Ok, where is my config. UPS is Smart-UPS RT 2000 XL, connected to Proxmox 3.3 host with serial cable. I don't know it uses a 'secret' protocol or not, but it works very well.
Set up server (host, which UPS connected to):
Step 1. Enable net server mode in /etc/nut/nut.conf:
MODE=netserver
Step...