[ntp:questions] How bad is USB?
Richard B. Gilbert
rgilbert88 at comcast.net
Fri May 1 13:29:31 UTC 2009
David J Taylor wrote:
> Richard B. Gilbert wrote:
>> You can try it and see what happens! The results may be sufficiently
>> good for your purposes. You will almost certainly not get microsecond
>> accuracy. If you are willing to settle for +/- 10 milliseconds you
>> can almost certainly get that.
>> There are (at least) two groups of people who hang out here:
>> a. The "chimeheads" in pursuit of the "one true time", and
>> b. The "synchs" who don't really care what time it is as long as all
>> their machines have the SAME time.
>> Both groups use the same tool, NTPD. It turns out that having a
>> stable and accurate source of time makes it much easier to get a
>> whole herd of machines to agree on what time it is.
> Yes, I /could/ try it, but I was rather hoping that someone else already
> had. I also think that it will be somewhere between 1 microsecond and
> 10 milliseconds, but just where?
> I'm in a third category (probably) - as long as my PCs "sound" to have
> the same time, I'm happy. Actually, I want UTC as well, but as I have a
> speaking clock working on two PCs it's "nice" when they both speak at
> the same time. I wonder how close that needs to be - a few
> milliseconds, perhaps? Here's what I actually get - all with Windows.
> - within about 250 microseconds on my serial/PPS ref-clock PCs
> - within about 2 milliseconds on my "Synced from ref-clock" PCs
> - within about 25 milliseconds on my Vista PC.
> Yes, I used to get better on the ref-clock PCs when running FreeBSD, but
> I've since been able to shut down that PC and just use a Windows box
> instead. It's good enough for me.
I have not tried to use USB devices as time sources. The conventional
wisdom here says "don't"! If I already had such a device I'd try it.
Given the conventional wisdom that USB is not suitable I'm not going to
waste my money or my time. YMMV!
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