test which resulted in under 1ms replies. Perhaps I should of tested the bandwidth too, but I don’t think it suffered from the change.
Now that I know its working I can save space & weight by implementing the real thing.