Troubleshooting eventual disconnections (Data stale)

There are numerous reports in the issue tracker, mailing list, and internet discussions about various vendors' USB devices losing connection over time. CyberPower (CPS) brand is mentioned maybe more often that others, but is certainly not alone.

Discussions at:

• https://wiki.archlinux.org/title/Network_UPS_Tools#CyberPower_UPS_keeps_disconnecting/reconnecting
• https://raspberrypi.stackexchange.com/questions/66611/nut-cyberpower-data-stale
• https://nmaggioni.xyz/2017/03/14/NUT-CyberPower-UPS/
• https://serverfault.com/questions/994338/how-can-i-fix-my-ups-repeatedly-disconnecting-and-reconnecting/995122#995122
• https://github.com/networkupstools/nut/issues/475
• https://github.com/networkupstools/nut/issues/786
• https://github.com/networkupstools/nut/issues/1029
• https://forum.netgate.com/topic/4220/trouble-with-nut-and-backups-xs-1500-via-usb/4?lang=en-US

...and countless others suggest that certain controllers may disconnect (often via USB reset) if there was no interaction for more than some time, e.g. 20 sec, while others may be overwhelmed by interactions too frequent and intense, causing the UPS controller to stall or reboot.

Such situations may need to balance pollinterval (for usbhid-ups; see manpages about equivalent settings for other drivers where available) to be frequent enough to keep the connection alive with their model, and modest enough to not outstay the welcome.

There was some work in the lead-up to NUT v2.8.0 release and/or soon after it, which should have improved the drivers' own ability to detect a connection loss/freeze and try to reconnect, e.g. PR #1335, PR #1430, PR #1632, and possibly others.

Also note that on some platforms it is possible to programmatically command the USB hardware (hubs, ports) to reset.

• Linux: in current NUT source tree there is a contributed usb_resetter instruction for the third-party script: https://github.com/networkupstools/nut/blob/master/scripts/usb_resetter/README.md
  • ...or conjure up something of that sort with usbutils package and its usbreset tool, see https://wiki.ubuntuusers.de/usbreset/
• Solaris/illumos: see an example use of cfgadm (as part of script to detect a hung driver and recycle the connection) in https://github.com/networkupstools/nut/blob/master/docs/solaris-usb.txt