From this article by Ivan Pepelnjak
Networking in 1980s: a Personal View
a diagram, a DECnet map from 1991, specifically the SLON network in Slovenia:
In those days a large group of enthusiasts build a country-wide DECnet network (SLOvenian Network – SLON – which happens to mean elephant in Slovenian) that had hundreds of nodes… all based on “let’s get together” mentality and people willing to buy a leased line to someone who had a spare RS-232 port.
Nearby on the same blog, and probably nearby in space and time too, I found these Z80 related posts:
Local Area Networking on RS-232
Ring Message Bus on RS-232