[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 13:11:43 UTC 2009

David Lord wrote:
> I'm intending buffering the pps to give 75r output to coax with
> another converter back to ttl at the server. The NMEA should manage
> the distance over twisted pair at 4800 baud.
> I'd rather have the option for two way in case the Garmin needs to be
> set to a different mode. I have a reel of utp I think should do.
> I'll have some sort of fan-out box to get a pps signal to each of
> servers that are powered up continuously.
> I've now seen an error in ntp log and suspect pps isn't enabled in
> kernel by default (NetBSD-5). I'll check tomorrow.
> Cheers
> David

David, if you have UTP cable, I think that's four twisted pairs, so I 
would just use one pair per signal (TX, RX, PPS)  and the remaining pair 
as an earth/+5V.  Unless you have a noisy electrical environment, I think 
that will be fine over 30m.  Screened would be better, if you have it. 
Fan-out would be nice - I've used two RS-232 input in parallel fed from 
one GPS 18 without problems.

I thought the offset figures you were quoting were for Windows - 
"between -74us and +62us".  For a PPS signal on a FreeBSD system I would 
expect much better.  I recall needing both the NMEA and the ATOM driver 
configured.  Oh, yes, I vaguely recall having to compile the kernel as 
well.  It was a few years ago....


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