[ntp:questions] Performance?

David R. Andersen k0rx at uiowa.edu
Wed Dec 24 16:09:45 UTC 2008


I have a Garmin 18LVC GPS set up at 9600 baud and emitting the GPGGA 
sentence.  PPS is connected to pin 1 of the DB9 (DCD pin).  I typically 
get 1-2 microsecond jitter with this setup and FreeBSD O/S.  ntp.conf 
statements to enable this clock are:

server 127.127.20.1 mode 18 prefer # mode 2 enables $GPGGA operation, 
mode 16 enables 9600 baud operation
fudge 127.127.20.1 time1 0.000 flag2 1 # flag2 0=pps edge on assert, 
1=pps edge on clear

The server mode 18 parameter does two things - enable GPGGA operation 
and enable 9600 baud operation.  Fudge flag2 should be set or cleared 
depending on your PPS signal.  time1 should be set as appropriate for 
your system.  Also, be sure to set symlinks as appropriate in /dev so 
that the clock driver can find the serial port.  With FreeBSD I have 
/dev/gps1 symlinked to /dev/cuad1.  With linux, /dev/gps1 should be 
symlinked to /dev/ttyS# where # is the number of the serial port you are 
using.

Dave

dhavey wrote:
> On Dec 22, 5:41 pm, hal-use... at ip-64-139-1-69.sjc.megapath.net (Hal
> Murray) wrote:
>   
>>> I'm using Garmin LVC's.  I used the windows config program on one of
>>> them and changed the baud rate to 9600, turned the PPS on, and enabled
>>> NEMA 3.x.
>>>       
>> The NMEA driver in ntpd is not expecting you to change the baud rate.
>> There may be some way to configure it to use other baud rates, but I
>> don't remember anything like that.
>>
>>     
>>> The other one I configured using cu.  I turned off all of the output
>>> sentences except for GPGG and turned the PPS on.
>>>       
>> The default setup for the NMEA driver is to use the GPRMC sentence.
>> It can use a few others.  Check the documentation or fix it to send
>> (only) the GPRMC sentence.
>>
>> --
>> These are my opinions, not necessarily my employer's.  I hate spam.
>>     
>
>
> The performance is still really bad:
>      remote           refid      st t when poll reach   delay
> offset  jitter
> ==============================================================================
> x192.168.0.2     69.25.96.13      2 u    1   16  377    0.130
> 4.307   0.417
>  192.168.0.26    192.168.0.2      2 u   47   16  210    0.000    0.000
> 130.357
>  192.168.0.27    .STEP.          16 u    -   16    0    0.000
> 0.000   0.000
> x192.168.0.29    .GPS.            1 u   11   16  376    0.086
> -199.29   0.172
> xGPS_NMEA(0)     .GPS.            0 l    4   16  377    0.000
> -220.73   2.039
>
>
>     remote           refid      st t when poll reach   delay   offset
> jitter
> ==============================================================================
>  192.168.0.26    192.168.0.2      2 u   13   16   42    0.007
> 15.204   0.409
>  192.168.0.27    192.168.0.29     2 u 1214   16    0    0.001
> 40.722   0.000
>  192.168.0.28    71.80.83.0       3 u    2   16  377    0.018
> 199.327   0.123
> *GPS_NMEA(0)     .GPS.            0 l    3   16  377    0.000
> 0.605   3.320
>
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> https://lists.ntp.org/mailman/listinfo/questions
>
>   


-- 

David R. Andersen
Professor
Department of Electrical and Computer Engineering
Department of Physics and Astronomy
The University of Iowa
Iowa City, IA 52242

Email: k0rx at uiowa.edu <mailto:k0rx at uiowa.edu>
Tel: (319) 335-2529
Fax: (319) 353-1115
Web: http://dx.eng.uiowa.edu/dave/




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