[ntp:questions] Stick to PPS, even if the prefer server fails

Unruh unruh-spam at physics.ubc.ca
Wed Mar 25 21:19:25 UTC 2009

"alkopedia at googlemail.com" <alkopedia at googlemail.com> writes:

>> 1. Immediately fire the Linux OS and replace with FreeBSD. It has PPS sup=
>port ex box and very good serial drivers. You need to disable the hardware/=
>driver FIFOs to get good serial time. This of course will be secondary to t=
>he PPS.

I have no idea why and whether kernel PPS code is any better ( or worse)
than say PPS discipline using the shm PPS refclock using parallel port
interrupt. Ie, both can discipline
to about 1-2usec level. The main problem is that the ntp model is too slow
reacting to temperature induced drifts.

>> 2. Use the parallel printer port rather than a serial port for the PPS. T=
>his avoids a level converter and hardware slew delays, as well as the contr=
>ol-lead state machine of the serial driver. Expect time within a few micros=
>econds with the PPS.

I agree here that parallel seems to be better in that the system reacts
faster to a parallel interrupt than a serial, especially if you can write a
special interrupt service routine and not have the driver service routing
imposing its delays into the chain.

>2. I don't use any level converters. The MAX3243C rs232 chip seems to
>handle TTL levels fine. In the last 12 hours the offset to PPS was
>between -5 and +5 microseconds.

>Except for the PPSTIME issue the setup is working quite well, but I
>have to check out the tos mindisp settings you've mentioned.

That has to do with the system allowing the serial nmea to take over in
case the PPS signal gets lost. It should do nothing if everything is

>The complete system doesn't serve time to others. It's intention is
>monitoring. I'm building this as a diploma thesis for a
>telecommunication company. They have a large number of ntp servers
>with GPS receivers (called SSU) across the country. So my system only
>watches the offsets, that these SSUs have to assure that they are OK.

They can monitor only to the msec (or 1/10 msec)  level unless the other systems are at the same place.

More information about the questions mailing list