I have been trying to implement a 10BASE-T Ethernet interface and in so doing, I have parsed the relevant portions of IEEE 802.3. While the standard does a marvelous job explaining the intricacies of the abstract interfaces between the, IMHO, overly-redundant sublayers, I was left with a bunch of questions and I thought this would be a good place to ask them.
First, what *EXACTLY* is the waveform of the signal? I got into Ethernet partially because I was convinced it used square waveforms which was great becase I though, since my FPGA uses sqare waves anyway, I could just cheat my way through encoding and decoding. But then I saw Figure14-10 and got a mild heart attack. Because that there on that figure (if I'm reading it correctly) can be best described as "sinusoid", NOT "square". My confusion only got bigger when I saw (and thought about) Figures 14-11 and 14-13 because those wave outlines are square, through- and-through.
Now, thankfully, I can still use a MOSFET-based physical receiver with the sine wave but if I need to produce a sinusoid on the transmitter output, that's going to be a bit trickier.
This may be a good time to mention that I'm doing this on my own, outside of any corporation/company and don't have access to waveform visualization equipment. Otherwise, I would have just taken my NIC and measured.
Second, why are maximum voltages for TP_IDL and link test pulse 3.1V when the peak voltage of signals is 2.5+-0.3V ?
And third, is it possible (to anyones knowledge) to implement autonegotiation but not step into any patents along the way? Since 10BASE- T is 22-23 years old, I suppose that all aplicable patents have expired, but I saw a couple of patents regarding autonegotiation that still have a few years in them. FWIW, I won't be making network equipment, just an interface on an otherwise non-networked device and don't expect to have big volumes.