[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Jun 12 21:14:30 UTC 2009

David J Taylor wrote:
> Unruh wrote:
> []
>> The nmea is virtually useless for accurate timing. The main thing that
>> the unit gives you is the PPS. You have to makes sure you do not
>> degrade it.
> The GPS 18x LVC only claims an accuracy of one microsecond for PPS in 
> the first place.  The RS-232 receivers may well have some filtering 
> limiting the risetime to a couple of microseconds (i.e. good enough for 

Note, if you actually do this you are deliberately grossly 
mis-terminating the line, not that that is necessarily a bad thing to do.

Incidentally, it is the transmitter's responsibility to control rise 
time, not the receiver's in RS232C.

> the maximum baud rate, typically 115,200 baud), so even if the final 

Baud rate should be irrelevant for modem control signals, but even for 
asynch data, the actual sampling resolution is 16 times better.

> risetime after the cable is a microsoecond or so, that's not going to 
> degrade the accuracy signifcantly for normal use.  Not an issue.

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