[ntp:questions] fudge time1 for gps-18x-LVC?

David J Taylor david-taylor at blueyonder.delete-this-bit.and-this-part.co.uk.invalid
Sat Feb 6 18:24:53 UTC 2010

"David Lord" <snews at lordynet.org> wrote in message 
news:7t5papF3b1U1 at mid.individual.net...
> David J Taylor wrote:
>> You're confusing me even more here!  Is a summary:
>> 1 - PPS connected to DCD (serial) - low offset
>> 2 - PPS connected to DCD (serial) and NACK (parallel) - low offset
>> 3 - GPS to serial, PPS to parallel alone, PPS loses sync.
>> 4 - GPS to serial with 650ms fudge, PPS to parallel alone - low offset
> That's correct.
> That's why I ran the network cable upstairs so I could confirm
> fudge was in right direction so I was not a second out one way
> or other.
> I wasn't expecting such a large fudge time being needed.
> David

OK, how long does the GPS sentence take to send at the baud rate you are 
using?  69 characters at 4800 baud - 480 chars/sec.  About 143 msec. 
Could it be that the NMEA driver is recognising the end of the character 
string and not the start?  What is the guaranteed delay time (if any) 
between PPS and the start of the string on the GPS 18?  I recall some 
scope plots:


and your 650ms is almost exactly the delay between the start of the PPS 
signal and the end of the transmitted serial data as shown here:


The delay before the /start/ of the serial data is 350ms, and he is 
sending two sentences....

An interesting page - my thanks to Gerhard Dangl.


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