[LinuxPPS] Some Info about the Project I'm working on

[Paul]

But how about running NTP on all the machines? One has a GPS signal and  
a PPS and so uses the nmea patch. The others use the atom patch, but  
can get the nearest second from the nmea machine. So ethernet lags and  
jitters don't really matter.

Of course you need the new patches (hint, hint).

Paul

On 07/09/07 23:05:56, Reg Clemens wrote:
> > Thank you all for your input.
> >
> > Hopefully the following will help clear up how the system is
> configured:
> > 1) PPS - CPU's 1, 2, and 3 receive PPS via I/O pins on their
> parallel
> > port
> > 2) GPS - Only CPU 1 receives via COM1.
> > 3) Processor Communications
> > 	a) CPU 1 - COM2 to  CPU 2 - COM1
> > 	b) CPU 1 - COM3 to  CPU 3 - COM1
> > 4) Accurate - As accurate as possible
> >
> > I really like the idea of using ethernet to sync the clocks. To do
> so
> > I'm thinking I'll incorporate a switch into the system. However, I'm
> > concerned about how reliable messages sent using ethernet are. Isn't
> > their some lag time and acknowledgment issues sending/receiving data
> via
> > ethernet?
> >
> > I truly appreciate all the help.
> >
> > Adam
> >
> > On Fri, 2007-09-07 at 18:45 +0200, Rodolfo Giometti wrote:
> > > On Fri, Sep 07, 2007 at 10:41:22AM -0600, clemens at dwf.com wrote:
> > >
> > > > Yes, but my reading of his message was that he had both the GPS
> data
> > > > and the PPS signal routed to all three machines, mabe Im wrong.
> > >
> > > Nonono, you are right! I agree with you, it was just to fix it. :)
> > >
> 
> OK, as Rodolfo mentioned, you would need both the PPS signal and the
> PPS data to run ntp on a given machine.  Since you only have the PPS
> signal going to two of the machines, ntp wont run on them.
> 
> Now as an aside, the ONCORE driver in ntp would in fact be ok with
> this
> situation if you just split the data channel three ways, but most of
> the other
> drivers (I think) would get hopelessly confused with the 'traffic'
> from the
> other two ntp drivers...
> 
> That said, the TCP/IP solution should work really well in your case.
> All three machines will be on the same stretch of ethernet cable, or
> on
> the same switch, so the time for a message to go from A->B will be the
> same as the time to go from B->A.  ntp takes this time into account in
> setting the times of the two machines on the ethernet, and its only
> when
> these time become long, or assymetrical (via strange routing on the
> external ethernet) that this is a problem.
> 
> your comment re:
>    4) Accurate - As accurate as possible
> 
> still leaves a lot of questions open in my mind.
> If you need miliseconds, no problem.
> If you need microseconds, its going to be tough.
> If you need nanoseconds, forget it.
>     (or in this case the one crystal for all three machines plus some
> significant
>      work to get things REALLY syncronized  would be necessary)
> 
> Good luck
> --
>                                         Reg.Clemens
>                                         reg at dwf.com
> 
> 
> 
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