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

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu Jun 11 15:22:25 UTC 2009

David Lord wrote:
> I'll scope pps down utp but doubt the pps will keep its rising
> edge. I'm having a box with indicator led and 5V regulator at end
> of the stock Garmin cable and coax + utp from that box downstairs.
> One pair will have +9V/0V to the regulator. It's a long while
> since I ran an rs232 cable downstairs but remember I had problems
> with higher data rates and 38400 bps being unreliable but 9600 bps
> just ok. That was also full rs232 levels rather than ttl.

Capacitive loading may slow down the edge a little, but I would expect 
that risetimes which are similar to RS-232 risetimes would be quite fine 
for PPS use, so a smooth one microsecond edge would be just fine. 
However, 38400 bps would be 26 us wide pulses, so if you had problems with 
that, perhaps it's not good enough for PPS.  I have a 5V regulator on one 
GPS 18, but decided to use the USB 5V approach for the other and it's been 

> Anyway getting better than 100us on Garmin vs better than 700ms
> with BR304 is a big enough step up already. Really all I need is
> that all pcs keep same time and several ms from utc is ok.
> Problem with ntp via broadband is sometimes a large download or
> build gives >100ms difference between servers and I expect
> feeding pps to each server will solve that problem.
> cheers
> David

With my broadband, I recall that PCs kept well within +/- 100ms, but I 
perhaps have a different usage pattern to you.  I like the idea of a 
buffered PPS feed to each server, and would be most interested to hear how 
it works out.  To be within a few ms though, having one stratum-1 server 
and the rest over the LAN would probably be adequate.  BTW: I'm using 
different minpoll and maxpoll values to have the benefit of both LAN 
stratum-1 and Internet fallback servers, without hammering the Internet 
servers too hard.

server  iburst maxpoll 6  prefer # startum-1
server  maxpoll 6  # second stratum-1

server 0.uk.pool.ntp.org minpoll 10
server 1.uk.pool.ntp.org minpoll 10


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