Check the voltages on your adapter. I use such adapters in the machine shop so machinists can share the CNC programs they write on a PC with their CNC controllers. The CNC controllers can be fussy about voltages, and some cheap RS232-USB adapters only generate +/-5vdc. It's within the RS232 spec and newer RS232 chips are happy with that, but older systems might want 12v or more. Another issue is handshake lines. Not all adapters provide all the handshake lines. Some are "3-wire" data-only with only ground, transmit, and receive connected. Some devices will want 5 or 7 wire connections, with RTS/CTS and DTR/DSR signals included. Check that the adapter you buy provides all the signals your device needs. Which service are you using to manage your UPS? Nut? Something else? They probaby have a mailing list, website, or wiki where you can find out what adapters work well with which UPS units. (Be sure to post back here when you get an answer.)