Was it perhaps DEC Baseway[1] system? It would arguably qualify as a form of DCS, with central VAX servers and running high level software, PDP-11 "shop floor" computers, which in turn talked with PLCs and related devices.
And additionally, TCP/IP was an optional layered product - either DEC's own UCX (Ultrix Communication eXtensions) or 3rd party
I guess they didn't buy the optional network software.
I remember working with this engineer writing in the lower level TCPIP stack on the deck - maybe in Fortran? Anyway, he didn't understand why he wasn't getting all the data transmitted and while I was no expert in TCP/IP, I was already familiar with packets and maximum segment size (MSS). I remember being impressed that he banged out a network stack in a couple weeks.
And additionally, TCP/IP was an optional layered product - either DEC's own UCX (Ultrix Communication eXtensions) or 3rd party
[1] https://bitsavers.org/pdf/dec/vax/vms/baseway/