[ntp:questions] Using mode byte of NMEA ref.clock driver ?

venu gopal neo.venu at gmail.com
Mon Jan 7 06:01:12 UTC 2008

Hi Folks,

I have been posting to the NTP 'questions mailing' list regarding
modification of
NMEA refclock driver. The current driver implementation supports 4800
GPGGA, GPGLL and GPRMC sentences.

We are using Accord's GPS Clock which generates NMEA sentences at 9600B.
It also generates 1PPS and a custom NMEA sentence with GPS time-stamp.

I've done few modifications to interface this with NTP.

1.  Support for 4800 and 9600
2.  Parsing of custom NMEA sentence

After going through the NMEA ref.clock driver, it seems that the
original author has choosen values 0(GPXXX), 1(GPRMC), 2(GPGGA) and
4(GPGLL) for mode field so that multiple sentences can be selected. As
posted earlier reg. using the mode byte /field to support Accord GPS
Clock, fourth bit cannot be used to denote the baudrate.

Fourth bit can denote GPZDG(8) (custom NMEA format) while the fifth
bit can be used for baudrate 0(4800) and 1(9600). But in this case
multiple sentences may be allowed only if the value of last four bits
is less than 8, because GPZDG gives GPS time and not UTC like rest of
them. So if GPZDG is selected, others sentences should not be

I need comments and suggestions from folks who have been maintaining
reference clock drivers regarding the suggested changes and its implications
if any.


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