I'm interested in designing a "simple" circuit that would bridge digital inputs or outputs over a distance with relatively low latency. The idea is to take several pins from a GPIO bus like Arduino or Raspberry Pi and reproduce their status remotely - let's say 8-bits in each direction, rather than freely configurable, and up to 30m distance and shoot for <1us latency (ideally no overhead that could cause non-deterministic behavior).
I like the idea of trying to leverage ethernet hardware to provide decent noise immunity, cheap standard cabling, and the possibility of providing remote power. The devices I find (and older questions on stack exchange) seem to always circle back to a fairly complex packet exchange and highly integrated ICs (or is that "highly ICs"). Most industrial solutions (e.g. Modbus or profinet) also stick to the packet model. I'd be happy to just use a clock and some 74-series shift registers, etc. to generate a serial train and decode it on the other end - just wondering if there's a "middle-bit" that would manage the level shifting and signal processing without getting into all the more complex ethernet layers (I suppose there may be some clock recovery - although I'd be happy to use part of the cable for a clock signal, or force the serial speed to be a standard ethernet speed and a start/stop bit if needed). I don't care about running through standard ethernet hardware (switches, etc. - those would all tend to introduce significant latencies), but would like to preserve the magnetics to obtain better isolation and power injection.
Thanks for any suggestions or references to existing hardware solutions or perhaps better suited protocols.