ezonpinzv DB9 9 Pin Female to 3.5mm Male Plug Serial Cable RS232 to 1/8 inch Conversion DB9 Female to 3.5mm Male Jack Plug Audio Adapter Serial Data Cable

Used something similar just the other day to configure an APC UPS. As others have mentioned, it's a serial cable...think very low speed data connection. Most likely pretty old...like 10+ years. Edit: Tons of examples on Amazon. There is a possibility something may require a slightly different pin out, so you'd want to confirm before using.