That uses 4 wires. I was curious to find out how it's done with two. Sites promoting Single Pair Ethernet are amazingly unhelpful.
The relevant standard is IEEE Std 802.3ch™‐2020 [1] This is actually the automotive standard for 10Gb/s over a single pair, but the industrial version seems to be the same thing with different connectors. The relevant section is 149.1.3, "Operation of 2.5GBASE-T1, 5GBASE-T1, and 10GBASE-T1".
It's a single twisted pair of wires, but with a grounded shielding braid. It's not an unshielded untwisted pair, like DSL, or unshielded twisted pairs, like CAT 5 and up. The shield is grounded through the connector. That keeps the noise level down to where 10Gb/s can work. The cables will be more expensive and will be harder to make up than CAT 5, etc. Cable length limits are rather short.
It uses a modem, of course. The basic signal is 4-level pulse amplitude modulation. There's echo cancellation, so sending doesn't blind receiving. That's how it does full duplex over one pair.
One end is master, the other is slave, and this is negotiated automatically at startup. The master end provides the bit clock, and the slave end synchronizes to it. That simplifies echo cancellation; the interference is the same on every bit. (OK, symbol.) At startup there's a link characterization process, where each end finds out exactly what voltage levels it is getting from the other end, and adjusts for that. That's called "training" for historical reasons.
There's 340 bits of forward error correction, because crosstalk from other cables might be a problem.
Although this is nominally a baseband system, actually all the information is in the 1GHz - 4 GHz part of the spectrum. The data is, as with most modems, XORed with a cyclic scramble pattern to eliminate the DC component. That leaves room for DC power over the same wires.
One cute feature - if the two wires are reversed, that's supposed to be detected at startup and corrected. But the connector, which came after the PHY layer spec, isn't reversible.
1000BASE-T uses 4 pairs and each pair is simultaneously used in both directions. 250Mbps per pair, 1Gbps total, full duplex.
You could trivially chop the 1000BASE-T standard down to 1 pair and get 250Mbps full duplex. That would be similar to what SPE does. Everything you just described applies to bog standard 1000BASE-T too - PAM, echo cancellation, error correction, scrambling, etc.
