It’s probably an RS-485 communication loop. IBM was fond of these in the 1970s through the 1990s. And RS-485 was used in a lot of industrial applications.
RS-485 has pretty good distance performance. Depending on the quality of the wiring it can travel up to 4000 feet without a repeater or short haul modem. That’s 10 times farther than Ethernet, and a pair of simple wires was a whole lot cheaper than fancy network cable. They’d run a pair of wires from the main loop controller computer to the first node, from the first to the second, and so on, eventually closing the loop by running the last pair back to the loop controller.
Each box had an address from 1-254. Communication was done by having the controller send a token to the first box, with a chain of messages attached that were addressed to whichever boxes needed the message. As the token was passed from box to box around the loop, the destinations would remove any messages sent to them, append any new messages they needed to send, then forward the token and rest of the chain along.
It was a precursor to IBM’s Token Ring, only at much lower speeds. We ran our first loops at 2400 or 4800 baud, but we replaced them in the 80’s with loops that ran at an astounding 28800 baud. (That’s a not-so-blazing 0.0000288 gigabit speed, for those of you comparing performance to your in-home fiber or broadband, or even a 5G cell phone.)
By the turn of the millennium, we had already replaced our loops with Ethernet, and our registers with PC based computers. It was clear that the aging loop technology was increasingly expensive compared to $10 Ethernet cards stamped out by the millions.
