We did this some years ago on an older PC, and we used COM1 ports to send over a 4 wire cable dragged through the roof, from PC to miller. If your PC is more modern (likely) and has no COM1 port then I have no solution. If your PCs/machines can have plugin COM1 cards then go that route. The wires get twisted as SEND->RECEIVE, and I use a COM1 plugin monitor with lights to see when wires were active. Lots of fiddling as I am no expert, but it *did* work in the end, and kept working. To send the target machine is put into receive and then the other sends by starting the handshake, when complete stop the sending machine, stop the target machine receive. It is all old-world DOS type interaction.
If you have no ability to support COM1 then I have no solution, sorry.