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

John Ackermann N8UR jra at febo.com
Mon Oct 20 21:26:01 UTC 2008

Bruce, unfortunately Linux doesn't play too nicely with PPS -- long
story, but you need to do patches to get even minimal functionality with
current kernels.

An option is to use a shared memory driver called shmpps (available from
http://time.qnan.org/).  I've done this in the past, but had a little
trouble keeping the driver running reliably; it would sometimes die
silently, causing loss of the PPS source.  However, I think I was using
an earlier version than the one that's on that web page.

Using this approach will let you use the PPS signal, but won't give you
coarse time via the serial data stream.  To get that, you can use the HP
driver in ntpd which supports the HP Z3801A/58503A command set.  I think
that the Jackson Fury emulates these commands, though I haven't tried it
with this driver.

But if you can, a FreeBSD system makes a *much* better ntp server.  I am
a Linux guy, but my primary ntp boxes all are slightly modified Soekris
4501A single-board computers running a stripped down FreeBSD.  They run
rings around anything else.

Hope this helps a bit.


Bruce Taylor said the following on 10/20/2008 04:09 PM:
> 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.
>   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.
> 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.
>   Apologies if my description is vague, as I'm just getting into the
> fun and joy of kernel hacking :-)
>   Many thanks for any advice,
>   - Bruce Taylor
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