[ntp:questions] Synching to a Legacy System as an External Clock Reference

Steve Kostecke kostecke at ntp.isc.org
Fri May 18 02:25:53 UTC 2007

On 2007-05-18, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:

> michael.antinucci at lmco.com wrote:

>> I have an old legacy external system that I need to synchronize with
>> that sends custom protocol UDP packets containing a single field in
>> the packet that contains the legacy system's current clock time (in
>> milliseconds).
> Since your internal systems are already synchronized using NTP, it
> should be fairly trivial to install ntpd on the external system and
> get it synchronized too.

Not if that old legacy system isn't an architecture that ntpd has been
ported to.

>> The current clock time starts at 0 when the system boots and
>> increments forever thereafter. The frequency of this message is at
>> least every 10 seconds, not 1 PPS like I would like.


>> My hope is to have my internal system's NTP server receive the 10
>> second clock pulses from the old legacy system over UDP and have my
>> internal system's NTP server synchronize with it.

Use a daemon that listens to the UDP clock pulses and, after suitable
preprocessing, sends them to ntpd via the SHM driver.

>> I read this fine article (which I think is the path to take) but I
>> need a clearer explanation on what to do.
>> http://www.ee.udel.edu/~mills/ntp/html/extern.html
> I think that, in the article you refer to, Dr. Mills is referring to a 
> system or a hardware clock that actually knows what time it is!

The stability of the time base is of primary importance. Numbering the
seconds is trivial in comparison.

> Your external system has no clue what time it is as we understand it. 
> It just ticks periodically and the period is apparently "approximately 
> ten seconds"

Actually, it sends a packet containing the clock counter (ostensibly
milliseconds since system startup) approximately every ten seconds.

That's not the same thing as "just tick[ing] periodically".

Steve Kostecke <kostecke at ntp.isc.org>
NTP Public Services Project - http://ntp.isc.org/

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