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

Jason Rabel jason at extremeoverclocking.com
Fri May 18 14:26:55 UTC 2007

>> Use a daemon that listens to the UDP clock pulses and, after suitable
>> preprocessing, sends them to ntpd via the SHM driver.
>That is the approach I had in mind. What worries me is the "suitble
>preprocessing" part as well as figuring out the exact parameters to
>pass to the ntpd via the SHM driver. Does the SHM require a 1HZ PPS or
>is my ~0.1HZ  pulse OK to use?

There is nothing in the driver documentation that says it has to be a 1PPS
for SHM. I would browse over the refclock_shm.c file, it's only about 300
lines of code. My coffee hasn't kicked in this morning yet otherwise I would
trace through it (maybe later today). The SHM driver is not an instant use
driver, you have to write some other interface that takes your incoming data
and writes it to the SHM segment (I think).

> I imagine it is quite stable. The old legacy system is a very large
> scale miltary system used for high-precesion data loading of weapon
> data. 

Never assume anything. You could blindly end up trying to sync to a time
source that is less stable than the existing PCs clocks.

> I really would like to synch my internal network to a external clock
> reference (say an Internet site or GPS unit) that is very stable so I
> can measure just how stable/precise/accurate the old legacy system is.
> Unfortunaley, that is not possible in the lab environment I am working
> in. So, since the single board computer I am using for the ntpd server
> has a cheap clock in it that I know drifts in a big way, I am unsure
> how to measure the accuracy of the old legacy system.

I have a Soekris Net4501 SBC and its is a lot more stable than any of my
regular PCs... The ntp drift file score has always been single digits with
the stock crystal! You could always mod one to allow an external
high-stability reference as detailed below.


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