Looking for ntp/PPS setup guide

[Jeff W. Boote]

Kevin Oberman wrote:
> The problem is that polling mode and PPS don't work properly together,
> so I have found that the TrueTime format provides the best results.
> ctime=off
> emul=truetime
> ctime=on

Specifically, If I remember correctly... the binary format used by the
palisades driver uses the same pin as PPS making them incompatible
(DCD?).

> I am still looking at the best choice for PPS setup...PPS_SYNC (flag3 1"
> or software PPL "flag3 1". Both do very well. I suspect that the absolute

PPL would be "flag3 0", right?

> time is closest with PPS_SYNC but the stability is often better with PLL
> discipline. If PLL proves the more stable, I will use a fudge of time1
> to correct for the offset. (In my last message bnl-owamp was running
> PPS_SYNC and the system I queries was running PLL. Note the 51
> microsecond offset. I don't know yet if that's real or an artifact of
> network delays.

That is an interesting idea. I have not tried that yet. I may try "flag3
0" on one or two of our systems and see if the stability improves. (We
have 11 of these in service at the moment... The places we have real
stability problems now is where we tend to loose CDMA service for
portions of a day.)

One thing we have noticed is that the TrueTime output seems to have a
fairly large offset relative to the PPS output of the device. (i.e. The
two outputs of the exact same device seem to have a non-trivial offset
from each other.) This causes a fair bit of instability. To compensate
for this, we have been using a fudge of time1 to correct the TrueTime
output. (11 msec's is what we have found works but I suspect this is PC
hardware and OS dependent.) I am not sure how much of this delay is due
to the polling nature of the TrueTime NTP driver verses the more event
driven PPS implementation in the kernel. To determine the 11 msec value,
we used the enable calibrate mode of NTP and waited for the offset to
stabilize for a couple of days.

(Another thing of note if you are using this EndRun device: Make sure
you set "flag2 1" for the PPS device. It is the "clear" of the PPS
signal that is synchronized.)

jeff