[ntp:questions] Re: NMEA refclock questions: baud-rate and PPS

John Pettitt jpp at cloudview.com
Sat Jan 28 20:07:50 UTC 2006

Hash: RIPEMD160

David J Taylor wrote:
> David J Taylor wrote:
>> Folks, I'm thinking of adding my new Garmin GPS 18 LVC just to see how
>> well it might function with serial input alone, so I have some
>> questions.
>> With the NMEA refclock driver (type 20):
>> - is it possible to select the baud rate without re-compiling or must
>> it be 4800?  I would have thought that having the higher baud rate
>> might perhaps improve jitter.
>> - is a PPS signal actually required, or will the driver function with
>> just the serial input (I realise that accuracy will be much worse
>> without the PPS, but his is just for testing).
>> - is it possible to use NTP just to observe a refclock, without
>> actually syncing time with it?
>> Thanks,
>> David
> OK, I did some testing!  I'd still like to know if you can select the COM 
> port and baud rate without recompiling.

Nope it's a hard define in refclock_nmea.c

#define SPEED232        B4800   /* uart speed (4800 bps) */

> - yes, it works without the PPS input, although the jitter seems as high 
> as 5 msec

Sounds about right - 1 byte = ~2ms @ 4800 - be sure to use a "real" serial port as a USB one will
add lots of jitter.  If you use the Garmin utils to turn off all the NMEA messages apart from the
one you want it will be a bit better (mine was ~1ms on FreeBSD) - compared to around ~8us with PPS.
> - set stratum to 10 to stop it syncing and just observe - would that work?

Sounds like a good strategy - I don't know if "noselect" works for refclocks (never tried it) but if
it does that will give you what you are looking for.

> David 


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