[ntp:questions] OK - You told me so! Amazing Sure gps performance on real serial

unruh unruh at invalid.ca
Thu Mar 29 19:01:49 UTC 2012

On 2012-03-29, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> Hi all,
> I finally got around to attaching a serial cable to the motherboard 
> header on my desktop computer and attaching the Sure gps to it.  This 
> NTP+GPS has been a long and winding road, and has been much more 
> difficult to get a handle on than I anticipated, including ordering a 
> number of parts and waiting what felt like eternity to get them.  
> Watched pot never boils, etc.
> WOW!  I'm blown away as I'm now getting + / - 50 MICROSECOND 
> performance.  Yes, I know, old hat to you guys, but still news to me.  I 
> can't believe decades old comm technology works so much better than 
> modern comm technology.

No. It is just that serial ports have interrupt lines and usb does not. 
Parallel ports do as well. But all those things are disappearing.
External interrupt signals are not the current thing. 

> Still, I wanted to wring out the maximum performance on USB since 3 of 
> my 4 computers don't have serial.  I eventually got around + / - 1ms, 
> which is pretty good considering the system limitations of USB.
> Unfortunately, my poor laptop has now been demoted to running with only 
> stratum 2 internet servers.  It's doing ok, and in fact, is better since 
> I have interpolation on.  However it's only making + / - approx 20 ms.  
> Also, what's bugging me is that there seems to be a continuous 20 ms 
> offset from zero.  This is certainly good enough for running from a 
> backup time source, but still confusing.  While I wanted to let the 
> internet servers be polled as much as 2 hours apart, I eventually 
> settled on letting them go back to the default polling intervals of 6 
> (64 seconds) to 10 (1024 seconds) to minimize offset fluctuations 
> between polling on the computer currently without the gps.  One problem 
> with having these intervals on the computer with the gps, is that the 
> polling interval gets continuously clamped to 64 seconds, which is 
> enough to get me banned from polling some systems.
> http://dl.dropbox.com/u/9879631/laptop%20internet%20ntp%202012-03-29.png
> http://dl.dropbox.com/u/9879631/ntp.conf%20laptop%20internet%20ntp%202012-03-29
> Eventually, I hope to have the desktop computer set up as a local time 
> server.  Before I do that, I have to configure all the other systems 
> like the laptop I was originally experimenting on, and figure out what 
> to do with firewalls and changing ip addresses via dhcp.
> Sincerely,
> Ron

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