[ntp:questions] Re: Toggling time sync between two servers

Hal Murray hmurray at suespammers.org
Mon Mar 27 05:08:10 UTC 2006


>As to why the two references that we have seem to be different at
>times, I believe this is most likely due to our third party software or
>possibly the fact that we have "enable pll" in our NTP configuration
>file.  From what I've been told by a co-worker, NTP can not read time
>off of a serial line with AIX version 3.4 and that we would need to
>upgrade to version 4 or 5.  This is why we have a third party
>application reading time off of the serial line and then disciplining
>the local clock on the NTP servers instead of having NTP do all of this
>for us.  If anyone knows this to be false, please let me know.

I don't know anything about AIX.

Most GPS clocks send a message over a serial port and also
flap a signal once per second: PPS, Pulse Per Second.
One edge of that pulse is on the second boundary.

The serial port isn't very good for timekeeping because of
the jitter.  It will probably get you within a few milliseconds.
Most kernels include serial port support that works with ntpd.

Using the PPS signal requires special support from the kernel.
The usual way is to connect it to one of the modem control
signals on the serial port.  PPS support is included with the normal
kernel sources on FreeBSD.  You probably have to turn it on in
your kernel building config file.  Linux requires a patch which
is readily available.

If accuracy of a few ms is good enough, you may not need the PPS
support in your kernel.

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