That subtractor is a digital system (i.e. made using logic gates..) The physical ethernet layer is purely analog. The subtractor you have referenced is not relevant to what you want to do :)
To keep it simple, the TX+ and TX- voltages are subtracted from each other to yield the actual ethernet TX signal. To do this, you can use a differential amplifier.
Getting differential amplifiers to work at 30MHz isn't very trivial. You would have to use very fast op-amps (gain bandwidth product >> 30MHz) which make it difficult if not impossible to do on a breadboard.
That being said, you might be able to use the TX/RX pairs from the mbed without the need of a differential amp. It could be possible to use TD- and RD+ (referenced to GND) to interface with the toslink transceivers. I'm not too familiar with the PHY implementation of the mbed so I would not recommend going this route as you may damage the PHY chip on the mbed.
I think it might be easier to use the transceiver chips that I mentioned above rather than building your own interface circuitry.
I hope that helps :)
I have a question for the group, where I have some experience with ethernet and such... I haven't the experience designing devices which use it as much.. So I'm wondering if anyone knows of a solution to use fiber optics instead of cat5/6 cables.....However, not in the traditional fiber manner... More what I mean is there a product that allows a fiber optic transceiver of some sort to be connected to the mbed for example in place of the normal magjack one would use for a copper cable? An extreme longshot I'm sure, especially since I don't recall any fiber switches, etc. that would accommodate that, or would they? Any hacks come to mind that might allow this?
Anyway, I'd love to use fiber in a project instead of copper for so many reasons... Any word is appreciated!
BTW- I am aware of media converters and the like but I would like to also keep the cost and part count down.... Also, this would be on a closed network of nothing but a few mbeds talking to each other.....