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Friday, August 3, 2012

Synchronized clocks vs. my school's poor little UPS

Back when we decided to upgrade my high school's Unix box, we also scraped together some money to put it on a UPS. This was at the urging of one of my friends. He knew all about these things since he had one at home. When we put the new machine online a couple of weeks later, we added that UPS to the mix.

Strange things started happening. On occasion, one of us would be up there working in the office where the box lived, and the UPS would go on battery for no apparent reason. The lights were still on, and indeed, it was life as normal throughout the school. Nothing had happened to the power. It just seemed to cycle onto battery once in a while, and nobody knew why.

Then, one day, it occurred to me: our clocks made this weird buzzing noise now and then, like before the top of an hour. Was the UPS doing it at the same time? Maybe they're related. Perhaps the clocks work by superimposing some kind of signal on the power line that really annoys the UPS?

I knew a bit about how X-10 worked, with its 120 kHz signal blasts at zero crossing, and figured maybe there was something similar for our clocks. The UPS must have just been very sensitive, and it was seriously annoyed by this. The bad part is that all of these short cycles really beat up our battery. It didn't have much runtime after suffering through this for a few months.

At some point, someone realized that the UPS had a little recessed button in the back for "sensitivity". Poking it a few times would make it less demanding about input power, and eventually we found a setting where it would live and let live when the clocks did their thing.

I decided to research this at last before writing this post. It turns out that (some) Simplex clocks use a high frequency signal on the power line for sync purposes. My "X-10 type signals" hypothesis was just a guess at the time, but it looks like I wasn't far off.

Obviously, I'll never know exactly what was going on, but all of the evidence points to this sort of thing. I don't have any system logs which could confirm whether it happened at x:57:54 hourly and 5:57:54 AM/PM daily, but it seems to fit with my memories of that time.

So, if you have something strange happening with your power and your equipment is in a rather old institutional type building (like a public school), then maybe you're just riding a clock sync pulse without even realizing it. What fun!