It uses a non-standard DB9 communication protoocol, which means we need a way to bridge the raw I/O to the PC, to do this I've used what I had on hand which is an old DigiSpark (Tiny).
The toolchain is a bit outdated but usable, but I've added an config.sh which configures arduino-cli to add it, then it can be uploaded with upload.sh.
Make sure the Digispark is connected correctly to the DB9 connector, and program it using upload.sh.
Then run python3 ups_man.py (will require pyserial), if all is well you will see \x08 repeatedly coming through the terminal.
If you wish you can make it a service so it runs on startup.
The default behaviour is to shutdown after 6 minutes in case of mains failure, and 30s in case of low battery. You can change this inside ups_main.py (it's very smol).