[ntp:questions] GPX18x LVC 3.50 firmware - high serial delay problem workround

David J Taylor david-taylor at blueyonder.co.uk.invalid
Fri Jan 14 08:40:25 UTC 2011


Folks,

You may recall that I had a problem with a Garmin GPS18x LVC after 
firmware upgrades, where the offset between the leading edge of the PPS 
signal and the end of the NMEA serial data exceeded one second.  With some 
help from Hal Murray who knows more of NTP than I do, we have worked round 
the problem as described here:

  http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm#gps-18x

The basic steps were:

- make the GPX18x LVC "noselect" so that NTP would not use it

- enable the peerstats to measure where NTP thought the 18x end of message 
occurred

- analyse the peerstats file to determine the apparent offset (which 
was -1.000 seconds, as it happened)

- add that offset (as +1.000 seconds) to the fudge time2 value for the 18x 
in the ntp.conf

- restart NTP

I hope that helps someone.

Cheers,
David 




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