You should probably state your data rate and timing requirements. I assume it's a steady stream of data which must be transmitted without interruption? Is the latency critical too or could generous buffering be used?
Ah, perhaps I wasn't too clear. There is no data rate as such - imagine a button at one end and a button input at the other. Or an LED. Perhaps daylight sensor (analog). Stuff like that - if I need to do actual data comms then I would do that via a pukka IP connection.
Also, presumably there is other traffic on the network -- do you need to share bandwidth?
Yes, normal LAN traffic. Currently is gigabit Ethernet and traffic is low: mobiles and satnavs periodically.
what are the "ad-hoc digital signals"?
Dunno. That's why they are ad-hoc
You can't really ask whether a suitable product is available without specifying that, right?
I believe that's what I did at the start, yes! However, whatever solution presents would, of course, define what I could connect up, so think of it more as trying not to narrow the possibilities to some specific thing (like how it's concentrated on serial data already!). I'm sure that if I succinctly describe a specific usage there will be a big discussion on how to perfectly sort that and nothing else will work with it.
Well they do make serial to Ethernet converters.
Yes, but that's NOT what I want to do (already have those if I needed them). I used that simply for illustration to show the idea of having none-IP stuff traverse a network, but I am quickly thinking that might've been too helpful
Does the network interface have to play nicely with existing Ethernet infrastructure
Yes.
or are you just looking to utilise the cables for sending signals
No, that's what it used to be. About twenty years ago I buried two runs of cat5, using one for network and t'other for alarm. Then because there was little use for the network (just a milling machine, and I preferred to keep that off the net for security) I used that cable for security cameras (four in the end, using baluns at each end). But now the alarm had an alternative route and we needed decent WiFi there so it made sense to convert the alarm cable over to Ethernet. But... I kind of got used to being able to use the unused pairs in the alarm cable for simple connections, so that's what I'm trying to replicate.