On 2021-06-25, Jim Pennino <j...@gonzo.specsol.net> wrote: > chris <chris-nos...@tridac.net> wrote: >> On 06/25/21 17:28, Jim Pennino wrote: > ... > > Actually what I plan to do is to put a $14 USB GPS on the machine that > already has a PPS GPS attached and do away with ALL external machines. > > If there are two GPS receivers attached to the machine I have a backup > if one receiver fails.
Two is in general bad, because your machine has no idea which the better one is and is likely to pick the GPS ratehr than the PPS. > > As GPS receivers are highly unlikely to fail in some wonky mode, e.g. time > being off by some large amount, but to fail completely, there is no need > for any other reference source while I replace the failed receiver. Since both are attached to the same machine, the probability of common mode errors become high. The cleaner unpluggin the line which feeds both receivers, etc. > > Now if there is a Carrington-class coronal mass ejection or WWIII > breaks out, I will lose all time references but I will have lots of > other things to worry about that are much more important than the > computer clock and it is likely that all internet access will also be > down. That of course is a very very general common mode error, and is extremely hard to counteract. More likely are those in your office, on your floor, or in your building. > > Then on two other machines I attach two $14 USB GPS receivers and no > external references. Remember pps is a factor of about 10000 more accurate than than NMEA GPS. > > These three machines then provide time for all other machines on my > network. The three machines will provide the redundancy needed for when > one of those machines gets rebooted for updates/upgrades. Again, make sure they are all on separate electrical circuits, prefereably also in separate buildings. > > Done. > > The only foreseeable change to that I might ever make is if and when USB > 3.0 GPS receivers with PPS become cheap and available, I might swap out > the USB receivers with one of those just to see how well they work. The usb level is irrelevant. It is the PPS that is important. And pps receivers are also coming down. In fact that UBLOCK probably has a PPS output, which the manufacturer never bothered to hook upon the puck. It is hard to feed ppd over usb with any accuracy. However a separate pps line which you can attach to some irq line on the computer is probably possible even for that cheap puck. > > Yes, this scheme only gets my machines to within 10s of milliseconds to > the actual time, but that is good enough for me. > > If I needed better, I would buy one of the $685 GPS GNSS Disciplined > Rubidium clocks off ebay and get time to the nanosecond. There is still a wide gap between namosecond and 10s of milliseconds. "If walking is too slow, I can always buy a X15 to get there." Actually the difference there is far less than the difference between ns and msec. > > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
