[ntp:questions] Garmin GPS 18x LVC huge offset between PPS and NMEA -- up to 700 ms

Rob van der Putten rob at sput.nl
Tue Dec 23 13:42:37 UTC 2008


Hi there


Some of my experiences with this Garmin and GPSD;

The Garmin isn't necessaryly set to defaults. Reset with: $PGRMO,,4
Then set the output pulse length to 200 ms; $PGRMC,,,,,,,,,,,,,9,
These should be terminated with a CrLf pair.

If connected directly to Minicom, the Garmin appears to be online once 
per second. This is because the PPS is tied to DCD.
The circuit below can be used to avoid line status confusion;

Garmin		Minicom

Male    	Female
		9P	25P

RXD-----RXD	2	 3

TXD-----TXD	3	 2

GND-----GND	5	 7

       +-DCD	1	 8
       |
       *-DTR	4	20
       |
       +-DSR	6	 6

       +-CTS	7	 5
       |
       +-RTS	8	 4

SH------SH

Start Minicom with '-o' to avoid sending an init string. Exit Minicom 
with 'Ctrl-A,Z,Q' instead of 'X' to avoid a reset.

GPSD wants to change theses settings. Disable this in /etc/default/gpsd 
with; DAEMON_OPTS="-b -n"
The NMEA data is off by some 170 ... 180 ms. This time keeps changing 
all the time;

server 127.127.28.0 minpoll 4
fudge 127.127.28.0 time1 0.178 refid GPSa
server 127.127.28.1 minpoll 4 prefer
fudge 127.127.28.1 refid PPSa

NTPD want to make shore that GPS time source isn't talking nonsense. 
This means you have to use an other time source as well.


Regards,
Rob




More information about the questions mailing list