[time-nuts] Dual PPS ntpd

Glenn Powers glenn at net127.com
Wed May 31 18:11:52 EDT 2006

The question of using Dual PPS inputs with ntpd/FreeBSD came up a  
long time ago on the ntp:questions list, but it doesn't look like it  
was ever resolved.

Has anyone here used ntpd with two PPS inputs?

What I want to do is to connect an Oncore GPS / 1 PPS  to serial0 and  
connect an undisciplined Rb. Osc. 1 PPS to serial1. (Even better  
would be to use the RTS line on serial0 for the 2nd PPS, but that  
would require some driver work.)

I'd like to use ntp to find the offset between the two PPS's and then  
apply that offset in ntpd to the second PPS. Then, if I lose GPS  
coverage, the second PPS should kick in with the offset applied.

I know the accuracy won't be as good as a directly disciplined Osc.,  
but I think it would good enough for ntp.

I think this design would qualify for the "Cheapest GPS Disciplined  
True Atomic Clock with NTP Server."


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