I got one of these surplus from an Exxon lab where they upgraded.

From the instrument company they had a 4-channel RS-232 "Autoswitch" you could wire into the serial port from 4 instruments, then the fifth cable ran to a dumb terminal CRT console, and you could access any one of the instruments by sending unique control codes to switch communication channels.

Those Autoswitches were about a $1000 option. Instead, most people who were doing something like this used a manual switch from Radio Shack that was about 50 bucks.

Alternatively, you could unplug the local CRT terminal and connect the serial cable to a phone modem instead. Your instruments then had a telephone number.

You could then call from anywhere to access the data and control the instruments. As long as you had a modem and terminal at any remote location. Long-distance charges may apply :)

Using 1980's design equipment.

In 2001 the Sony smartphone made it possible to remotely call in on a cellular, link it to your laptop by infrared, and use Hyperterminal in Windows XP instead of a local CRT if you wanted to also. They had this to an extent before XP, Bluetooth, and USB anyway.

But with a manual switch you couldn't switch instruments remotely 24/7 unless there was somebody on site 24/7, not without an autoswitch.

At Exxon, 4 channels was not enough, and there were AT&T rack units like this, way more powerful, with built-in modems for direct connection to the phone lines.

Never did use the rack one I got since all my instruments were in the same lab at the time, and before I could make a move, ethernet became dominant which joined each instrument's PC quite well when it was built into Windows, instead of having to join instruments together in some way beforehand.

All I had to do so I could have a PC for each of the old RS-232 instruments was to "just" write my own code to accomplish both, "concurrently" for instruments never designed for DOS interfacing, much less Windows.

Bonding twelve 56K modems together to set dial-up broadband records