By null modem cable I mean, female-to-female serial cross-over cable that uses DB-9 connectors. They use the RS-232C protocol. I mostly use null modem cables to upload firmware to TVs..and control them. Most TVs that have a RS-232C (Serial) port can be controlled using telnet (Like a wired remote, but more powerful).
I had a couple of retractable phone cables in the bottom of my drawer at work I thought I'd never use again, but I kept them anyway. As I was looking at them trying to figure out what I could do with them, I remembered that the RS-232C instructions in LG TV manuals say you only need 3 wires. I looked at one of the schematics and sure enough, inside the TV only the 2, 3, and 5 pins are connected. So I knew a retractable phone cable or Ethernet cable would work.
This will only work if your RS-232C devices use 4 or less wires. I haven't seen a retractable cable with more than 4 wires, but they may exist.
The second picture just shows two other types of retractable phone cables. I paid less than a $1 each for those. The wires in those are really small and hard to work with, so use the type I use in the instructions if you can.