On 7/21/13 6:59 AM, David J Taylor wrote:
My own notes on the GPS-18 LVC are here:
http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm
You may be able to gather something about the electrical characteristics
from that note - the device will happily feed to PC "RS-232" ports
connected in parallel (driving receive signals, that is) without problem.
I see you're driving a LED through a 1.5k resistor, and still getting
above the notional 3V threshold for the RS232 input, so that PPS must be
able to source a fair amount of current. So it's definitely not a 1 k
pullup and an open collector driving it.
I recall that during initial testing it takes some time for the PPS to
start, so think it needs at least a minimal lock. I have less
recollection about how long the PPS takes to stop in the event of lack
of satellite signal, but it does stop at some time and does not continue
for ever. Being crystal-controlled, I imagine that the free-running PPS
would be pretty accurate, not 10% out as you suggest.
Oh, I didn't actually think it would be 10% off.. more like a few ppm,
depending on temperature. I was wondering more what would happen if you
were indoors for a couple days, then went back out, what the box does.
realistically, I don't expect that it smoothly brings it back into line,
rather, I would expect it to "jump" to the correct time and timing.
I think, realistically, I should be looking at the actual sentences
coming out and figuring out whether our "GPS synchronized" state has
changed, and maybe deal with it appropriately.
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