BE550GQ: How should Windows react to a loose USB signaling cable?

[rseiler_apc]

I should say up front that the UPS itself is fine and that this question is merely about the cable's relationship to Windows.

I saw Windows 8.1 behave in a very unexpected way to a USB signaling cable that had somehow gotten unsecured (but not completely disconnected) on the UPS side. I didn't know the cable was loose, so it was quite some time before I thought to check it.

On a desktop system with just the native Microsoft UPS support (it appears in Device Manager, fully charged battery appears on the system tray), and no Powerchute, what should happen if the signaling cable is suddenly pulled or is in a halfway state?  Should the machine automatically shutdown, as happened here (see below)?  It was behaving very much like a laptop does when you try to bring it up with a battery that's nearly dead. The state of the cable somehow tricked Windows into thinking that power was about to fail and that it needs to shutdown immediately.

Background:

This started when one day I walked up to the PC, which was on and in Windows but with a sleeping monitor, and found that no mouse movement or key presses would allow me to see Windows. The monitor had turned on, but the screen was dark. I'd never seen this happen before (it's not the same thing as the monitor not coming out of sleep--it did come out of sleep).

So I rebooted it, and while it was booting, Windows displayed a "Windows is now shutting down" message even before the desktop had quite loaded. This was repeatable: turn on the machine, see Windows partially boot, see Windows say that it's shutting down, see Windows turn off the machine.

Eventually, I realized that it was the cable, and I reseated it and the problem went away. I then perused Event Viewer and found the passages you see below corresponding to each shutdown.

The kernel power manager has initiated a shutdown transition.

Shutdown Reason: Kernel API

The kernel power manager has initiated a shutdown transition.

Shutdown Reason: Battery

[BillP]

My first thought is I wonder what pins were making contact and which were not (on the RJ-50 connector in the UPS data port) may impact how it reacted. I also don't know if Windows has a setting to shut down if the PC loses communication with the UPS and that is what triggered the shutdown (which some of our other software allows that). Just some ideas/feedback I had based on what I know.

[rseiler_apc]

Yes, I meant to try testing that by pulling the cable, but I haven't yet. I'd be quite puzzled if that's the default behavior, but it's certainly possible.

[rseiler_apc · Accepted solution]

There was a surprising postscript to this story: it happened again. And this time, the cable was very much secured (in fact, taped for good measure).

I've since removed the cable to restore order, since I don't really understand what's going on with it. One day, tightening it stops the problem, and a couple days later only removing it does. No idea.