[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.
> Richard,
> 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.
>  http://www.satsignal.eu/mrtg/daily_ntp.html
> - 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.
> Cheers,
> David

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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