| You're looking at a Raspberry Pi Pico which has been hooked up with a make-shift differential driver[1] using two GPIO's[2] where one outputs the inverse signal of the other. The differential signal is connected directly to a RJ-45 jack, which is why the warning about not connecting Power over Ethernet[3] gear is written in bold as it would destroy the Pi. Normally Ethernet is AC coupled[4] through transformers, though capacitors can be used[5], which protects the local device from any DC voltage on the transmission line. Power over Ethernet exploits this by explicitly feeding a DC voltage over the lines. That voltage is way above what the GPIO's on the Pico can tolerate, and would cause the Pico to relase the magic smoke[6]. Not having any AC coupling on the Pico works for short cables if the other side is properly AC coupled. The scope plots shows the differential signal as driven by the Pico. Ethernet is Manchester encoded[7], a self-clocking signal without DC component, which makes it easy to AC couple. The fixed preamble used by Ethernet is so the receiver can recover the clock signal used by the sender in order to correctly read the incoming signal. Using a self-clocking encoding means the senders clock pulses doesn't have to be sent as a separate signal. What you don't see is any way to receive data. As noted this is the bare bones needed to send data. [1]: https://en.wikipedia.org/wiki/Differential_signalling [2]: https://en.wikipedia.org/wiki/General-purpose_input/output [3]: https://en.wikipedia.org/wiki/Power_over_Ethernet [4]: https://www.intel.com/content/www/us/en/docs/programmable/68... [5]: https://ww1.microchip.com/downloads/en/AppNotes/ANLAN120-UNG... [6]: https://en.wikipedia.org/wiki/Magic_smoke [7]: https://en.wikipedia.org/wiki/Manchester_code |