[Beowulf] precise synchronization of system clocks

Donald Becker becker at scyld.com
Tue Sep 30 12:01:57 PDT 2008

On Tue, 30 Sep 2008, Lawrence Stewart wrote:

> On Sep 30, 2008, at 12:07 PM, Lux, James P wrote:
> > On 9/30/08 8:37 AM, "Robert G. Brown" <rgb at phy.duke.edu> wrote:
> > The grottiest grungy GPS receiver can probably do 100ns on its 1pps  
> > tick,
> > and most are in the 20ns range.  There ARE receivers that have  
> > systematic
> > errors (i.e. Some sort of sawtooth in the error) and, of course,  
> > there are
> > countless schemes to compensate in one way or another.
> So someone with some grad students and soldering irons should hook up  
> the 1 pps
> output of a GPS to, say, the carrier detect pin on the unused RS232  
> ports of all their
> nodes, and write some software...

Note that you need a "component" GPS receiver, hooked up with a bunch of 
wires.  The 1 PPS output isn't available from your typical packaged 
handheld receiver.  Those output serial NMEA lines that might be a full 
second out of date.

And once you think about dealing with a 1 PPS signal, you find that you 
don't even need a GPS.  Just a pulse generator.  And you can do some other 
clever things.  Or just skip ahead and read about Purdue PAPERS -- figure 
out why it was very appealing but failed.

Meanwhile, I'll try to find out where I can plug a serial cable into a 
modern server...

Donald Becker				becker at scyld.com
Penguin Computing / Scyld Software
www.penguincomputing.com		www.scyld.com
Annapolis MD and San Francisco CA

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