As part of my home network setup, I have an APC UPS (XS 1500 LCD if you’re curious). I have the UPS monitoring cable hooked up to my Ubuntu home server and run apcupsd to monitor and control the UPS behavior. After I upgraded to Ubuntu 18.04, I didn’t pay attention to the UPS setup and recently noticed that apcupsd was reporting comms failure with the UPS (other symptoms included a defunct apccontrol process – from apcupsd unsuccessfully trying to recover the comms failure).
A little bit of digging revealed that the problem was in my /etc/apcupsd/apcupsd.conf – it had the line stating DEVICE /dev/ttyS0 but you should not set the DEVICE parameter if you’re connecting via a USB cable. Simply commenting out that line and restarting the apcupsd fixed the problem. Now I just need to replace the batteries since it’s been 5 years since the last battery replacement 🙂