Hi, On Tuesday 12 May 2009 18.59:42 Michael Martin wrote: > Serial to Ethernet adapter and communicate with the Linix host using a > virtual serial driver. While I have no problem getting the GPS TX data > stream. the SHMPPS driver does not see the PPS signal
This is just fundamentally wrong.
The idea of PPS is that you have an exact timestamp when the second changes.
While USB is already stretching this assumption to the limits, you're at
least still working with dedicated hardware on the time server.
Now imagine how much will happen to your PPS signal until it comes into the
virtual driver: sampling by the serial -> eth adapter, wrapping in whatever
protocol this uses, non-deterministic delay introduced by switch(es) on the
road, possibly additional delay because your ethernet port was just busy
receiving another packet (an ntp request, for example :-) etc. etc.
If you can't get the GPS signal to the time server, why not get a small
embedded box running just ntp and move it to the GPS recceiver? You
obviously have ethernet there, so then run the ntp protocol to the server
where you actually need your time. Accuracy will be *much* higher.
Small embedded comupters running linux don't cost much these days and yeat
just a few watts of power. You could run it off a flash (no moving parts!),
so it could probably run years without any maintenance. Assuming you block
everything except ntp packets (put in a size limit, and forbid remote
configuration to ntpd), you could probably even ignore security updates
(except, of course, issues with ntp itself and some parts of the low level
networking kernel stuff.)
Just a thought.
Another idea is to use a radio receiver instead of a GPS one. You can often
still get radio signal in server rooms even in basements.
--vbi
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