[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.
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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