To begin with I thought about some sort of Serial->IP converter, or an old PC that could run as a "terminal server" (in the proper, traditional sense... not the new fangled Windows RDP sense). So off I went Googling.

Then I stopped, engaged a braincell and remembered that in 100Mbit networks only two out of four pairs are actually used to send Ethernet data. Which leaves me four wires for something else. Coincidentally, that day I also found a hack someone had done to transport Ethernet, Power and Serial down a single piece of Cat5. This provided a useful wiring diagram and some inspiration.

My plan was to create the most basic version of a serial cable possible - simply the TX/RX and ground wires. No handshaking or flow control lines. This cable would be utter mince for sending high speed data, but since the terminal only communicates at 19200bps that isn't an issue.

Many places on the Internet will describe the wiring in a network cable, and the wiring in a serial cable. Combining the two isn't that difficult, all I'd need to make is some sort of splitter box that took a network connection, serial data and munged the two down a regular piece of Cat5, then another box that split the two out again.

One splitter box would be plugged into the serial port on my server and to the switch underneath. The "out" end of the splitter would then connect to the main cable that runs downstairs, with the other splitter on the end. I'm fairly sure nothing too bad will happen if a PC is plugged into the "unsplit" connection, but I don't feel like testing it out.