David Lord <[email protected]> writes: >David J Taylor wrote: >> David Lord wrote: >> [] >>> First I don't think there's any problem getting the nmea down >>> twisted pair at 4800 baud. >> >> .. agreed, but ..... >> >>> To play safe with the ttl level pps I've setup to use a simple >>> emitter follower that can drive into 75r coax. >>> >>> I didn't want to use line drivers as I don't have any handy that >>> work off single 5V supply. >>> >>> I need a converter back to ttl before going to pc and don't have >>> any spare gates in the box with compulsory flashing led and >>> ticker, so will use another couple of transistors wired as >>> Schmitt trigger. >>> >>> >>> David >> >> I think you're needlessly overcomplicating the PPS signal. I have the >> GPS 18LVC feeding /two/ parallel RS-232 interfaces, /and/ a low-current >> LED, so for a 30m length I think you would get away with a simple wire >> connection. However, have fun with the soldering iron!
>Are you getting better than 10us offsets though? :-) I get an average of 2usec from pps driving the parallel interrupt ( with "myown" interupt service daemon). I think much of that is a) ntp's handling of rate fluctuations, and b)natural variation in the interrupt handling due to computer issues. My gps 18 has about a 20m run to the computer, and is simply a "bar wire ( well one of the Cat5e twisted pair wires) connection. >I've no idea how much signal degradation there would be using >utp for pps and will both put it on spice and keep the spare >pair in cable free to give it a try. >However I'm making up various pcbs so an extra couple of tiny >jobs won't add much to the fun and is playing safe in case I >end up mounting aerials on roof or in garden, although that >looks very much less likely to be needed now. >> If you're seeing 4us offset now, that may be as good as it gets. You >> may need a more constant temperature to see better. >That's another project, putting the servers in a cupboard >with temperature control. Alternative of locating the crystals >and fitting heater is also a possibility. Cupboard idea has >advantage of cutting down noise and will help keep the pcs >from overheating. Putting in a thermistor on top of the clock crystal and read the temperature and use it to predict the rate variation. That will probably give far better results than using a temperature controll. Also using chrony ( which now has an shm refclock input at least in beta). will give better results than ntp. >David _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
