I'm working on a design where it needs to use only circular DIN connectors [...]
Umm, but why?? Even if you can get good signal integrity for Ethernet over a DIN connector, won't the users hate you forever since you force them to use (buy? make?) custom cables? What is the compelling reason against using a standard RJ45 connector?
Seems not unheard of...
https://www.globalsources.com/DIN-cable/Mini-DIN-8pin-to-RJ45-cable-1141079898p.htmSee a few for MIDI, which I guess makes sense; maybe some phones have used it too, not sure? RS-232?
To be sure, M12 and such are far more commonly used for Ethernet.
I guess there's power (not PoE) and other signals, which wouldn't exactly be compatible with standard.
Since there is no RJ45 chassis, does a Bob Smith termination just connect to the casing of the DIN plug (which is connected to board ground)?
RJ45 chassis..?
Chassis chassis. Is this in a metal box? (If not, maybe it should be, or at least a shield/plane to tie together all the connector grounds?) Presumably the shell/shield also connects there, or at least with some RF bypassing (to afford some galvanic isolation -- alleviate ground loop).
If it's completely and utterly floating, like, a battery powered box (or line powered, for that matter), no other connections, plastic case, single connector -- BS would return to the shell, if anything at all.
Note that, because your other wires are in the same cable, they should be terminated the same way, for the same reason... this may not be easily arranged depending on what the "signals" are, and the "power" may need a CMC to introduce that impedance at high frequency.
Tim