[time-nuts] Getting NTP/Linux to work with GPS-DO?

Björn Gabrielsson bg at lysator.liu.se
Mon Oct 20 20:42:39 UTC 2008

Hi Bruce,

On Mon, 2008-10-20 at 16:09 -0400, Bruce Taylor wrote:
> Folks,
>   Has anyone gotten a Jackson Labs 'Fury' gps-do to work as a time/pps
> source for Linux?
>   Our lab is looking to use both the stable 10MHz clock for various
> LOs on a radio-telescope, and also get reasonably tight timing for a
> software correllator to use in discriminating pulsar signals.
>   Our Linux guy has lost most of his hair out trying to get the NTP
> daemon to receive the PPS signal and thus get NTP to see the GPS-DO as
> a stratum 0 source.

Try a 2.4-kernel, see below.

>   Our configuration is: vanilla Linux from kernel.org (version 2.6.23)
> running on an intel platform.  We've applied the LinuxPPS kernel
> patch, and we're feeding the serial port with the Fury in NMEA mode.

LinuxPPS is _very_ much work in progress. You are on the bleeding edge!
I am not sure the dust has settled enough for stable performance.

If your LAB have the option -- and some hair left -- try the latest 2.4
kernel with a working PPSKit-patch.


Do not know if there are later ones than 2.1.7 somewhere else.

I have 2.4.3X-something kernel running on a lab laptop with a standard
NMEA serial stream with PPS on DCD, that converges down to sub 10us
offset within a minute or two. (There might be reasons not to trust the
NTP stats down at this level, but...)

> We made the required internal mod on the Fury to feed the PPS signal
> out on the serial port, and can see it blinking using a scope.

Most serial ports I have tested will tolerate a standard PPS-pulse even
if its voltage levels are (way) noncompliant with RS232. YMMV!

To see if you get the pulses through the serial port, you might run
the radioclkd in test mode.

That will show you both edges on signals attached to DCD, CTS or DSR.
This approach could also be used to time your signals from a user space

>   Apologies if my description is vague, as I'm just getting into the
> fun and joy of kernel hacking :-)

Good luck!

>   Many thanks for any advice,
>   - Bruce Taylor
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